1.   Anatomy of the brain.

2.   Sensory organs.

3.   Peripheral nervous system. Autonomic nervous system.

Lesson # 5

 

Theme 1.  Anatomy of the brain.

The Brain [encephalon, cerebrum] during the fourth week develops from three primary cerebral vesicles: prosencephalon, mesencephalon and rhomboencephalon. Prosencephalon forms two secondary cerebral vesicles: telencephalon and diencephalon. Rhomboencephalon gives origin for medulla oblongata and metencephalon. Mesencephalon separates from rhomboencephalon by isthmus.

Metencephalon developes into pons and cerebellum. Midbrain comprises tectum and pedunculi cerebri. Diencephalon contains thalamus and hypothalamus. Telencephalon (forebrain) gives origin for rhinencephalon, basal nuclei of gray matter, pallium of hemispheres, corpus callosum and fornix.

 



Scheme showing the connections of the several parts of the brain.

 

The brain can be divided into two cerebral hemispheres, brain stem and cerebellum. A hemisphere has a base, dorsolateral and medial surfaces. 12 pairs of the cranial nerves exit from the encephalon base (see table below).

 

On the brain base students should find:

·        medulla oblongata

·        pons

·        medii cerebellar pedunculi

·        cerebral pedunculi, interpeduncular fos­sa lies between the two peduncles. Its floor is perforated by large numbers of blood vessels, the posterior perforated sub­stance

·        2 mammillary bodies are located forward from interpeduncular fos­sa

·        tuber cinereum with infundibulum that carries the hypophysis

·        optic tracts which join each other and form optic chiasm

·        optic nerve

·        olfactory triangles with anterior perforated sub­stance

·        olfactory tracts

·        olfactory bulbs

 

Cranial Nerves:

Num-ber

Name

 Position

Foramen

Function

I

Olfactory

olfactory bulb

openings of the cribriform plate of ethmoid bone

smell

II

Optic

optic chiasm

optic canal

vision

III

Oculomotor

interpeduncular fos­sa

superior orbital fissure

eye muscles

IV

Trochlear

superior medullary velum

superior orbital fissure

eye muscles

V

Trigeminal

between the pons and medii cerebellar pedunculi

(1)     Ophthalmic: superior orbital fissure;

(2)     Maxillary: foramen rotundum;

(3)     Mandibular: foramen ovale

face sensation & mastication

VI

Abducent

between the pons and pyramids

superior orbital fissure

eye muscles

VII

Facial

cerebellopontine angle

internal acoustic meatus, stylomastoid foramen

face muscles; salivary & lacrimal glands

VIII

Vestibulo-cochlear

cerebellopontine angle

internal acoustic meatus

hearing & balance

IX

Glosso-pharyngeal

dorsal lateral sulcus of medulla oblongata

jugular foramen

pharynx; tongue & parotid gland

X

Vagus

dorsal lateral sulcus of medulla oblongata lower X pair

jugular foramen

pharynx, larynx & viscera

XI

Accessory

dorsal lateral sulcus of medulla oblongata lower XI pair

jugular foramen

neck muscles (Trapezius/sternocleidomastoid)

XII

Hypoglossal

between olive and pyramid

hypoglossal canal

tongue & neck muscles

 

 

Brain or Encephalon. General Considerations and Divisions.—The brain, is contained within the cranium, and constitutes the upper, greatly expanded part of the central nervous system. In its early embryonic condition it consists of three hollow vesicles, termed the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon; and the parts derived from each of these can be recognized in the adult.



Base of brain.

 

 Thus in the process of development the wall of the hind-brain undergoes modification to form the medulla oblongata, the pons, and cerebellum, while its cavity is expanded to form the fourth ventricle. The mid-brain forms only a small part of the adult brain; its cavity becomes the cerebral aqueduct (aqueduct of Sylvius), which serves as a tubular communication between the third and fourth ventricles; while its walls are thickened to form the corpora quadrigemina and cerebral peduncles. The fore-brain undergoes great modification: its anterior part or telencephalon expands laterally in the form of two hollow vesicles, the cavities of which become the lateral ventricles, while the surrounding walls form the cerebral hemispheres and their commissures; the cavity of the posterior part or diencephalon forms the greater part of the third ventricle, and from its walls are developed most of the structures which bound that cavity.

The telencephalon includes: (1) the cerebral hemispheres with their cavities, the lateral ventricles; and (2) the pars optica hypothalami and the anterior portion of the third ventricle (already described under the diencephalon). As previously stated (see page 744), each cerebral hemisphere may be divided into three fundamental parts, viz., the rhinencephalon, the corpus striatum, and the neopallium. The rhinencephalon, associated with the sense of smell, is the oldest part of the telencephalon, and forms almost the whole of the hemisphere in some of the lower animals, e. g., fishes, amphibians, and reptiles. In man it is rudimentary, whereas the neopallium undergoes great development and forms the chief part of the hemisphere.

 

The Cerebral Hemispheres.—The cerebral hemispheres constitute the largest part of the brain, and, when viewed together from above, assume the form of an ovoid mass broader behind than in front, the greatest transverse diameter corresponding with a line connecting the two parietal eminences. The hemispheres are separated medially by a deep cleft, named the longitudinal cerebral fissure, and each possesses a central cavity, the lateral ventricle.

  The Longitudinal Cerebral Fissure (fissura cerebri longitudinalis; great longitudinal fissure) contains a sickle-shaped process of dura mater, the falx cerebri. It front and behind, the fissure extends from the upper to the under surfaces of the hemispheres and completely separates them, but its middle portion separates them for only about one-half of their vertical extent; for at this part they are connected across the middle line by a great central white commissure, the corpus callosum.

  In a median sagittal section (720) the cut corpus callosum presents the appearance of a broad, arched band. Its thick posterior end, termed the splenium, overlaps the mid-brain, but is separated from it by the tela chorioidea of the third ventricle and the pineal body. Its anterior curved end, termed the genu, gradually tapers into a thinner portion, the rostrum, which is continued downward and backward in front of the anterior commissure to join the lamina terminalis. Arching backward from immediately behind the anterior commissure to the under surface of the splenium is a second white band named the fornix: between this and the corpus callosum are the laminæ and cavity of the septum pellucidum.

 

Surfaces of the Cerebral Hemispheres.—Each hemisphere presents three surfaces: lateral, medial, and inferior.

  The lateral surface is convex in adaptation to the concavity of the corresponding half of the vault of the cranium. The medial surface is flat and vertical, and is separated from that of the opposite hemisphere by the great longitudinal fissure and the falx cerebri. The inferior surface is of an irregular form, and may be divided into three areas: anterior, middle, and posterior. The anterior area, formed by the orbital surface of the frontal lobe, is concave, and rests on the roof of the orbit and nose; the middle area is convex, and consists of the under surface of the temporal lobe: it is adapted to the corresponding half of the middle cranial fossa. The posterior area is concave, directed medialward as well as downward, and is named the tentorial surface, since it rests upon the tentorium cerebelli, which intervenes between it and the upper surface of the cerebellum.

  These three surfaces are separated from each other by the following borders: (a) supero-medial, between the lateral and medial surfaces; (b) infero-lateral, between the lateral and inferior surfaces; the anterior part of this border separating the lateral from the orbital surface, is known as the superciliary border; (c) medial occipital, separating the medial and tentorial surfaces; and (d) medial orbital, separating the orbital from the medial surface. The anterior end of the hemisphere is named the frontal pole; the posterior, the occipital pole; and the anterior end of the temporal lobe, the temporal pole. About 5 cm. in front of the occipital pole on the infero-lateral border is an indentation or notch, named the preoccipital notch.

  The surfaces of the hemispheres are moulded into a number of irregular eminences, named gyri or convolutions, and separated by furrows termed fissures and sulci. The furrows are of two kinds, complete and incomplete. The former appear early in fetal life, are few in number, and are produced by infoldings of the entire thickness of the brain wall, and give rise to corresponding elevations in the interior of the ventricle. They comprise the hippocampal fissure, and parts of the calcarine and collateral fissures. The incomplete furrows are very numerous, and only indent the subjacent white substance, without producing any corresponding elevations in the ventricular cavity.

The medulla oblongata also called as cerebral bulb, located between the py­ramidal decussation and the lower margin of the pons, forms the transi­tion from the spinal cord to the brain. The anterior median fissure separates the pyramids. The anterior and posterior lateral sulcus boundary the olives. On the posterior surface funiculi thicken to form the tuberculum nuclei cuneati and the gracilis, which are bordered in the midline by the pos­terior median sulcus. Posterior surface of the medulla oblongata form a lower part of rhomboid fossa.

Gray substance of the medulla oblongata is presented by:

·        Olivar nuclei

·        gracilis and cuneatus nuclei

·        cardiac, vasomotor and respiratory centers

·        nuclei of the IX-XII cranial nerves

White substance of the medulla oblongata consists of ascending tracts that form medial lemniscus and descending tracts and reticular formation.

 

The hind-brain or rhombencephalon occupies the posterior fossa of the cranial cavity and lies below a fold of dura mater, the tentorium cerebelli. It consists of (a) the myelencephalon, comprising the medulla oblongata and the lower part of the fourth ventricle; (b) the metencephalon, consisting of the pons, cerebellum, and the intermediate part of the fourth ventricle; and (c) the isthmus rhombencephali, a constricted portion immediately adjoining the mid-brain and including the superior peduncles of the cerebellum, the anterior medullary velum, and the upper part of the fourth ventricle.

Mesencephalon

Mesencephalon ventrally is formed by the cerebral peduncles and dorsally lamina tecti. The cerebral aqueduct (Sylvius) is a cavity of the mesencephalon. On the dorsal surface of the mesencephalon lies the quadrigeminal plate, with two upper and two lower hillocks, the superior (subcortical visual centre) and inferior colliculi (subcortical hearing centre). Brachii from the superior colliculi extend to the lateral geniculate body, brachii from the inferior colliculi extend to the medial geniculate body.

Cerebral peduncles are the thick cords that extend from pons to the serebral hemispheres. The interpeduncular fossa lies between the two peduncles. Its floor is perforated by large numbers of blood vessels, the posterior perforated sub­stance. In sulcus on the medial surface of the peduncles there is oculomotor nerve [III]. In section of the peduncles substantia nigra separates the tegmen and the base. The base is formed by conducting tracts that pass from cortex cerebri to the spinal cord, medulla oblongata and pons. There are nucleus ruber (red) in the tegmen, on the level of superior colliculi oculomotor [III] (motor) and accessory (Yakubovycha, Edinger-Westphal /parasympathetic/) nuclei are located. On the level of inferior colliculi motor trochlear [IV] is located.

Cerebral aqueduct (Sylvius) is a narrow canal that connects III and IV ventricles and surrounded by gray substance.

 

Nuclei of the V-XII Cranial Nerves

No

Name

Nucleus

Function

Location

V

Trigeminal

Motor nucleus

mesencephalic, pontine, spinal nuclei

Motor

sensory

Pons,

mesencephalon, pons, spinal cord

VI

Abducent

Abducent

Motor

Colliculus facialis (superficially)

VII

Facial

Facial (forms genu)

nucleus of the tractus solitarius

superior salivatory

Motor

Sensory

parasympathetic

Colliculus facialis (deep),

Pons

VIII

Vestibulo-cochlear

Superior, inferior, medial and lateral vestibular (4),

Ventral and dorsal cochlear (2)

All sensory

Vestibular area

lateral recess

IX

Glosso-pharyngeal

Ambiguus

nucleus of the tractus solitarius

Inferior salivatory

Motor

Sensory

Parasympathetic

Medial eminence

Medulla oblongata

X

Vagus

Ambiguus

nucleus of the tractus solitarius

dorsal nucleus

Motor

Sensory

parasympathetic

Medial eminence

Medulla oblongata

Trigone of the vagus nerve

XI

Accessory

Cranial nucleus,

spinal nucleus

Motor

Motor

Medial eminence

spinal cord

XII

Hypoglossal

hypoglossal

Motor

Trigone of the hypoglossal nerve

   The nuclei of the trigeminal nerve (nuclei n. trigemini) in the pons are two in number: a motor and a sensory. The motor nucleus is situated in the upper part of the pons, close to its posterior surface and along the line of the lateral margin of the fourth ventricle. It is serially homologous with the nucleus ambiguus and the dorso-lateral cell group of the anterior column of the spinal cord. The axis-cylinder processes of its cells form the motor root of the trigeminal nerve. The mesencephalic root arises from the gray substance of the floor of the cerebral aqueduct, joins the motor root and probably conveys fibers of muscle sense from the temporal, masseter and pterygoid muscles. It is not altogether clear whether the mesencephalic root is motor or sensory. The sensory nucleus is lateral to the motor one, and beneath the superior peduncle. Some of the sensory fibers of the trigeminal nerve end in this nucleus; but the greater number descend, under the name of the spinal tract of the trigeminal nerve, to end in the substantia gelatinosa of Rolando. The roots, motor and sensory, of the trigeminal nerve pass through the substance of the pons and emerge near the upper margin of its anterior surface.

1.      The nucleus of the abducent nerve (nucleus n. abducentis) is a circular mass of gray substance situated close to the floor of the fourth ventricle, above the striæ medullares and subjacent to the medial eminence: it lies a little lateral to the ascending part of the facial nerve. The fibers of the abducent nerve pass forward through the entire thickness of the pons on the medial side of the superior olivary nucleus, and between the lateral fasciculi of the cerebrospinal fibers, and emerge in the furrow between the lower border of the pons and the pyramid of the medulla oblongata.

  4. The nucleus of the facial nerve (nucleus n. fascialis) is situated deeply in the reticular formation of the pons, on the dorsal aspect of the superior olivary nucleus, and the roots of the nerve derived from it pursue a remarkably tortuous course in the substance of the pons. At first they pass backward and medialward until they reach the rhomboid fossa, close to the median sulcus, where they are collected into a round bundle; this passes upward and forward, producing an elevation, the colliculus facialis, in the rhomboid fossa, and then takes a sharp bend, and arches lateralward through the substance of the pons to emerge at its lower border in the interval between the olive and the inferior peduncle of the medulla oblongata.

  5. The nucleus of the cochlear nerve consists of: (a) the lateral cochlear nucleus, corresponding to the tuberculum acusticum on the dorso-lateral surface of the inferior peduncle; and (b) the ventral or accessory cochlear nucleus, placed between the two divisions of the nerve, on the ventral aspect of the inferior peduncle.

  The nuclei of the vestibular nerve. (a) The medial (dorsal or chief vestibular nucleus), corresponding to the lower part of the area acustica in the rhomboid fossa; the caudal end of this nucleus is sometimes termed the descending or spinal vestibular nucleus. (b) The lateral or nucleus of Deiters, consisting of large cells and situated in the lateral angle of the rhomboid fossa; the dorso-lateral part of this nucleus is sometimes termed the nucleus of Bechterew.

  The fibers of the vestibular nerve enter the medulla oblongata on the medial side of those of the cochlear, and pass between the inferior peduncle and the spinal tract of the trigeminal. They then divide into ascending and descending fibers. The latter end by arborizing around the cells of the medial nucleus, which is situated in the area acustica of the rhomboid fossa. The ascending fibers either end in the same manner or in the lateral nucleus, which is situated lateral to the area acustica and farther from the ventricular floor. Some of the axons of the cells of the lateral nucleus, and possibly also of the medial nucleus, are continued upward through the inferior peduncle to the roof nuclei of the opposite side of the cerebellum, to which also other fibers of the vestibular root are prolonged without interruption in the nuclei of the medulla oblongata. A second set of fibers from the medial and lateral nuclei end partly in the tegmentum, while the remainder ascend in the medial longitudinal fasciculus to arborize around the cells of the nuclei of the oculomotor nerve.

 

Diencephalon. Structure of thalamus, epithalamus, metathalamus. Hypothalamus. Third ventricle.

Diencephalon comprises the thalamencephalon and hypothalamus. Thalamencephalon consists of thalamus opticus, epithalamus and metathalamus. Hypothalamus formed by front optic part and back (olfactory) part.

Thalamus opticus is a paired body, which consists of gray substance. In the front it carries the anterior tubercle. The posterior extremity pulvinar is expanded, directed backward and lateralward, and overlaps the superior colliculus. Each thalamus about 4 cm. in length, and presents two extremities, an anterior and a posterior, and four surfaces, superior, inferior, medial, and lateral. Medial surfaces of the right and left thalamus communicated by interthalamic adhesion. Upper surfaces of the thalamus are bordered by stria medullaris (medial) and stria terminalis (lateral). Gray substance forms the anterior, medial, dorsal and ventrolateral groups of nuclei, which are separated each from other by white substance.

Epithalamus consists of the pineal body (Epiphysis), and the habenulae with trigonum habenulae, the posterior commissure. Pineal body is connected with thalamus by the habenulae.

   The Metathalamus comprises the geniculate bodies, a medial and a lateral. The medial geniculate body (corpus geniculatum mediale) lies under cover of the pulvinar of the thalamus. The inferior brachium from the inferior colliculus attaches to the medial geniculate bodies. The lateral geniculate body (corpus geniculatum laterale) is an oval elevation on the lateral part of the pulvinar. The superior brachium from the superior colliculus attaches to the lateral geniculate bodies.

Anterior part of the Hypothalamus consists of the optic chiasm and tuber cinereum with infundibulum that carries the hypophysis. Posterior part consists of the mammillary bodies and subthalamic region that carries the corpus subthalamicum (nucleus of Luis). The third ventricle, the cavity of the diencephalon has 6 walls:

·        lateral walls formed by medial surface of the thalamus

·        lower wall (floor) formed by hypothalamic region. There are infundibuli recess and optic recess

·        anterior wall formed by terminal lamina, columna fornicis and anterior cerebral commissura

·        anterior wall is formed by the habenular commissure and posterior commissure. There is suprapineal reces

·        upper wall (roof) formed by tela choroidea of the III ventricle with plexus choroideus

 

There is interventricular foramen (of Monro) between anterior thalamic tubercle and columna fornicis. Foramen communicates the III ventricle with the lateral ventricles of cerebrum. Cerebral aqueduct connects the III ventricle with the cavity of the IV ventricle.

Pineal body

The pineal body (corpus pineale; epiphysis) is a small, conical, reddish-gray body lies in the depression between the superior colliculi and has an endocrine role. Habenulae extend from the epiphysis to the right and left thalamus. The pineal recess of the third ventricle is located near base of the epiphysis. The epiphysis covered by capsule externally, the septa separate glandular parenchyma into lobuli. Special glandular pinealocytes and gliocytes are the cells of the epiphysis. Often there is “sand” in the gland of adults. The epiphysis produces hormone which inhibits the hypophysis activity until puberty age and takes part in regulation of the metabolism.

Hypophysis

The hypophysis (pituitary endocrine gland) is a reddish-gray, somewhat oval mass. It is attached to the end of the infundibulum, and is situated in the fossa hypophyseos of the sphenoidal bone, where it is retained by a circular fold of dura mater, the diaphragma sellae. The hypophysis consists of anterior (adenohypophysis) part and posterior (neuorohypophysis) part. Adenohypophysis has three portions: anterior (or pars distalis), pars intermedia and pars tuberalis. Neuorohypophysis has pars nervosa and infundibulum. Adenohypophysis (pars distalis) secretes somatotropin, adrenocorticotropin, thyrotropin, folliculotropin, prolactin and luteotropin. Pars intermedia produces melanocytestimulating hormone. Neuorohypophysis secretes vasopressin and oxytocyn both of which are produced in the hypothalamus.

The diencephalon is connected above and in front with the cerebral hemispheres; behind with the mid-brain. Its upper surface is concealed by the corpus callosum, and is covered by a fold of pia mater, named the tela chorioidea of the third ventricle; inferiorly it reaches to the base of the brain.

  The diencephalon comprises: (1) the thalamencephalon; (2) the pars mamillaris hypothalami; and (3) the posterior part of the third ventricle. For descriptive purposes, however, it is more convenient to consider the whole of the third ventricle and its boundaries together; this necessitates the inclusion, under this heading, of the pars optica hypothalami and the corresponding part of the third ventricle—structures which properly belong to the telencephalon.

 

The Thalamencephalon.—The thalamencephalon comprises: (1) the thalamus; (2) the metathalamus or corpora geniculata; and (3) the epithalamus, consisting of the

trigonum habenulæ, the pineal body, and the posterior commissure.

 



Dissection showing the ventricles of the brain.

 

  The Thalami (optic thalamus) ( 716, 717) are two large ovoid masses, situated one on either side of the third ventricle and reaching for some distance behind that cavity. Each measures about 4 cm. in length, and presents two extremities, an anterior and a posterior, and four surfaces, superior, inferior, medial, and lateral.

  The anterior extremity is narrow; it lies close to the middle line and forms the posterior boundary of the interventricular foramen.

  The posterior extremity is expanded, directed backward and lateralward, and overlaps the superior colliculus. Medially it presents an angular prominence, the pulvinar, which is continued laterally into an oval swelling, the lateral geniculate body, while beneath the pulvinar, but separated from it by the superior brachium, is a second oval swelling, the medial geniculate body.

  The superior surface is free, slightly convex, and covered by a layer of white substance, termed the stratum zonale. It is separated laterally from the caudate nucleus by a white band, the stria terminalis, and by the terminal vein. It is divided into a medial and a lateral portion by an oblique shallow furrow which runs from behind forward and medialward and corresponds with the lateral margin of the fornix; the lateral part forms a portion of the floor of the lateral ventricle, and is covered by the epithelial lining of this cavity; the medial part is covered by the tela chorioidea of the third ventricle, and is destitute of an epithelial covering. In front, the superior is separated from the medial surface by a salient margin, the tænia thalami, along which the epithelial lining of the third ventricle is reflected on to the under surface of the tela chorioidea. Behind, it is limited medially by a groove, the sulcus habenulæ, which intervenes between it and a small triangular area, termed the trigonum habenulæ.

  The inferior surface rests upon and is continuous with the upward prolongation of the tegmentum (subthalamic tegmental region), in front of which it is related to the substantia innominata of Meynert.

 

  The medial surface constitutes the upper part of the lateral wall of the third ventricle, and is connected to the corresponding surface of the opposite thalamus by a flattened gray band, the massa intermedia (middle or gray commissure). This mass averages about 1 cm. in its antero-posterior diameter: it sometimes consists of two parts and occasionally is absent. It contains nerve cells and nerve fibers; a few of the latter may cross the middle line, but most of them pass toward the middle line and then curve lateralward on the same side.

  The lateral surface is in contact with a thick band of white substance which forms the occipital part of the internal capsule and separates the thalamus from the lentiform nucleus of the corpus striatum.

 

Structure.—The thalamus consists chiefly of gray substance, but its upper surface is covered by a layer of white substance, named the stratum zonale, and its lateral surface by a similar layer termed the lateral medullary lamina. Its gray substance is incompletely subdivided into three parts—anterior, medial, and lateral—by a white layer, the medial medullary lamina. The anterior part comprises the anterior tubercle, the medial part lies next the lateral wall of the third ventricle while the lateral and largest part is interposed between the medullary laminæ and includes the pulvinar. The lateral part is traversed by numerous fibers which radiate from the thalamus into the internal capsule, and pass through the latter to the cerebral cortex. These three parts are built up of numerous nuclei, the connections of many of which are imperfectly known.

 



Coronal section of brain through intermediate mass of third ventricle.

 

Connections.—The thalamus may be regarded as a large ganglionic mass in which the ascending tracts of the tegmentum and a considerable proportion of the fibers of the optic tract end, and from the cells of which numerous fibers (thalamocortical) take origin, and radiate to almost every part of the cerebral cortex. The lemniscus, together with the other longitudinal strands of the tegmentum, enters its ventral part: the thalamomammillary fasciculus (bundle of Vicq d’Azyr), from the corpus mammillare, enters in its anterior tubercle, while many of the fibers of the optic tract terminate in its posterior end. The thalamus also receives numerous fibers (corticothalamic) from the cells of the cerebral cortex. The fibers that arise from the cells of the thalamus form four principal groups or stalks: (a) those of the anterior stalk pass through the frontal part of the internal capsule to the frontal lobe; (b) the fibers of the posterior stalk (optic radiations) arise in the pulvinar and are conveyed through the occipital part of the internal capsule to the occipital lobe; (c) the fibers of the inferior stalk leave the under and medial surfaces of the thalamus, and pass beneath the lentiform nucleus to the temporal lobe and insula; (d) those of the parietal stalk pass from the lateral nucleus of the thalamus to the parietal lobe. Fibers also extend from the thalamus into the corpus striatum—those destined for the caudate nucleus leave the lateral surface, and those for the lentiform nucleus, the inferior surface of the thalamus.

 

  The Metathalamus (719) comprises the geniculate bodies, which are two in number—a medial and a lateral—on each side.

  The medial geniculate body (corpus geniculatum mediale; internal geniculate body; postgeniculatum) lies under cover of the pulvinar of the thalamus and on the lateral aspect of the corpora quadrigemina. Oval in shape, with its long axis directed forward and lateralward, it is lighter in color and smaller in size than the lateral. The inferior brachium from the inferior colliculus disappears under cover of it while from its lateral extremity a strand of fibers passes to join the optic tract. Entering it are many acoustic fibers from the lateral lemniscus. The medial geniculate bodies are connected with one another by the commissure of Gudden, which passes through the posterior part of the optic chiasma.

  The lateral geniculate body (corpus geniculatum laterale; external geniculate body; pregeniculatum) is an oval elevation on the lateral part of the posterior end of the thalamus, and is connected with the superior colliculus by the superior brachium. It is of a dark color, and presents a laminated arrangement consisting of alternate layers of gray and white substance. It receives numerous fibers from the optic tract, while other fibers of this tract pass over or through it into the pulvinar. Its cells are large and pigmented; their axons pass to the visual area in the occipital part of the cerebral cortex.

  The superior colliculus, the pulvinar, and the lateral geniculate body receive many fibers from the optic tracts, and are therefore intimately connected with sight, constituting what are termed the lower visual centers. Extirpation of the eyes in a newly born animal entails an arrest of the development of these centers, but has no effect on the medial geniculate bodies or on the inferior colliculi. Moreover, the latter are well-developed in the mole, an animal in which the superior colliculi are rudimentary.

  The Epithalamus comprises the trigonum habenulæ, the pineal body, and the posterior commissure.

  The trigonum habenulæ is a small depressed triangular area situated in front of the superior colliculus and on the lateral aspect of the posterior part of the tænia thalami. It contains a group of nerve cells termed the ganglion habenulæ. Fibers enter it from the stalk of the pineal body, and others, forming what is termed the habenular commissure, pass across the middle line to the corresponding ganglion of the opposite side. Most of its fibers are, however, directed downward and form a bundle, the fasciculus retroflexus of Meynert, which passes medial to the red nucleus, and, after decussating with the corresponding fasciculus of the opposite side, ends in the interpeduncular ganglion.

  The pineal body (corpus pineale; epiphysis) is a small, conical, reddish-gray body which lies in the depression between the superior colliculi. It is placed beneath the splenium of the corpus callosum, but is separated from this by the tela chorioidea of the third ventricle, the lower layer of which envelops it. It measures about 8 mm. in length, and its base, directed forward, is attached by a stalk or peduncle of white substance. The stalk of the pineal body divides anteriorly into two laminæ, a dorsal and a ventral, separated from one another by the pineal recess of the third ventricle. The ventral lamina is continuous with the posterior commissure; the dorsal lamina is continuous with the habenular commissure and divides into two strands the medullary striæ, which run forward, one on either side, along the junction of the medial and upper surfaces of the thalamus to blend in front with the columns of the fornix.

  The posterior commissure is a rounded band of white fibers crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct. Its fibers acquire their medullary sheaths early, but their connections have not been definitely determined. Most of them have their origin in a nucleus, the nucleus of the posterior commissure (nucleus of Darkschewitsch), which lies in the central gray substance of the upper end of the cerebral aqueduct, in front of the nucleus of the oculomotor nerve. Some are probably derived from the posterior part of the thalamus and from the superior colliculus, while others are believed to be continued downward into the medial longitudinal fasciculus.

  The Hypothalamus (720) includes the subthalamic tegmental region and the structures forming the greater part of the floor of the third ventricle, viz., the corpora mammillaria, tuber cinereum, infundibulum, hypophysis, and optic chiasma.

  The subthalamic tegmental region consists of the upward continuation of the tegmentum; it lies on the ventro-lateral aspect of the thalamus and separates it from the fibers of the internal capsule. The red nucleus and the substantia nigra are prolonged into its lower part; in front it is continuous with the substantia innominata of Meynert, medially with the gray substance of the floor of the third ventricle.

  It consists from above downward of three strata: (1) stratum dorsale, directly applied to the under surface of the thalamus and consisting of fine longitudinal fibers; (2) zona incerta, a continuation forward of the formatio reticularis of the tegmentum; and (3) the corpus subthalamicum (nucleus of Luys), a brownish mass presenting a lenticular shape on transverse section, and situated on the dorsal aspect of the fibers of the base of the cerebral peduncle; it is encapsuled by a lamina of nerve fibers and contains numerous medium-sized nerve cells, the connections of which are as yet not fully determined.

  The corpora mammillaria (corpus albicantia) are two round white masses, each about the size of a small pea, placed side by side below the gray substance of the floor of the third ventricle in front of the posterior perforated substance. They consist of white substance externally and of gray substance internally, the cells of the latter forming two nuclei, a medial of smaller and a lateral of larger cells. The white substance is mainly formed by the fibers of the columns of the fornix, which descend to the base of the brain and end partly in the corpora mammillaria. From the cells of the gray substance of each mammillary body two fasciculi arise: one, the thalamomammillary fasciculus (bundle of Vicq d’Azyr), passes upward into the anterior nucleus of the thalamus; the other is directed downward into the tegmentum. Afferent fibers are believed to reach the corpus mammillare from the medial lemniscus and from the tegmentum.

 



Median sagittal section of brain. The relations of the pia mater are indicated by the red color.

 

  The tuber cinereum is a hollow eminence of gray substance situated between the corpora mammillaria behind, and the optic chiasma in front. Laterally it is continuous with the anterior perforated substances and anteriorly with a thin lamina, the lamina terminalis. From the under surface of the tuber cinereum a hollow conical process, the infundibulum, projects downward and forward and is attached to the posterior lobe of the hypophysis.

  In the lateral part of the tuber cinereum is a nucleus of nerve cells, the basal optic nucleus of Meynert, while close to the cavity of the third ventricle are three additional nuclei. Between the tuber cinereum and the corpora mammillaria a small elevation, with a corresponding depression in the third ventricle, is sometimes seen. Retzius has named it the eminentia saccularis, and regards it as a representative of the saccus vasculosus found in this situation in some of the lower vertebrates.

  The hypophysis (pituitary body) (721) is a reddish-gray, somewhat oval mass, measuring about 12.5 mm. in its transverse, and about 8 mm. in its antero-posterior diameter. It is attached to the end of the infundibulum, and is situated in the fossa hypophyseos of the sphenoidal bone, where it is retained by a circular fold of dura mater, the diaphragma sella; this fold almost completely roofs in the fossa, leaving only a small central aperture through which the infundibulum passes.

 



The hypophysis cerebri, in position. Shown in sagittal section.

 

Optic Chiasma (chiasma opticum; optic commissure).—The optic chiasma is a flattened, somewhat quadrilateral band of fibers, situated at the junction of the floor and anterior wall of the third ventricle. Most of its fibers have their origins in the retina, and reach the chiasma through the optic nerves, which are continuous with its antero-lateral angles. In the chiasma, they undergo a partial decussation (722); the fibers from the nasal half of the retina decussate and enter the optic tract of the opposite side, while the fibers from the temporal half of the retina do not undergo decussation, but pass back into the optic tract of the same side. Occupying the posterior part of the commissure, however, is a strand of fibers, the commissure of Gudden, which is not derived from the optic nerves; it forms a connecting link between the medial geniculate bodies.

 

Optic Tracts.—The optic tracts are continued backward and lateralward from the postero-lateral angles of the optic chiasma. Each passes between the anterior perforated substance and the tuber cinereum, and, winding around the ventrolateral aspect of the cerebral peduncle, divides into a medial and a lateral root. The former comprises the fibers of Gudden’s commissure. The lateral root consists mainly of afferent fibers which arise in the retina and undergo partial decussation in the optic chiasma, as described; but it also contains a few fine efferent fibers which have their origins in the brain and their terminations in the retina. When traced backward, the afferent fibers of the lateral root are found to end in the lateral geniculate body and pulvinar of the thalamus, and in the superior colliculus; and these three structures constitute the lower visual centers. Fibers arise from the nerve cells in these centers and pass through the occipital part of the internal capsule, under the name of the optic radiations, to the cortex of the occipital lobe of the cerebrum, where the higher or cortical visual center is situated. Some of the fibers of the optic radiations take an opposite course, arising from the cells of the occipital cortex and passing to the lower visual centers. Some fibers are detached from the optic tract, and pass through the cerebral peduncle to the nucleus of the oculomotor nerve. These may be regarded as the afferent branches for the Sphincter pupillæ and Ciliaris muscles. Other fibers have been described as reaching the cerebellum through the superior peduncle; while others, again, are lost in the pons.

 

The Third Ventricle (ventriculus tertius).—The third ventricle is a median cleft between the two thalami. Behind, it communicates with the fourth ventricle through the cerebral aqueduct, and in front with the lateral ventricles through the interventricular foramen. Somewhat triangular in shape, with the apex directed backward, it has a roof, a floor, an anterior and a posterior boundary and a pair of lateral walls.

  The roof (723) is formed by a layer of epithelium, which stretches between the upper edges of the lateral walls of the cavity and is continuous with the epithelial lining of the ventricle. It is covered by and adherent to a fold of pia mater, named the tela chorioidea of the third ventricle, from the under surface of which a pair of vascular fringed processes, the choroid plexuses of the third ventricle, project downward, one on either side of the middle line, and invaginate the epithelial roof into the ventricular cavity.

  The floor slopes downward and forward and is formed mainly by the structures which constitute the hypothalamus: from before backward these are: the optic chiasma, the tuber cinereum and infundibulum, and the corpora mammillaria. Behind the last, the floor is formed by the interpeduncular fossa and the tegmenta of the cerebral peduncles. The ventricle is prolonged downward as a funnel-shaped recess, the recessus infundibuli, into the infundibulum, and to the apex of the latter the hypophysis is attached.

  The anterior boundary is constituted below by the lamina terminalis, a thin layer of gray substance stretching from the upper surface of the optic chiasma to the rostrum of the corpus callosum; above by the columns of the fornix and the anterior commissure. At the junction of the floor and anterior wall, immediately above the optic chiasma, the ventricle presents a small angular recess or diverticulum, the optic recess. Between the columns of the fornix, and above the anterior commissure, is a second recess termed the vulva. At the junction of the roof and anterior wall of the ventricle, and situated between the thalami behind and the columns of the fornix in front, is the interventricular foramen (foramen of Monro) through which the third communicates with the lateral ventricles.

 

 

  The posterior boundary is constituted by the pineal body, the posterior commissure and the cerebral aqueduct. A small recess, the recessus pinealis, projects into the stalk of the pineal body, while in front of and above the pineal body is a second recess, the recessus suprapinealis, consisting of a diverticulum of the epithelium which forms the ventricular roof.

  Each lateral wall consists of an upper portion formed by the medial surface of the anterior two-thirds of the thalamus, and a lower consisting of an upward continuation of the gray substance of the ventricular floor. These two parts correspond to the alar and basal laminæ respectively of the lateral wall of the fore-brain vesicle and are separated from each other by a furrow, the sulcus of Monro, which extends from the interventricular foramen to the cerebral aqueduct (pages 741 and 742). The lateral wall is limited above by the tænia thalami. The columns of the fornix curve downward in front of the interventricular foramen, and then run in the lateral walls of the ventricle, where, at first, they form distinct prominences, but subsequently are lost to sight. The lateral walls are joined to each other across the cavity of the ventricle by a band of gray matter, the massa intermedia

 

Theme 2. Sensory organs.

Eye consists of eyeball, auxiliary eye organs and the optic nerve. Eyeball has nucleus and wall. Auxiliary eye apparatus includes eyelids, muscles of eyeball, lacrimal apparatus, orbital fasciae, vessels and nerves.

Eyeball is serrounded by adiposal body of orbite, muscles of eyeball and orbital fascia. Bony orbit is covered by periorbita. It has an anterior pole, posterior pole, and axis. Axis courses between poles. Optic axis starts from anterior pole to central fossa of the retina. Line that is found transversal on surface of eyeball and is found in the middle to distance between poles is called equator, and line passing perpendicularly to equator is called meridian.

Eyeball wall consists of three coats: fibrous (external), vascular (middle) and internal (retina).

Fibrous coat of eyeball subdivides on transparent cornea (anteriorly) and sclera (the rast). Venous sinus of sclera (Schlemm`s canal) localised between cornea and sclera.

Vascular eye coat has: 1] proper vascular coat ‘choroidea’, which connects with sclera and delimited by perivascular space. 2] Ciliary body consists of ciliary corona and by 70 ciliary processes. There is ciliary muscle in ciliary body, its contraction provides eye accommodation. 3] Iris carries the round oriface in centre - pupilla. Smooth muscles, which form a pupil muscle-sphincter and pupil muscle-dilator are round the pupil.

Internal coat of eyeball – ‘retina’. There are external pigmental layer and internal nervous layer in visual part of the retina. According to function they distinguish posterior larger visual part of retina, which contains rods and cones, and lesser blind part of retina. There are neither rods nor cones in blind part.

 

Ora serrata is the boundary between optic and blind parts, which accords with transition of choroid into ciliary body. In posterior part of retina is found a disc of the optic nerve that has a small concavity. Macula is located in the centre of retina. Central fossa is the place of best sight sharpness, where is observed most rods and cones.

Nucleus of eyeball consists of vitreous body, lens, and aqueous humor in anterior and posterior chambers.

Vitreous body represents by transparent mass without any vessels. It occupies largest portion of eyeball behind lens.

The transparent lens consists of tight layers of proteins. The thin, clear lens capsule encloses the lens and provides attachment for the suspensory ligament (zonular fibers).

Anterior chamber of eyeball placed between posterior surface of cornea surface and anterior surface of the iris. Posterior chamber is found between posterior surface by iris and anterior surface of lens. The anterior and posterior chambers are filled by aqueous humor, which produced by ciliary processes of ciliary body and unite each other by the medium of pupil. Between cornea and iris is found iridocorneal corner, which is filled by pectinate ligament with the Fontana`s spaces. Aqueous humor draines from anterior chamber through fountain spaces to the Schlemm`s canal (venous sinus of sclera).

Organon Visus; The Eye

The bulb of the eye (bulbus oculi; eyeball), or organ of sight, is contained in the cavity of the orbit, where it is protected from injury and moved by the ocular muscles. Associated with it are certain accessory structures, viz., the muscles, fasciæ, eyebrows, eyelids, conjunctiva, and lacrimal apparatus.

The bulb of the eye is imbedded in the fat of the orbit, but is separated from it by a thin membranous sac, the fascia bulbi (page 1024). It is composed of segments of two spheres of different sizes. The anterior segment is one of a small sphere; it is transparent, and forms about one-sixth of the bulb. It is more prominent than the posterior segment, which is one of a larger sphere, and is opaque, and forms about five-sixths of the bulb. The term anterior pole is applied to the central point of the anterior curvature of the bulb, and that of posterior pole to the central point of its posterior curvature; a line joining the two poles forms the optic axis. The axes of the two bulbs are nearly parallel, and therefore do not correspond to the axes of the orbits, which are directed forward and lateralward. The optic nerves follow the direction of the axes of the orbits, and are therefore not parallel; each enters its eyeball 3 mm. to the nasal side and a little below the level of the posterior pole. The bulb measures rather more in its transverse and antero-posterior diameters than in its vertical diameter, the former amounting to about 24 mm., the latter to about 23.5 mm.; in the female all three diameters are rather less than in the male; its antero-posterior diameter at birth is about 17.5 mm., and at puberty from 20 to 21 mm.

Development.—The eyes begin to develop as a pair of diverticula from the lateral aspects of the forebrain. These diverticula make their appearance before the closure of the anterior end of the neural tube; after the closure of the tube they are known as the optic vesicles. They project toward the sides of the head, and the peripheral part of each expands to form a hollow bulb, while the proximal part remains narrow and constitutes the optic stalk (863, 864). The ectoderm overlying the bulb becomes thickened, invaginated, and finally severed from the ectodermal covering of the head as a vesicle of cells, the lens vesicle, which constitutes the rudiment of the crystalline lens. The outer wall of the bulb becomes thickened and invaginated, and the bulb is thus converted into a cup, the optic cup, consisting of two strata of cells (864). These two strata are continuous with each other at the cup margin, which ultimately overlaps the front of the lens and reaches as far forward as the future aperture of the pupil. The invagination is not limited to the outer wall of the bulb, but involves also its postero-inferior surface and extends in the form of a groove for some distance along the optic stalk, so that, for a time, a gap or fissure, the choroidal fissure, exists in the lower part of the cup (865). Through the groove and fissure the mesoderm extends into the optic stalk and cup, and in this mesoderm a bloodvessel is developed; during the seventh week the groove and fissure are closed and the vessel forms the central artery of the retina. Sometimes the choroidal fissure persists, and when this occurs the choroid and iris in the region of the fissure remain undeveloped, giving rise to the condition known as coloboma of the choroid or iris.

  The retina is developed from the optic cup. The outer stratum of the cup persists as a single layer of cells which assume a columnar shape, acquire pigment, and form the pigmented layer of the retina; the pigment first appears in the cells near the edge of the cup. The cells of the inner stratum proliferate and form a layer of considerable thickness from which the nervous elements and the sustentacular fibers of the retina, together with a portion of the vitreous body, are developed. In that portion of the cup which overlaps the lens the inner stratum is not differentiated into nervous elements, but forms a layer of columnar cells which is applied to the pigmented layer, and these two strata form the pars ciliaris and pars iridica retinæ.

  The cells of the inner or retinal layer of the optic cup become differentiated into spongioblasts and germinal cells, and the latter by their subdivisions give rise to neuroblasts. From the spongioblasts the sustentacular fibers of Müller, the outer and inner limiting membranes, together with the groundwork of the molecular layers of the retina are formed. The neuroblasts become arranged to form the ganglionic and nuclear layers. The layer of rods and cones is first developed in the central part of the optic cup, and from there gradually extends toward the cup margin. All the layers of the retina are completed by the eighth month of fetal life.

  The optic stalk is converted into the optic nerve by the obliteration of its cavity and the growth of nerve fibers into it. Most of these fibers are centripetal, and grow backward into the optic stalk from the nerve cells of the retina, but a few extend in the opposite direction and are derived from nerve cells in the brain. The fibers of the optic nerve receive their medullary sheaths about the tenth week after birth. The optic chiasma is formed by the meeting and partial decussation of the fibers of the two optic nerves. Behind the chiasma the fibers grow backward as the optic tracts to the thalami and mid-brain.

 

The crystalline lens is developed from the lens vesicle, which recedes within the margin of the cup, and becomes separated from the overlying ectoderm by mesoderm. The cells forming the posterior wall of the vesicle lengthen and are converted into the lens fibers, which grow forward and fill up the cavity of the vesicle (866). The cells forming the anterior wall retain their cellular character, and form the epithelium on the anterior surface of the adult lens. By the second month the lens is invested by a vascular mesodermal capsule, the capsula vasculosa lentis; the bloodvessels supplying the posterior part of this capsule are derived from the hyaloid artery; those for the anterior part from the anterior ciliary arteries; the portion of the capsule which covers the front of the lens is named the pupillary membrane. By the sixth month all the vessels of the capsule are atrophied except the hyaloid artery, which disappears during the ninth month; the position of this artery is indicated in the adult by the hyaloid canal, which reaches from the optic disk to the posterior surface of the lens. With the loss of its bloodvessels the capsula vasculosa lentis disappears, but sometimes the pupillary membrane persists at birth, giving rise to the condition termed congenital atresia of the pupil.


  The vitreous body is developed between the lens and the optic cup. The lens rudiment and the optic vesicle are at first in contact with each other, but after the closure of the lens vesicle and the formation of the optic cup the former withdraws itself from the retinal layer of the cup; the two, however, remain connected by a network of delicate protoplasmic processes. This network, derived partly from the cells of the lens and partly from those of the retinal layer of the cup, constitutes the primitive vitreous body (867, 868). At first these protoplasmic processes spring from the whole of the retinal layer of the cup, but later are limited to the ciliary region, where by a process of condensation they appear to form the zonula ciliaris. The mesoderm which enters the cup through the choroidal fissure and around the equator of the lens becomes intimately united with this reticular tissue, and contributes to form the vitreous body, which is therefore derived partly from the ectoderm and partly from the mesoderm.

  The anterior chamber of the eye appears as a cleft in the mesoderm separating the lens from the overlying ectoderm. The layer of mesoderm in front of the cleft forms the substantia propria of the cornea, that behind the cleft the stroma of the iris and the pupillary membrane. The fibers of the ciliary muscle are derived from the mesoderm, but those of the Sphincter and Dilatator pupillæ are of ectodermal origin, being developed from the cells of the pupillary part of the optic cup.

  The sclera and choroid are derived from the mesoderm surrounding the optic cup.

  The eyelids are formed as small cutaneous folds (866, 867), which about the middle of the third month come together and unite in front of the cornea. They remain united until about the end of the sixth month.

  The lacrimal sac and nasolacrimal duct result from a thickening of the ectoderm in the groove, nasooptic furrow, between the lateral nasal and maxillary processes. This thickening forms a solid cord of cells which sinks into the mesoderm; during the third month the central cells of the cord break down, and a lumen, the nasolacrimal duct, is established. The lacrimal ducts arise as buds from the upper part of the cord of cells and secondarily establish openings (puncta lacrimalia) on the margins of the lids. The epithelium of the cornea and conjunctiva, and that which lines the ducts and alveoli of the lacrimal gland, are of ectodermal origin, as are also the eyelashes and the lining cells of the glands which open on the lid-margins.

 

There is ciliary muscle

in ciliary body, its contraction provides eye accommodation.

 

The refracting media are three, viz.:

Aqueous humor.

Vitreous body.

Crystalline lens.

 

The Aqueous Humor (humor aqueus).—The aqueous humor fills the anterior and posterior chambers of the eyeball. It is small in quantity, has an alkaline reaction, and consists mainly of water, less than one-fiftieth of its weight being solid matter, chiefly chloride of sodium.

 The Vitreous Body (corpus vitreum).—The vitreous body forms about four-fifths of the bulb of the eye. It fills the concavity of the retina, and is hollowed in front, forming a deep concavity, the hyaloid fossa, for the reception of the lens. It is transparent, of the consistence of thin jelly, and is composed of an albuminous fluid enclosed in a delicate transparent membrane, the hyaloid membrane. It has been supposed, by Hannover, that from its surface numerous thin lamellæ are prolonged inward in a radiating manner, forming spaces in which the fluid is contained. In the adult, these lamellæ cannot be detected even after careful microscopic examination in the fresh state, but in preparations hardened in weak chromic acid it is possible to make out a distinct lamellation at the periphery of the body. In the center of the vitreous body, running from the entrance of the optic nerve to the posterior surface of the lens, is a canal, the hyaloid canal, filled with lymph and lined by a prolongation of the hyaloid membrane. This canal, in the embryonic vitreous body, conveyed the arteria hyaloidea from the central artery of the retina to the back of the lens. The fluid from the vitreous body is nearly pure water; it contains, however, some salts, and a little albumin.

  The hyaloid membrane envelopes the vitreous body. The portion in front of the ora serrata is thickened by the accession of radial fibers and is termed the zonula ciliaris (zonule of Zinn). Here it presents a series of radially arranged furrows, in which the ciliary processes are accommodated and to which they adhere, as is shown by the fact that when they are removed some of their pigment remains attached to the zonula. The zonula ciliaris splits into two layers, one of which is thin and lines the hyaloid fossa; the other is named the suspensory ligament of the lens: it is thicker, and passes over the ciliary body to be attached to the capsule of the lens a short distance in front of its equator. Scattered and delicate fibers are also attached to the region of the equator itself. This ligament retains the lens in position, and is relaxed by the contraction of the meridional fibers of the Ciliaris muscle, so that the lens is allowed to become more convex. Behind the suspensory ligament there is a sacculated canal, the spatia zonularis (canal of Petit), which encircles the equator of the lens; it can be easily inflated through a fine blowpipe inserted under the suspensory ligament.

  No bloodvessels penetrate the vitreous body, so that its nutrition must be carried on by vessels of the retina and ciliary processes, situated upon its exterior.

The Crystalline Lens (lens crystallina).—The crystalline lens, enclosed in its capsule, is situated immediately behind the iris, in front of the vitreous body, and encircled by the ciliary processes, which slightly overlap its margin.

  The capsule of the lens (capsula lentis) is a transparent, structureless membrane which closely surrounds the lens, and is thicker in front than behind. It is brittle but highly elastic, and when ruptured the edges roll up with the outer surface innermost. It rests, behind, in the hyaloid fossa in the forepart of the vitreous body; in front, it is in contact with the free border of the iris, but recedes from it at the circumference, thus forming the posterior chamber of the eye; it is retained in its position chiefly by the suspensory ligament of the lens, already described.

  The lens is a transparent, biconvex body, the convexity of its anterior being less than that of its posterior surface. The central points of these surfaces are termed respectively the anterior and posterior poles; a line connecting the poles constitutes the axis of the lens, while the marginal circumference is termed the equator.

 

Structure.—The lens is made up of soft cortical substance and a firm, central part, the nucleus (884). Faint lines (radii lentis) radiate from the poles to the equator. In the adult there may be six or more of these lines, but in the fetus they are only three in number and diverge from each other at angles of 120° (885); on the anterior surface one line ascends vertically and the other two diverge downward; on the posterior surface one ray descends vertically and the other two diverge upward. They correspond with the free edges of an equal number of septa composed of an amorphous substance, which dip into the substance of the lens. When the lens has been hardened it is seen to consist of a series of concentrically arranged laminæ, each of which is interrupted at the septa referred to. Each lamina is built up of a number of hexagonal, ribbon-like lens fibers, the edges of which are more or less serrated—the serrations fitting between those of neighboring fibers, while the ends of the fibers come into apposition at the septa. The fibers run in a curved manner from the septa on the anterior surface to those on the posterior surface. No fibers pass from pole to pole; they are arranged in such a way that those which begin near the pole on one surface of the lens end near the peripheral extremity of the plane on the other, and vice versa. The fibers of the outer layers of the lens are nucleated, and together form a nuclear layer, most distinct toward the equator. The anterior surface of the lens is covered by a layer of transparent, columnar, nucleated epithelium. At the equator the cells become elongated, and their gradual transition into lens fibers can be traced (887).



The crystalline lens, hardened and divided.

  In the fetus, the lens is nearly spherical, and has a slightly reddish tint; it is soft and breaks down readily on the slightest pressure. A small branch from the arteria centralis retinæ runs forward, as already mentioned, through the vitreous body to the posterior part of the capsule of the lens, where its branches radiate and form a plexiform network, which covers the posterior surface of the capsule, and they are continuous around the margin of the capsule with the vessels of the pupillary membrane, and with those of the iris. In the adult, the lens is colorless, transparent, firm in texture, and devoid of vessels. In old age it becomes flattened on both surfaces, slightly opaque, of an amber tint, and increased in density (886).

 

Vessels and Nerves.—The arteries of the bulb of the eye are the long, short, and anterior ciliary arteries, and the arteria centralis retinæ. They have already been described (see p. 571).

  The ciliary veins are seen on the outer surface of the choroid, and are named, from their arrangement, the venæ vorticosæ; they converge to four or five equidistant trunks which pierce the sclera midway between the sclero-corneal junction and the porus opticus. Another set of veins accompanies the anterior ciliary arteries. All of these veins open into the ophthalmic veins.

  The ciliary nerves are derived from the nasociliary nerve and from the ciliary ganglion.

 

Additional eye structures include: extrinsic muscles of eyeball, eyebrows, eyelids, conjuctiva, lacrimal apparatus.

Extraocular Musculature

Levator palpebrae superioris

•Origin: inferior aspect of the lesser wing of sphenoid (adjacent to the common annular tendon) •Insertion:

1.medial and lateral walls of the orbit 2.superior tarsus

•Action: elevates the eyelid •Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)

 

Lateral rectus

•Origin:

1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal

•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action: abducts eye •Blood: branches of ophthalmic artery •Nerve: abducens nerve (VI cranial)

 

Medial rectus

•Origin:

1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal

•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action: adducts eye •Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)

 

Superior rectus

•Origin:

1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal

•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action:

1.elevates 2.medially rotates 3.adducts the eye

•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)

 

Superior rectus

•Origin:

1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal

•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action:

1.elevates 2.medially rotates 3.adducts the eye

•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)

 

Inferior rectus

•Origin:

1.common annular tendon (which comes off the body and lesser wing of sphenoid) 2.margins of the optic canal

•Insert: posterior to the sclerocorneal junction (each muscle inserting along its own directional axis) •Action:

1.depress 2.laterally rotates 3.adducts the eye

•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)

Superior oblique

•Origin: body of sphenoid •Insert: upper lateral quadrant of the posterior half of the sclera (via the trochlea, as a pulley) •Action:

1.depress 2.medially rotates 3.abducts the eye

•Blood: branches of ophthalmic artery •Nerve: trochlear nerve (IV cranial)

 

Inferior oblique

•Origin: orbital surface of maxilla •Insert: lower lateral quadrant of the posterior half of the sclera (via the suspensory ligament, as a pulley) •Action:

1.elevates 2.laterally rotates 3.abducts the eye

•Blood: branches of ophthalmic artery •Nerve: oculomotor nerve (III cranial)

 

They originate in common annular tendom and insert into sclera and can rotate eyeball on frontal and vertical axis. As result pupilla moves up, down laterally and medially. Superior oblique muscle psses through a pulleylike cartilagenous loop, the trochlea, before attaching to the eyeball. Levator palpebrae superioris muscle elevates upper eyelid.

Periosteum of orbit is ‘periorbita’ forming the cover of bones and passes through the optic canal into dura mater encephali. Eyeball is enveloped by vagina of eyeball (Tenon`s capsule). Adiposal body of orbit localised between vagina of eyeball and periorbita, which formes elastic pillow for eyeball.

Superior and inferior eyelids cover and protect eyeball. Front surface of eyelids is covered by skin. Posterior surface of the eyelids and anterior free surface of eyeball are covered by thin conjuctiva. Last forms superior and inferior sac of conjuctiva.

Tarsal plates, composed of dense regular connective tissue, are important in maintaining the shape of the eyelids. Yeybrows (supercilium) consist of short, sick hair positioned transversally above both eyes along the superior orbital ridges of the scull.

Lacrimal apparatus consists of lacrimal gland lying in superolateral portion of the orbit and a series of lacrimal ducts that drain the secretion into the nasal cavity. The excretory ductuli of lacrimal gland (10-15) open into conjuctival sac of upper yeylid (superior rivus). With each blink of the eyelids, tears passe medially and downward and drains into lake and two small openings, called lacrimal puncta on both sides of the lacrimal caruncle. From here, tears drains through the lacrimal canaliculus into the lacrimal sac and continious through the nasolacrimal duct to the inferior meatus of the nasal cavity.

 

 

Visual tract

Optic nerve is a part of visual analyser. Three neurons of visual tract are located in retina: 1 - fotoreceptors rods and cones, 2 – bipolar cells and 3 – ganglionic (multipolar) cells. Axons of third neurons form the II Optic nerve, which passes through the optic canal and get the cranial cavity. Medial fibres of the optic nerve pass to the opposite side and form  the optic chiasma. Lateral fibres part do not cross each other and keep their own side. Then fibres of the optic nerve form optic tract wich get the subcortical sight centres (lateral geniculate body and superior colliculus of midbrain). Then 4th  neurons are located under pulvinar thalami. Their axons run through the posterior leg of internal capsule (visual radiation) and reach cortical visual analyser in calcarine sulcus (occipital lobe).

Axons of optic tract contact with cells in accessory oculomotor /parasympathetic/ nucleus (Yakubovych-Edinger-Westphal`s) by means of intermediate neuron. There are link for realising the pupillar reflex and accomodation. Axons of fifth neurons run in composition of oculomotor nerve get a ciliary ganglion, where sixth neurons positioned. Their axons pass with short ciliary nerves into eyeball and give innervating for ciliary and sphincter pupillae muscles. This reflex does not depend on our will and consciousness.

 

EAR (HEARING AND EQUILIBRIUM ORGAN)

Ear subdivides on auricle (outer ear), middle ear and internal ear. Auricle and external auditory meatus belong to outer ear. Middle ear contains a tympanic cavity and auditory tube (Eustachian). Internal ear composes an osseous labyrinth and membranous labyrinth.

AURICLE contains a cartilage covered by skin. In inferior part a cartilage is absent there is auricular lobule (earlobe). Also auricle has a helix, triangular fossa, antihelix, concha, tragus, antitragus.

External auditory meatus is open outside, in depth from cavity of middle ear it dissociates by tympanic membrane. External auditory meatus has cartilaginous part and inner osseous part. Cartilaginous part composes one-third length of auditory meatus. Osseous part occupies two thirds of auditory meatus. Auditory meatus is curved S-like and for its straightening attached to examination of tympanic membrane necessary to draw off auricle posterior, up and outside.

The external ear consists of the expanded portion named the auricula or pinna, and the external acoustic meatus. The former projects from the side of the head and serves to collect the vibrations of the air by which sound is produced; the latter leads inward from the bottom of the auricula and conducts the vibrations to the tympanic cavity.

  The Auricula or Pinna (904) is of an ovoid form, with its larger end directed upward. Its lateral surface is irregularly concave, directed slightly forward, and presents numerous eminences and depressions to which names have been assigned. The prominent rim of the auricula is called the helix; where the helix turns downward behind, a small tubercle, the auricular tubercle of Darwin, is frequently seen; this tubercle is very evident about the sixth month of fetal life when the whole auricula has a close resemblance to that of some of the adult monkeys. Another curved prominence, parallel with and in front of the helix, is called the antihelix; this divides above into two crura, between which is a triangular depression, the fossa triangularis. The narrow-curved depression between the helix and the antihelix is called the scapha; the antihelix describes a curve around a deep, capacious cavity, the concha, which is partially divided into two parts by the crus or commencement of the helix; the upper part is termed the cymba conchæ, the lower part the cavum conchæ. In front of the concha, and projecting backward over the meatus, is a small pointed eminence, the tragus, so called from its being generally covered on its under surface with a tuft of hair, resembling a goat’s beard. Opposite the tragus, and separated from it by the intertragic notch, is a small tubercle, the antitragus. Below this is the lobule, composed of tough areolar and adipose tissues, and wanting the firmness and elasticity of the rest of the auricula.

  The cranial surface of the auricula presents elevations which correspond to the depressions on its lateral surface and after which they are named, e. g., eminentia conchæ, eminentia triangularis, etc.



The auricula. Lateral surface.

Structure.—The auricula is composed of a thin plate of yellow fibrocartilage, covered with integument, and connected to the surrounding parts by ligaments and muscles; and to the commencement of the external acoustic meatus by fibrous tissue.

  The skin is thin, closely adherent to the cartilage, and covered with fine hairs furnished with sebaceous glands, which are most numerous in the concha and scaphoid fossa. On the tragus and antitragus the hairs are strong and numerous. The skin of the auricula is continuous with that lining the external acoustic meatus.

  The cartilage of the auricula (cartilago auriculæ; cartilage of the pinna) (905, 906) consists of a single piece; it gives form to this part of the ear, and upon its surface are found the eminences and depressions above described. It is absent from the lobule; it is deficient, also, between the tragus and beginning of the helix, the gap being filled up by dense fibrous tissue. At the front part of the auricula, where the helix bends upward, is a small projection of cartilage, called the spina helicis, while in the lower part of the helix the cartilage is prolonged downward as a tail-like process, the cauda helicis; this is separated from the antihelix by a fissure, the fissura antitragohelicina. The cranial aspect of the cartilage exhibits a transverse furrow, the sulcus antihelicis transversus, which corresponds with the inferior crus of the antihelix and separates the eminentia conchæ from the eminentia triangularis. The eminentia conchæ is crossed by a vertical ridge (ponticulus), which gives attachment to the Auricularis posterior muscle. In the cartilage of the auricula are two fissures, one behind the crus helicis and another in the tragus.

  The ligaments of the auricula (ligamenti auricularia [Valsalva]; ligaments of the pinna) consist of two sets: (1) extrinsic, connecting it to the side of the head; (2) intrinsic, connecting various parts of its cartilage together.

  The extrinsic ligaments are two in number, anterior and posterior. The anterior ligament extends from the tragus and spina helicis to the root of the zygomatic process of the temporal bone. The posterior ligament passes from the posterior surface of the concha to the outer surface of the mastoid process.

  The chief intrinsic ligaments are: (a) a strong fibrous band, stretching from the tragus to the commencement of the helix, completing the meatus in front, and partly encircling the boundary of the concha; and (b) a band between the antihelix and the cauda helicis. Other less important bands are found on the cranial surface of the pinna.

  The muscles of the auricula (906) consist of two sets: (1) the extrinsic, which connect it with the skull and scalp and move the auricula as a whole; and (2) the intrinsic, which extend from one part of the auricle to another.

  The extrinsic muscles are the Auriculares anterior, superior, and posterior.

  The Auricularis anterior (Attrahens aurem), the smallest of the three, is thin, fan-shaped, and its fibers are pale and indistinct. It arises from the lateral edge of the galea aponeurotica, and its fibers converge to be inserted into a projection on the front of the helix.

  The Auricularis superior (Attolens aurem), the largest of the three, is thin and fan-shaped. Its fibers arise from the galea aponeurotica, and converge to be inserted by a thin, flattened tendon into the upper part of the cranial surface of the auricula.

  The Auricularis posterior (Retrahens aurem) consists of two or three fleshy fasciculi, which arise from the mastoid portion of the temporal bone by short aponeurotic fibers. They are inserted into the lower part of the cranial surface of the concha.

  Actions.—In man, these muscles possess very little action: the Auricularis anterior draws the auricula forward and upward; the Auricularis superior slightly raises it; and the Auricularis posterior draws it backward.



The muscles of the auricula.

 

  The intrinsic muscles are the:

Helicis major.

Antitragicus.

Helicis minor.

Transversus auriculæ.

Tragicus.

Obliquus auriculæ.

 

  The Helicis major is a narrow vertical band situated upon the anterior margin of the helix.

  It arises below, from the spina helicis, and is inserted into the anterior border of the helix, just where it is about to curve backward.

  The Helicis minor is an oblique fasciculus, covering the crus helicis.

  The Tragicus is a short, flattened vertical band on the lateral surface of the tragus.

  The Antitragicus arises from the outer part of the antitragus, and is inserted into the cauda helicis and antihelix.

  The Transversus auriculæ is placed on the cranial surface of the pinna. It consists of scattered fibers, partly tendinous and partly muscular, extending from the eminentia conchæ to the prominence corresponding with the scapha.

  The Obliquus auriculæ, also on the cranial surface, consists of a few fibers extending from the upper and back part of the concha to the convexity immediately above it.

  Nerves.—The Auriculares anterior and superior and the intrinsic muscles on the lateral surface are supplied by the temporal branch of the facial nerve, the Auricularis posterior and the intrinsic muscles on the cranial surface by the posterior auricular branch of the same nerve.

  The arteries of the auricula are the posterior auricular from the external carotid, the anterior auricular from the superficial temporal, and a branch from the occipital artery.

  The veins accompany the corresponding arteries.

  The sensory nerves are: the great auricular, from the cervical plexus; the auricular branch of the vagus; the auriculotemporal branch of the mandibular nerve; and the lesser occipital from the cervical plexus.

External and middle ear, opened from the front. Right side.

  The External Acoustic Meatus (meatus acusticus externus; external auditory canal or meatus) extends from the bottom of the concha to the tympanic membrane (907, 908). It is about 4 cm. in length if measured from the tragus; from the bottom of the concha its length is about 2.5 cm. It forms an S-shaped curve, and is directed at first inward, forward, and slightly upward (pars externa); it then passes inward and backward (pars media), and lastly is carried inward, forward, and slightly downward (pars interna). It is an oval cylindrical canal, the greatest diameter being directed downward and backward at the external orifice, but nearly horizontally at the inner end. It presents two constrictions, one near the inner end of the cartilaginous portion, and another, the isthmus, in the osseous portion, about 2 cm. from the bottom of the concha. The tympanic membrane, which closes the inner end of the meatus, is obliquely directed; in consequence of this the floor and anterior wall of the meatus are longer than the roof and posterior wall.

  The external acoustic meatus is formed partly by cartilage and membrane, and partly by bone, and is lined by skin.

  The cartilaginous portion (meatus acusticus externus cartilagineus) is about 8 mm. in length; it is continuous with the cartilage of the auricula, and firmly attached to the circumference of the auditory process of the temporal bone. The cartilage is deficient at the upper and back part of the meatus, its place being supplied by fibrous membrane; two or three deep fissures are present in the anterior part of the cartilage.

  The osseous portion (meatus acusticus externus osseus) is about 16 mm. in length, and is narrower than the cartilaginous portion. It is directed inward and a little forward, forming in its course a slight curve the convexity of which is upward and backward. Its inner end is smaller than the outer, and sloped, the anterior wall projecting beyond the posterior for about 4 mm.; it is marked, except at its upper part, by a narrow groove, the tympanic sulcus, in which the circumference of the tympanic membrane is attached. Its outer end is dilated and rough in the greater part of its circumference, for the attachment of the cartilage of the auricula. The front and lower parts of the osseous portion are formed by a curved plate of bone, the tympanic part of the temporal, which, in the fetus, exists as a separate ring (annulus tympanicus,) incomplete at its upper part (page 146).



Horizontal section through left ear; upper half of section.

  The skin lining the meatus is very thin; adheres closely to the cartilaginous and osseous portions of the tube, and covers the outer surface of the tympanic membrane. After maceration, the thin pouch of epidermis, when withdrawn, preserves the form of the meatus. In the thick subcutaneous tissue of the cartilaginous part of the meatus are numerous ceruminous glands, which secrete the ear-wax; their structure resembles that of the sudoriferous glands.

 

Relations of the Meatus.—In front of the osseous part is the condyle of the mandible, which however, is frequently separated from the cartilaginous part by a portion of the parotid gland. The movements of the jaw influence to some extent the lumen of this latter portion. Behind the osseous part are the mastoid air cells, separated from the meatus by a thin layer of bone.

  The arteries supplying the meatus are branches from the posterior auricular, internal maxillary, and temporal.

  The nerves are chiefly derived from the auriculotemporal branch of the mandibular nerve and the auricular branch of the vagus.

 

Tympanic cavity positioned in thickness of temporal pyramid and has the following walls:

1.     tegmental wall (superior);

2.     jugular wall (inferior);

3.     labyrinthic wall (medial), where found 2 windows: vestibular (oval) window and cochlear (round) window. Vestibular window  is closed by base stapes. Round window is tightened by secondary tympanic membrane;

4.     mastoid wall (posterior). On it located stapedius muscle. Superiorly posterior wall continues into mastoid cave, the mastoid cells open in it;

5.     carotid wall (anterior),  a tympanic foramen of auditory tube and muscle-tensor of tympanic membrane are found here;

6.     membranous wall (lateral) is formed tympanic membrane. Epitympanic recess contains a head of malleus and body of the incus.

The middle ear or tympanic cavity is an irregular, laterally compressed space within the temporal bone. It is filled with air, which is conveyed to it from the nasal part of the pharynx through the auditory tube. It contains a chain of movable bones, which connect its lateral to its medial wall, and serve to convey the vibrations communicated to the tympanic membrane across the cavity to the internal ear.

  The tympanic cavity consists of two parts: the tympanic cavity proper, opposite the tympanic membrane, and the attic or epitympanic recess, above the level of the membrane; the latter contains the upper half of the malleus and the greater part of the incus. Including the attic, the vertical and antero-posterior diameters of the cavity are each about 15 mm. The transverse diameter measures about 6 mm. above and 4 mm. below; opposite the center of the tympanic membrane it is only about 2 mm. The tympanic cavity is bounded laterally by the tympanic membrane; medially, by the lateral wall of the internal ear; it communicates, behind, with the tympanic antrum and through it with the mastoid air cells, and in front with the auditory tube (907).

  The Tegmental Wall or Roof (paries tegmentalis) is formed by a thin plate of bone, the tegmen tympani, which separates the cranial and tympanic cavities. It is situated on the anterior surface of the petrous portion of the temporal bone close to its angle of junction with the squama temporalis; it is prolonged backward so as to roof in the tympanic antrum, and forward to cover in the semicanal for the Tensor tympani muscle. Its lateral edge corresponds with the remains of the petrosquamous suture.

  The Jugular Wall or Floor (paries jugularis) is narrow, and consists of a thin plate of bone (fundus tympani) which separates the tympanic cavity from the jugular fossa. It presents, near the labyrinthic wall, a small aperture for the passage of the tympanic branch of the glossopharyngeal nerve.



Right tympanic membrane as seen through a speculum.

  The Membranous or Lateral Wall (paries membranacea; outer wall) is formed mainly by the tympanic membrane, partly by the ring of bone into which this membrane is inserted. This ring of bone is incomplete at its upper part, forming a notch (notch of Rivinus), close to which are three small apertures: the iter chordæ posterius, the petrotympanic fissure, and the iter chordæ anterius.

  The iter chordæ posterius (apertura tympanica canaliculi chordæ) is situated in the angle of junction between the mastoid and membranous wall of the tympanic cavity immediately behind the tympanic membrane and on a level with the upper end of the manubrium of the malleus; it leads into a minute canal, which descends in front of the canal for the facial nerve, and ends in that canal near the stylo-mastoid foramen. Through it the chorda tympani nerve enters the tympanic cavity.

  The petrotympanic fissure (fissura petrotympanica; Glaserian fissure) opens just above and in front of the ring of bone into which the tympanic membrane is inserted; in this situation it is a mere slit about 2 mm. in length. It lodges the anterior process and anterior ligament of the malleus, and gives passage to the anterior tympanic branch of the internal maxillary artery.

  The iter chordæ anterius (canal of Huguier) is placed at the medial end of the petrotympanic fissure; through it the chorda tympani nerve leaves the tympanic cavity.

  The Tympanic Membrane (membrana tympani) (909, 910) separates the tympanic cavity from the bottom of the external acoustic meatus. It is a thin, semitransparent membrane, nearly oval in form, somewhat broader above than below, and directed very obliquely downward and inward so as to form an angle of about fifty-five degrees with the floor of the meatus. Its longest diameter is downward and forward, and measures from 9 to 10 mm.; its shortest diameter measures from 8 to 9 mm. The greater part of its circumference is thickened, and forms a fibrocartilaginous ring which is fixed in the tympanic sulcus at the inner end of the meatus. This sulcus is deficient superiorly at the notch of Rivinus, and from the ends of this notch two bands, the anterior and posterior malleolar folds, are prolonged to the lateral process of the malleus. The small, somewhat triangular part of the membrane situated above these folds is lax and thin, and is named the pars flaccida; in it a small orifice is sometimes seen. The manubrium of the malleus is firmly attached to the medial surface of the membrane as far as its center, which it draws toward the tympanic cavity; the lateral surface of the membrane is thus concave, and the most depressed part of this concavity is named the umbo.

The tympanic membrane viewed from within. (Testut.) The malleus has been resected immediately beyond its lateral process, in order to show the tympanomalleolar folds and the membrana flaccida. 1. Tympanic membrane. 2. Umbo. 3. Handle of the malleus. 4. Lateral process. 5. Anterior tympanomalleolar fold. 6. Posterior tympanomalleolar fold. 7. Pars flaccida. 8. Anterior pouch of Tröltsch. 9. Posterior pouch of Tröltsch. 10. Fibrocartilaginous ring. 11. Petrotympanic fissure. 12. Auditory tube. 13. Iter chordæ posterius. 14. Iter chordæ anterius. 15. Fossa incudis for short crus of the incus. 16. Prominentia styloidea.

Structure.—The tympanic membrane is composed of three strata: a lateral (cutaneous), an intermediate (fibrous), and a medial (mucous). The cutaneous stratum is derived from the integument lining the meatus. The fibrous stratum consists of two layers: a radiate stratum, the fibers of which diverge from the manubrium of the malleus, and a circular stratum, the fibers of which are plentiful around the circumference but sparse and scattered near the center of the membrane. Branched or dendritic fibers, as pointed out by Grüber, are also present especially in the posterior half of the membrane.

 

Vessels and Nerves.—The arteries of the tympanic membrane are derived from the deep auricular branch of the internal maxillary, which ramifies beneath the cutaneous stratum; and from the stylomastoid branch of the posterior auricular, and tympanic branch of the internal maxillary, which are distributed on the mucous surface. The superficial veins open into the external jugular; those on the deep surface drain partly into the transverse sinus and veins of the dura mater, and partly into a plexus on the auditory tube. The membrane receives its chief nerve supply from the auriculotemporal branch of the mandibular; the auricular branch of the vagus, and the tympanic branch of the glossopharyngeal also supply it. 150



The right membrana tympani with the hammer and the chorda tympani, viewed from within, from behind, and from above.

  The Labyrinthic or Medial Wall (paries labyrinthica; inner wall) (913) is vertical in direction, and presents for examination the fenestræ vestibuli and cochleæ, the promontory, and the prominence of the facial canal.

  The fenestra vestibuli (fenestra ovalis) is a reniform opening leading from the tympanic cavity into the vestibule of the internal ear; its long diameter is horizontal, and its convex border is upward. In the recent state it is occupied by the base of the stapes, the circumference of which is fixed by the annular ligament to the margin of the foramen.

  The fenestra cochleæ (fenestra rotunda) is situated below and a little behind the fenestra vestibuli, from which it is separated by a rounded elevation, the promontory. It is placed at the bottom of a funnel-shaped depression and, in the macerated bone, leads into the cochlea of the internal ear; in the fresh state it is closed by a membrane, the secondary tympanic membrane, which is concave toward the tympanic cavity, convex toward the cochlea. This membrane consists of three layers: an external, or mucous, derived from the mucous lining of the tympanic cavity; an internal, from the lining membrane of the cochlea; and an intermediate, or fibrous layer.

  The promontory (promontorium) is a rounded hollow prominence, formed by the projection outward of the first turn of the cochlea; it is placed between the fenestræ, and is furrowed on its surface by small grooves, for the lodgement of branches of the tympanic plexus. A minute spicule of bone frequently connects the promontory to the pyramidal eminence.

  The prominence of the facial canal (prominentia canalis facialis; prominence of aqueduct of Fallopius) indicates the position of the bony canal in which the facial nerve is contained; this canal traverses the labyrinthic wall of the tympanic cavity above the fenestra vestibuli, and behind that opening curves nearly vertically downward along the mastoid wall.

  The mastoid or posterior wall (paries mastoidea) is wider above than below, and presents for examination the entrance to the tympanic antrum, the pyramidal eminence, and the fossa incudis.

  The entrance to the antrum is a large irregular aperture, which leads backward from the epitympanic recess into a considerable air space, named the tympanic or mastoid antrum (see page 142). The antrum communicates behind and below with the mastoid air cells, which vary considerably in number, size, and form; the antrum and mastoid air cells are lined by mucous membrane, continuous with that lining the tympanic cavity. On the medial wall of the entrance to the antrum is a rounded eminence, situated above and behind the prominence of the facial canal; it corresponds with the position of the ampullated ends of the superior and lateral semicircular canals.

  The pyramidal eminence (eminentia pyramidalis; pyramid) is situated immediately behind the fenestra vestibuli, and in front of the vertical portion of the facial canal; it is hollow, and contains the Stapedius muscle; its summit projects forward toward the fenestra vestibuli, and is pierced by a small aperture which transmits the tendon of the muscle. The cavity in the pyramidal eminence is prolonged downward and backward in front of the facial canal, and communicates with it by a minute aperture which transmits a twig from the facial nerve to the Stapedius muscle.

  The fossa incudis is a small depression in the lower and back part of the epitympanic recess; it lodges the short crus of the incus.

  The Carotid or Anterior Wall (paries carotica) is wider above than below; it corresponds with the carotid canal, from which it is separated by a thin plate of bone perforated by the tympanic branch of the internal carotid artery, and by the deep petrosal nerve which connects the sympathetic plexus on the internal carotid artery with the tympanic plexus on the promontory. At the upper part of the anterior wall are the orifice of the semicanal for the Tensor tympani muscle and the tympanic orifice of the auditory tube, separated from each other by a thin horizontal plate of bone, the septum canalis musculotubarii. These canals run from the tympanic cavity forward and downward to the retiring angle between the squama and the petrous portion of the temporal bone.

  The semicanal for the Tensor tympani (semicanalis m. tensoris tympani) is the superior and the smaller of the two; it is cylindrical and lies beneath the tegmen tympani. It extends on to the labyrinthic wall of the tympanic cavity and ends immediately above the fenestra vestibuli.

  The septum canalis musculotubarii (processus cochleariformis) passes backward below this semicanal, forming its lateral wall and floor; it expands above the anterior end of the fenestra vestibuli and terminates there by curving laterally so as to form a pulley over which the tendon of the muscle passes.

  The auditory tube (tuba auditiva; Eustachian tube) is the channel through which the tympanic cavity communicates with the nasal part of the pharynx. Its length is about 36 mm., and its direction is downward, forward, and medialward, forming an angle of about 45 degrees with the sagittal plane and one of from 30 to 40 degrees with the horizontal plane. It is formed partly of bone, partly of cartilage and fibrous tissue (819, 915).

  The osseous portion (pars osseo tubæ auditivæ) is about 12 mm. in length. It begins in the carotid wall of the tympanic cavity, below the septum canalis musculotubarii, and, gradually narrowing, ends at the angle of junction of the squama and the petrous portion of the temporal bone, its extremity presenting a jagged margin which serves for the attachment of the cartilaginous portion.

  The cartilaginous portion (pars cartilaginea tubæ auditivæ), about 24 mm. in length, is formed of a triangular plate of elastic fibrocartilage, the apex of which is attached to the margin of the medial end of the osseous portion of the tube, while its base lies directly under the mucous membrane of the nasal part of the pharynx, where it forms an elevation, the torus tubarius or cushion, behind the pharyngeal orifice of the tube. The upper edge of the cartilage is curled upon itself, being bent laterally so as to present on transverse section the appearance of a hook; a groove or furrow is thus produced, which is open below and laterally, and this part of the canal is completed by fibrous membrane. The cartilage lies in a groove between the petrous part of the temporal and the great wing of the sphenoid; this groove ends opposite the middle of the medial pterygoid plate. The cartilaginous and bony portions of the tube are not in the same plane, the former inclining downward a little more than the latter. The diameter of the tube is not uniform throughout, being greatest at the pharyngeal orifice, least at the junction of the bony and cartilaginous portions, and again increased toward the tympanic cavity; the narrowest part of the tube is termed the isthmus. The position and relations of the pharyngeal orifice are described with the nasal part of the pharynx. The mucous membrane of the tube is continuous in front with that of the nasal part of the pharynx, and behind with that of the tympanic cavity; it is covered with ciliated epithelium and is thin in the osseous portion, while in the cartilaginous portion it contains many mucous glands and near the pharyngeal orifice a considerable amount of adenoid tissue, which has been named by Gerlach the tube tonsil. The tube is opened during deglutition by the Salpingopharyngeus and Dilatator tubæ. The latter arises from the hook of the cartilage and from the membranous part of the tube, and blends below with the Tensor veli palatini.

 

Three auditory ossicles (malleus, incus and stapes), and two muscles are placed in tympanic cavity. Malleus has a head and manubrium with anterior and lateral processes. Muscle-tensor of tympanic membrane handle fastened to malleus. Incus consists of body, short and long legs. Body of incus adjoins to head of malleus, forming incusоmalleus jont. Long leg unites with stapes. Stapes has a head, anterior leg and posterior leg and base stapes, which closes a vestibular window. Musculus stapedius fastens to posterior leg of stapes. Muscles of tympanic cavity regulate auditory ossicles and prevent their oscillation during loud sounds.

 

Osseous labyrinth consists of cochlea, vestibulum and semicircular canals.

Vestibulum represents by cavity, its lateral wall carries vestibular and cochlear fenestrae (windows). Vestibular (oval) fenestra containes a base of stapes, and a cochlear (round) window is closed by the secondary tympanic membrane. There are 5 foramens of the semicircle canals in posterior wall of vestibulum, anterior wall has a big foramen conducting into cochlear canal. Crest of internal wall separates a spherical recess from elliptic recess. Internal foramen of vestibular canalicule opens in elliptic recess.

Osseous semicircular canals represented by three arched tubes lying in three mutually perpendicular planes. There are anterior, posterior and lateral semicircular canals. Each semicircular canal has broadened part in its base anterior, posterior and lateral osseous ampule. Semicircle canals join the vestibulum by the medium of osseous legs. Those legs containing ampule are called ampular legs. The legs of the anterior and posterior semicircular canal fuse together into one. As result the semicircle osseous canals unite with vestibulum by five foramina.

Cochlea (snail shell) lies anteriorly from vestibulum, represented by osseous tube forming two and half turns round cochlear axis (modiolus). There is osseous spiral plate inside the cochlea, apex of the cochlea called cupula. In cochlear base internal foramen of cochlear canaliculi is found.

Membranous labyrinth is inserted in osseous labyrinth, has the lesser dimensions and repeats the course of osseous labyrinth. The perilymphatic space is situated between internal surface of osseous labyrinth and external surface of membranous one and filled with liquid - perilymph. Membranous labyrinth is filled with the endolymph. Membranous labyrinth consists of vestibular part (a), semicircular canals (b) and cochlear duct (c).

A.    Vestibular labyrinth consists of utriculus and sacculus. Utriculus lies in elliptic recess of osseous labyrinth and connects with semicircular ducts, and a sacculus lies in spherical recess of osseous labyrinth and connects with cochlear duct by communicating duct. Utriculus and saccule communicate each other by the medium of utriculosaccular duct. From last endolymphatic duct starts that passes in vestibular canalicule. Endolymphatic duct passing from external foramen of vestibular canaliculus on posterior surface of pyramide of temporal bone, reaches endolymphatic sacculus placed under cerebral dura mater.

B.    Semicircular ducts are inserted in osseous semicircular canals. So there are anterior, posterior and lateral semicircular ducts. They carry anterior, posterior and lateral membranous ampulae. Receptors of balance of rotating located in cristae ampullares. Receptors of balance located in static maculae in utriculus and sacculus.

C.    Cochlear duct positioned in spiral canal and starts from vestibular osseous labyrinth and finishes in blind end. On transversal cut a cochlear duct has triangle shape and enclosed by external, superior and inferior walls. External wall fused together with periosteum of spiral canal; an inferior wall is a tympanic wall, it supplementes the spiral plate; superior wall is vestibular wall. Cochlear duct occupies middle part of osseous spiral canal and separates tympanic scala from vestibular scala. Spiral organ (Corti) localised in cochlear duct on spiral membrane, which belongs to peripheral part of auditory analyser.

Sound waves are received by tympanic membrane from auricle and external acoustic meatus. Oscillation of tympanic membrane is transfered to auditory ossicles - malleus, incus and stapes. Base of stapes, which covers a window vestibularа, begins oscillates the perilymph. Oscillation passes through vestibular scala, helicotrema and tympanic scala. Then vestibular wall (Reisner`s membrane) starts to vibrate. Last forces the oscillation of the endolymph in cochlear duct. This vibration is received by sensory haircells of spiral (Korti) organ.

The internal ear is the essential part of the organ of hearing, receiving the ultimate distribution of the auditory nerve. It is called the labyrinth, from the complexity of its shape, and consists of two parts: the osseous labyrinth, a series of cavities within the petrous part of the temporal bone, and the membranous labyrinth, a series of communicating membranous sacs and ducts, contained within the bony cavities.

 The Vestibule (vestibulum).—The vestibule is the central part of the osseous labyrinth, and is situated medial to the tympanic cavity, behind the cochlea, and in front of the semicircular canals. It is somewhat ovoid in shape, but flattened transversely; it measures about 5 mm. from before backward, the same from above downward, and about 3 mm. across. In its lateral or tympanic wall is the fenestra vestibuli, closed, in the fresh state, by the base of the stapes and annular ligament. On its medial wall, at the forepart, is a small circular depression, the recessus sphæricus, which is perforated, at its anterior and inferior part, by several minute holes (macula cribrosa media) for the passage of filaments of the acoustic nerve to the saccule; and behind this depression is an oblique ridge, the crista vestibuli, the anterior end of which is named the pyramid of the vestibule. This ridge bifurcates below to enclose a small depression, the fossa cochlearis, which is perforated by a number of holes for the passage of filaments of the acoustic nerve which supply the vestibular end of the ductus cochlearis. As the hinder part of the medial wall is the orifice of the aquæductus vestibuli, which extends to the posterior surface of the petrous portion of the temporal bone. It transmits a small vein, and contains a tubular prolongation of the membranous labyrinth, the ductus endolymphaticus, which ends in a cul-de-sac between the layers of the dura mater within the cranial cavity. On the upper wall or roof is a transversely oval depression, the recessus ellipticus, separated from the recessus sphæricus by the crista vestibuli already mentioned. The pyramid and adjoining part of the recessus ellipticus are perforated by a number of holes (macula cribrosa superior). The apertures in the pyramid transmit the nerves to the utricle; those in the recessus ellipticus the nerves to the ampullæ of the superior and lateral semicircular ducts. Behind are the five orifices of the semicircular canals. In front is an elliptical opening, which communicates with the scala vestibuli of the cochlea.



Interior of right osseous labyrinth.

The Bony Semicircular Canals (canales semicirculares ossei).—The bony semicircular canals are three in number, superior, posterior, and lateral, and are situated above and behind the vestibule. They are unequal in length, compressed from side to side, and each describes the greater part of a circle. Each measures about 0.8 mm. in diameter, and presents a dilatation at one end, called the ampulla, which measures more than twice the diameter of the tube. They open into the vestibule by five orifices, one of the apertures being common to two of the canals.

 

  The superior semicircular canal (canalis semicircularis superior), 15 to 20 mm. in length, is vertical in direction, and is placed transversely to the long axis of the petrous portion of the temporal bone, on the anterior surface of which its arch forms a round projection. It describes about two-thirds of a circle. Its lateral extremity is ampullated, and opens into the upper part of the vestibule; the opposite end joins with the upper part of the posterior canal to form the crus commune, which opens into the upper and medial part of the vestibule.

  The posterior semicircular canal (canalis semicircularis posterior), also vertical, is directed backward, nearly parallel to the posterior surface of the petrous bone; it is the longest of the three, measuring from 18 to 22 mm.; its lower or ampullated end opens into the lower and back part of the vestibule, its upper into the crus commune already mentioned.

  The lateral or horizontal canal (canalis semicircularis lateralis; external semicircular canal) is the shortest of the three. It measures from 12 to 15 mm., and its arch is directed horizontally backward and lateralward; thus each semicircular canal stands at right angles to the other two. Its ampullated end corresponds to the upper and lateral angle of the vestibule, just above the fenestra vestibuli, where it opens close to the ampullated end of the superior canal; its opposite end opens at the upper and back part of the vestibule. The lateral canal of one ear is very nearly in the same plane as that of the other; while the superior canal of one ear is nearly parallel to the posterior canal of the other.

 

  The Cochlea.—The cochlea bears some resemblance to a common snail-shell; it forms the anterior part of the labyrinth, is conical in form, and placed almost horizontally in front of the vestibule; its apex (cupula) is directed forward and lateralward, with a slight inclination downward, toward the upper and front part of the labyrinthic wall of the tympanic cavity; its base corresponds with the bottom of the internal acoustic meatus, and is perforated by numerous apertures for the passage of the cochlear division of the acoustic nerve. It measures about 5 mm. from base to apex, and its breadth across the base is about 9 mm. It consists of a conical shaped central axis, the modiolus; of a canal, the inner wall of which is formed by the central axis, wound spirally around it for two turns and three-quarters, from the base to the apex; and of a delicate lamina, the osseous spiral lamina, which projects from the modiolus, and, following the windings of the canal, partially subdivides it into two. In the recent state a membrane, the basilar membrane, stretches from the free border of this lamina to the outer wall of the bony cochlea and completely separates the canal into two passages, which, however, communicate with each other at the apex of the modiolus by a small opening named the helicotrema.

  The modiolus is the conical central axis or pillar of the cochlea. Its base is broad, and appears at the bottom of the internal acoustic meatus, where it corresponds with the area cochleæ; it is perforated by numerous orifices, which transmit filaments of the cochlear division of the acoustic nerve; the nerves for the first turn and a half pass through the foramina of the tractus spiralis foraminosus; those for the apical turn, through the foramen centrale. The canals of the tractus spiralis foraminosus pass up through the modiolus and successively bend outward to reach the attached margin of the lamina spiralis ossea. Here they become enlarged, and by their apposition form the spiral canal of the modiolus, which follows the course of the attached margin of the osseous spiral lamina and lodges the spiral ganglion (ganglion of Corti). The foramen centrale is continued into a canal which runs up the middle of the modiolus to its apex. The modiolus diminishes rapidly in size in the second and succeeding coil.

  The bony canal of the cochlea takes two turns and three-quarters around the modiolus. It is about 30 mm. in length, and diminishes gradually in diameter from the base to the summit, where it terminates in the cupula, which forms the apex of the cochlea. The beginning of this canal is about 3 mm. in diameter; it diverges from the modiolus toward the tympanic cavity and vestibule, and presents three openings. One, the fenestra cochleæ, communicates with the tympanic cavity—in the fresh state this aperture is closed by the secondary tympanic membrane; another, of an elliptical form, opens into the vestibule. The third is the aperture of the aquæductus cochleæ, leading to a minute funnel-shaped canal, which opens on the inferior surface of the petrous part of the temporal bone and transmits a small vein, and also forms a communication between the subarachnoid cavity and the scala tympani.

  The osseous spiral lamina (lamina spiralis ossea) is a bony shelf or ledge which projects from the modiolus into the interior of the canal, and, like the canal, takes two-and three-quarter turns around the modiolus. It reaches about half-way toward the outer wall of the tube, and partially divides its cavity into two passages or scalæ, of which the upper is named the scala vestibuli, while the lower is termed the scala tympani. Near the summit of the cochlea the lamina ends in a hook-shaped process, the hamulus laminæ spiralis; this assists in forming the boundary of a small opening, the helicotrema, through which the two scalæ communicate with each other. From the spiral canal of the modiolus numerous canals pass outward through the osseous spiral lamina as far as its free edge. In the lower part of the first turn a second bony lamina, the secondary spiral lamina, projects inward from the outer wall of the bony tube; it does not, however, reach the primary osseous spiral lamina, so that if viewed from the vestibule a narrow fissure, the vestibule fissure, is seen between them.

  The osseous labyrinth is lined by an exceedingly thin fibro-serous membrane; its attached surface is rough and fibrous, and closely adherent to the bone; its free surface is smooth and pale, covered with a layer of epithelium, and secretes a thin, limpid fluid, the perilymph. A delicate tubular process of this membrane is prolonged along the aqueduct of the cochlea to the inner surface of the dura mater.

 

The Membranous Labyrinth (labyrinthus membranaceus) (924, 925, 926).—The membranous labyrinth is lodged within the bony cavities just described, and has the same general form as these; it is, however, considerably smaller, and is partly separated from the bony walls by a quantity of fluid, the perilymph. In certain places it is fixed to the walls of the cavity. The membranous labyrinth contains fluid, the endolymph, and on its walls the ramifications of the acoustic nerve are distributed.

  Within the osseous vestibule the membranous labyrinth does not quite preserve the form of the bony cavity, but consists of two membranous sacs, the utricle, and the saccule.

 

The Utricle (utriculus).—The utricle, the larger of the two, is of an oblong form, compressed transversely, and occupies the upper and back part of the vestibule, lying in contact with the recessus ellipticus and the part below it. That portion which is lodged in the recess forms a sort of pouch or cul-de-sac, the floor and anterior wall of which are thickened, and form the macula acustica utriculi, which receives the utricular filaments of the acoustic nerve. The cavity of the utricle communicates behind with the semicircular ducts by five orifices. From its anterior wall is given off the ductus utriculosaccularis, which opens into the ductus endolymphaticus.

 

The Saccule (sacculus).—The saccule is the smaller of the two vestibular sacs; it is globular in form, and lies in the recessus sphæricus near the opening of the scala vestibuli of the cochlea. Its anterior part exhibits an oval thickening, the macula acustica sacculi, to which are distributed the saccular filaments of the acoustic nerve. Its cavity does not directly communicate with that of the utricle. From the posterior wall a canal, the ductus endolymphaticus, is given off; this duct is joined by the ductus utriculosaccularis, and then passes along the aquæductus vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior surface of the petrous portion of the temporal bone, where it is in contact with the dura mater. From the lower part of the saccule a short tube, the canalis reuniens of Hensen, passes downward and opens into the ductus cochlearis near its vestibular extremity (924).



The membranous labyrinth.

 

VIDEO 1

 

VIDEO 2

 

The Semicircular Ducts (ductus semicirculares; membranous semicircular canals), (925, 926).—The semicircular ducts are about one-fourth of the diameter of the osseous canals, but in number, shape, and general form they are precisely similar, and each presents at one end an ampulla. They open by five orifices into the utricle, one opening being common to the medial end of the superior and the upper end of the posterior duct. In the ampullæ the wall is thickened, and projects into the cavity as a fiddle-shaped, transversely placed elevation, the septum transversum, in which the nerves end.

  The utricle, saccule, and semicircular ducts are held in position by numerous fibrous bands which stretch across the space between them and the bony walls.

—The walls of the utricle, saccule, and semicircular ducts consist of three layers. The outer layer is a loose and flocculent structure, apparently composed of ordinary fibrous tissue containing bloodvessels and some pigment-cells. The middle layer, thicker and more transparent, forms a homogeneous membrana propria, and presents on its internal surface, especially in the semicircular ducts, numerous papilliform projections, which, on the addition of acetic acid, exhibit an appearance of longitudinal fibrillation. The inner layer is formed of polygonal nucleated epithelial cells. In the maculæ of the utricle and saccule, and in the transverse septa of the ampullæ of the semicircular ducts, the middle coat is thickened and the epithelium is columnar, and consists of supporting cells and hair cells. The former are fusiform, and their deep ends are attached to the membrana propria, while their free extremities are united to form a thin cuticle. The hair cells are flask-shaped, and their deep, rounded ends do not reach the membrana propria, but lie between the supporting cells. The deep part of each contains a large nucleus, while its more superficial part is granular and pigmented. The free end is surmounted by a long, tapering, hair-like filament, which projects into the cavity. The filaments of the acoustic nerve enter these parts, and having pierced the outer and middle layers, they lose their medullary sheaths, and their axis-cylinders ramify between the hair cells.


Right human membranous labyrinth, removed from its bony enclosure and viewed from the antero-lateral aspect.

  Two small rounded bodies termed otoconia, each consisting of a mass of minute crystalline grains of carbonate of lime, held together in a mesh of gelatinous tissue, are suspended in the endolymph in contact wish the free ends of the hairs projecting from the maculæ. According to Bowman, a calcareoutmaterial is also sparingly scattered in the cells lining the ampullæ of the semicircular ducts.

VIDEO

The Ductus Cochlearis (membranous cochlea; scala media).—The ductus cochlearis consists of a spirally arranged tube enclosed in the bony canal of the cochlea and lying along its outer wall.

  As already stated, the osseous spiral lamina extends only part of the distance between the modiolus and the outer wall of the cochlea, while the basilar membrane stretches from its free edge to the outer wall of the cochlea, and completes the roof of the scala tympani. A second and more delicate membrane, the vestibular membrane (Reissneri) extends from the thickened periosteum covering the osseous spiral lamina to the outer wall of the cochlea, where it is attached at some little distance above the outer edge of the basilar membrane. A canal is thus shut off between the scala tympani below and the scala vestibuli above; this is the ductus cochlearis or scala media (928). It is triangular on transverse section, its roof being formed by the vestibular membrane, its outer wall by the periosteum lining the bony canal, and its floor by the membrana basilaris and the outer part of the lamina spiralis ossea. Its extremities are closed; the upper is termed the lagena and is attached to the cupula at the upper part of the helicotrema; the lower is lodged in the recessus cochlearis of the vestibule. Near the lower end the ductus cochlearis is brought into continuity with the saccule by a narrow, short canal, the canalis reuniens of Hensen (924). On the membrana basilaris is situated the spiral organ of Corti. The vestibular membrane is thin and homogeneous, and is covered on its upper and under surfaces by a layer of epithelium. The periosteum, forming the outer wall of the ductus cochlearis, is greatly thickened and altered in character, and is called the spiral ligament. It projects inward below as a triangular prominence, the basilar crest, which gives attachment to the outer edge of the basilar membrane; immediately above the crest is a concavity, the sulcus spiralis externus. The upper portion of the spiral ligament contains numerous capillary loops and small bloodvessels, and is termed the stria vascularis.

  The osseous spiral lamina consists of two plates of bone, and between these are the canals for the transmission of the filaments of the acoustic nerve. On the upper plate of that part of the lamina which is outside the vestibular membrane, the periosteum is thickened to form the limbus laminæ spiralis (929), this ends externally in a concavity, the sulcus spiralis internus, which represents, on section, the form of the letter C; the upper part, formed by the overhanging extremity of the limbus, is named the vestibular lip; the lower part, prolonged and tapering, is called the tympanic lip, and is perforated by numerous foramina for the passage of the cochlear nerves. The upper surface of the vestibular lip is intersected at right angles by a number of furrows, between which are numerous elevations; these present the appearance of teeth along the free surface and margin of the lip, and have been named by Huschke the auditory teeth (930). The limbus is covered by a layer of what appears to be squamous epithelium, but the deeper parts of the cells with their contained nuclei occupy the intervals between the elevations and between the auditory teeth. This layer of epithelium is continuous on the one hand with that lining the sulcus spiralis internus, and on the other with that covering the under surface of the vestibular membrane.


Diagrammatic longitudinal section of the cochlea.

Basilar Membrane.—The basilar membrane stretches from the tympanic lip of the osseous spiral lamina to the basilar crest and consists of two parts, an inner and an outer. The inner is thin, and is named the zona arcuata: it supports the spiral organ of Corti. The outer is thicker and striated, and is termed the zona pectinata. The under surface of the membrane is covered by a layer of vascular connective tissue; one of the vessels in this tissue is somewhat larger than the rest, and is named the vas spirale; it lies below Corti’s tunnel.

  The spiral organ of Corti (organon spirale [Corti]; organ of Corti) (931, 932) is composed of a series of epithelial structures placed upon the inner part of the basilar membrane. The more central of these structures are two rows of rod-like bodies, the inner and outer rods or pillars of Corti. The bases of the rods are supported on the basilar membrane, those of the inner row at some distance from those of the outer; the two rows incline toward each other and, coming into contact above, enclose between them and the basilar membrane a triangular tunnel, the tunnel of Corti. On the inner side of the inner rods is a single row of hair cells, and on the outer side of the outer rods three or four rows of similar cells, together with certain supporting cells termed the cells of Deiters and Hensen. The free ends of the outer hair cells occupy a series of apertures in a net-like membrane, the reticular membrane, and the entire organ is covered by the tectorial membrane.

  RODS OF CORTI.—Each of these consists of a base or foot-plate, and elongated part or body, and an upper end or head; the body of each rod is finely striated, but in the head there is an oval non-striated portion which stains deeply with carmine. Occupying the angles between the rods and the basilar membrane are nucleated cells which partly envelop the rods and extend on to the floor of Corti’s tunnel; these may be looked upon as the undifferentiated parts of the cells from which the rods have been formed.


The lamina reticularis and subjacent structures. (Schematic.) A. Internal rod of Corti, with a, its plate. B. External rod (in yellow). C. Tunnel of Corti. D. Membrana basilaris. E. Inner hair cells. 1, 1’. Internal and external borders of the membrana reticularis. 2, 2’, 2”. The three rows of circular holes (in blue). 3. First row of phalanges (in yellow). 4, 4’, 4”. Second, third, and fourth rows of phalanges (in red). 6, 6’, 6”. The three rows of outer hair cells (in blue). 7, 7’, 7”. Cells of Deiters. 8. Cells of Hensen and Claudius.

  The inner rods number nearly 6000, and their bases rest on the basilar membrane close to the tympanic lip of the sulcus spiralis internus. The shaft or body of each is sinously curved and forms an angle of about 60 degrees with the basilar membrane. The head resembles the proximal end of the ulna and presents a deep concavity which accommodates a convexity on the head of the outer rod. The head-plate, or portion overhanging the concavity, overlaps the head-plate of the outer rod.

  The outer rods, nearly 4000 in number, are longer and more obliquely set than the inner, forming with the basilar membrane an angle of about 40 degrees. Their heads are convex internally; they fit into the concavities on the heads of the inner rods and are continued outward as thin flattened plates, termed phalangeal processes, which unite with the phalangeal processes of Deiters’ cells to form the reticular membrane.

 

Hair Cells.—The hair cells are short columnar cells; their free ends are on a level with the heads of Corti’s rods, and each is surmounted by about twenty hair-like processes arranged in the form of a crescent with its concavity directed inward. The deep ends of the cells reach about half-way along Corti’s rods, and each contains a large nucleus; in contact with the deep ends of the hair cells are the terminal filaments of the cochlear division of the acoustic nerve. The inner hair cells are arranged in a single row on the medial side of the inner rods, and their diameters being greater than those of the rods it follows that each hair cell is supported by more than one rod. The free ends of the inner hair cells are encircled by a cuticular membrane which is fixed to the heads of the inner rods. Adjoining the inner hair cells are one or two rows of columnar supporting cells, which, in turn, are continuous with the cubical cells lining the sulcus spiralis internus. The outer hair cells number about 12,000, and are nearly twice as long as the inner. In the basal coil of the cochlea they are arranged in three regular rows; in the apical coil, in four, somewhat irregular, rows.

  Between the rows of the outer hair cells are rows of supporting cells, called the cells of Deiters; their expanded bases are planted on the basilar membrane, while the opposite end of each presents a clubbed extremity or phalangeal process. Immediately to the outer side of Deiters’ cells are five or six rows of columnar cells, the supporting cells of Hensen. Their bases are narrow, while their upper parts are expanded and form a rounded elevation on the floor of the ductus cochlearis. The columnar cells lying outside Hensen’s cells are termed the cells of Claudius. A space exists between the outer rods of Corti and the adjacent hair cells; this is called the space of Nuel.

  The reticular lamina (932) is a delicate frame-work perforated by rounded holes which are occupied by the free ends of the outer hair cells. It extends from the heads of the outer rods of Corti to the external row of the outer hair cells, and is formed by several rows of “minute fiddle-shaped cuticular structures,” called phalanges, between which are circular apertures containing the free ends of the hair cells. The inner most row of phalanges consists of the phalangeal processes of the outer rods of Corti; the outer rows are formed by the modified free ends of Deiters’ cells.

  Covering the sulcus spiralis internus and the spiral organ of Corti is the tectorial membrane, which is attached to the limbus laminæ spiralis close to the inner edge of the vestibular membrane. Its inner part is thin and overlies the auditory teeth of Huschke; its outer part is thick, and along its lower surface, opposite the inner hair cells, is a clear band, named Hensen’s stripe, due to the intercrossing of its fibers. The lateral margin of the membrane is much thinner. Hardesty 151 considers the tectorial membrane as the vibrating mechanism in the cochlea. It is inconceivably delicate and flexible; far more sensitively flexible in the transverse than in the longitudinal direction and the readiness with which it bends when touched is beyond description. It is ectodermal in origin. It consists of fine colorless fibers embedded in a transparent matrix (the matrix may be a variety of soft keratin), of a soft collagenous, semisolid character with marked adhesiveness. The general transverse direction of the fibers inclines from the radius of the cochlea toward the apex.

  The acoustic nerve (n. acusticus; auditory nerve or nerve of hearing) divides near the bottom of the internal acoustic meatus into an anterior or cochlear and a posterior or vestibular branch.

  The vestibular nerve (n. vestibularis) supplies the utricle, the saccule, and the ampullæ of the semicircular ducts. On the trunk of the nerve, within the internal acoustic meatus, is a ganglion, the vestibular ganglion (ganglion of Scarpa); the fibers of the nerve arise from the cells of this ganglion. On the distal side of the ganglion the nerve splits into a superior, an inferior, and a posterior branch. The filaments of the superior branch are transmitted through the foramina in the area vestibularis superior, and end in the macula of the utricle and in the ampullæ of the superior and lateral semicircular ducts; those of the inferior branch traverse the foramina in the area vestibularis inferior, and end in the macula of the saccule. The posterior branch runs through the foramen singulare at the postero-inferior part of the bottom of the meatus and divides into filaments for the supply of the ampulla of the posterior semicircular duct.

  The cochlear nerve (n. cochlearis) divides into numerous filaments at the base of the modiolus; those for the basal and middle coils pass through the foramina in the tractus spiralis foraminosis, those for the apical coil through the canalis centralis, and the nerves bend outward to pass between the lamellæ of the osseous spiral lamina. Occupying the spiral canal of the modiolus is the spiral ganglion of the cochlea (ganglion of Corti), consisting of bipolar nerve cells, which constitute the cells of origin of this nerve. Reaching the outer edge of the osseous spiral lamina, the fibers of the nerve pass through the foramina in the tympanic lip; some end by arborizing around the bases of the inner hair cells, while others pass between Corti’s rods and across the tunnel, to end in a similar manner in relation to the outer hair cells. The cochlear nerve gives off a vestibular branch to supply the vestibular end of the ductus cochlearis; the filaments of this branch pass through the foramina in the fossa cochlearis (page 1048).

Vessels.—The arteries of the labyrinth are the internal auditory, from the basilar, and the stylomastoid, from the posterior auricular. The internal auditory artery divides at the bottom of the internal acoustic meatus into two branches: cochlear and vestibular. The cochlear branch subdivides into twelve or fourteen twigs, which traverse the canals in the modiolus, and are distributed, in the form of a capillary net-work, in the lamina spiralis and basilar membrane. The vestibular branches are distributed to the utricle, saccule, and semicircular ducts.

  The veins of the vestibule and semicircular canals accompany the arteries, and, receiving those of the cochlea at the base of the modiolus, unite to form the internal auditory veins which end in the posterior part of the superior petrosal sinus or in the transverse sinus.

Auditory tract

1.     Body of first neuron of auditory tract positioned in cochlear ganglion (spiral cochlear ganglion). The peripheral process of first neurons terminates in spiral organ and the central process neurons form cochlear part of VІІІ cranial nerve. It passes through the internal auditory meatus into cranial cavity, where terminates by synapse with second neuron.

2.     The bodies of second neurons of cochlear nerve are found in anterior and posterior cochlear nucleus in lateral recess of rhomboid fossa. Axons of second neurons form fascicles having a name trapezoid body. These fibres terminate partly in superior olivar nucleus. One from posterior cochlear nucleus form striae medullaris of fourth ventricle.

3.     The third neuron of auditory tract positioned in superior olivar nucleus. Their axons a lateral lemniscus, which runs through the isthmus of rhombencephalon (triangle of lemniscus) and reach the subcortical hearing centres.

The bodies of fourth neurons of auditory tract located in medial geniculate body and inferior colliculus of midbrain. Then axons run through the posterior third of posterior leg of internal capsule and reach Geshla gyrus (cortical hearing analyser in superior temporal gyrus). Axons of the fourth neurons also pass from inferior colliculus to the anterior funiculus of spinal cord as vestibulospinal tract. Instantaneous reaction in response to sound (protective reflex) realises by means of this pathway.

OLFACTOry ORGAN

Olfactory receptors placed in olfactory region of nasal cavity (in superior nasal meatus). Receptors (1st neuron) associated with epithelial supporting cells. The peripheral process of olfactory cells carry the olfactory cilia and the central process form 15-20 olfactory nerves (1st cranial nerve), which pass through the foramens in cribriform plate and reach the olfactory bulb. The axons of 2d neurons runs through the olfactory tract terminate in olfactory triangle and anterior perforating substance, where the bodies of the 3d neurons lie. Axons of the 3d neurons get the uncus and other part of limbic system, which is cortical olfactory analyser.

TASTE ORGAN

In man gustatory buds (2000 in number) are situated in mucous membrane of the tongue, palatine, pharynx, epiglottis. Most of gustatory buds localised in vallatae, foliatae and fungiform papillae of the tongue. In front 2/3 part of tongue tasting impulses are perceived by fibres of chorda tympani (intermediate nerve), in back 1/3 portion of tongue – by glossopharyngeal nerve, in lingual root and epiglottis – by fibres of vagus nerve.

The central process of first neurons, that are situated in mouth cavity, pass in composition of VII, IX, X cranial nerves to tasting sensory nucleus that positioned in medulla oblongata - nucleus tractus solitarius. Axons of second neurons run to the thalamus, where the third neuron is situated. Axons of third neurons terminate in uncus (cortex of cerebrum), where is situated a cortical taste analyzer.

GENERAL COVER

Includes a skin and hypodermic stratum. Skin forms general bodies cover, which defends the organism from influence of external environment. It carries out regulation of warmly, metabolism, breathing, excretes the sweat and fat and is as depot of power resources. Skin is sensory organ and built from two layers:

1.     epidermis (superficial layer);

2.     dermis (deep layer, or proper skin), which consists of fibrous connective tissue, elastic and muscular fibers.

Blood and lymphatic capillaries, and terminal nervous bodies present in superior dermal layer. Lower stratum of dermis passes into hypodermic base where agglomeration of adipose cells is situated.

Skin color depends on pigment (melanin), which positioned in innermost layer of epidermis. Hair and nails derive from epidermis.

Hairs cover the skin (except palms, soles, transitional part of lips, head and preputium of penis, minor pudenda labia). Hair has a shaft and root. Last lies in deepness of skin and terminates by hair bulb, which provides growth of hair. Root of hair lies hair follicle where sebaceous gland opens and erector pili muscle attaches.

Nail is flattened elastic structures of a horny texture plate lying in connective tissue nail bed where it begins to growth. In nail there distinguish a nail root, nail body and free edge, which stick out form nail bed border, and also cover margin and lateral margin.

Skin Glands. Derivates of skin - sebaceous glands and sweat glands - are learned in histology course carefully.

Mammary gland is modified sweat gland, which lies on fascia of major pectoral muscle on level of the ІІІ-VІ ribs. Body of breasts consists of lobes of mammary gland, which contain 15-20 lobules of mammary gland, dissevered one from one by bands of connective tissue. The smallest lobules consist of a cluster of rounded alveoli, which open into the smallest branches of the lactiferous ducts; these ducts communicate by to form larger ducts, and these end in a single canal, corresponding with one of the chief subdivisions of the gland. They converge toward the areola, beneath which they form dilatations sinus, which serve as reservoirs for the milk, and, at the base of the papillæ. Lobes are disposed relatively to mammary nipple radially, and the mammary ducts open on the top of nipple. Mammary areola positioned round mammary nipple covered areolar tubercles where open areolar glands.

Hypodermic stratum contains nervous and venous plexus, lymphatic vessels and hypodermic adipose tissue, which carry out function of thermal isolation and depot of energetic reserve. Also a soft connective tissue there is situated.

 Theme 3.  Peripheral nervous system. Autonomic nervous system.

Autonomic nervous system - this is part of nervous system, which provides innervation of all vessels, internal organs, smooth muscles and glandular epithelium including. It co-ordinates work of all of internal organs, regulates the metabolic and trophic processes in all organs and walls, supports organism homeostasis. Function of autonomous department controlled by influence of higher autonomous (vegetative) centres, which are contained in cerebellum, hypothalamus, and basal nuclei of forebrain and in cortex of cerebrum. This department has a row of peculiarities, which distinguish it from somatic nervous system:

-         group localization of autonomic (vegetative) nuclei in central nervous system:

ü     higher autonomic center – hypothalamus, frontal cortex of the hemisphere, cerebellum

ü     midbrain

ü     rhomboid fossa

ü     spinal cord

-         congestion of effectory neuron bodies in ganglia;

-         2 neurons tract from central autonomous (vegetative) system to organ:

-         1st neuron – preganglionic, 2nd neuron - postganglionic;

-         major part of peripheral fibers of autonomous part does not have myelin coat ;

-         speed of nervous impulse on autonomous part is lesser, than in somatic nervous system;

Autonomous nervous system has sympathetic and parasympathetic divisions.

The Hypothalamus includes the subthalamic tegmental region and the structures forming the greater part of the floor of the third ventricle, viz., the corpora mammillaria, tuber cinereum, infundibulum, hypophysis, and optic chiasma. The subthalamic tegmental region consists of the upward continuation of the tegmentum; it lies on the ventro-lateral aspect of the thalamus and separates it from the fibers of the internal capsule. The red nucleus and the substantia nigra are prolonged into its lower part; in front it is continuous with the substantia innominata of Meynert, medially with the gray substance of the floor of the third ventricle. It consists from above downward of three strata: (1) stratum dorsale, directly applied to the under surface of the thalamus and consisting of fine longitudinal fibers; (2) zona incerta, a continuation forward of the formatio reticularis of the tegmentum; and (3) the corpus subthalamicum (nucleus of Luys), a brownish mass presenting a lenticular shape on transverse section, and situated on the dorsal aspect of the fibers of the base of the cerebral peduncle; it is encapsuled by a lamina of nerve fibers and contains numerous medium-sized nerve cells, the connections of which are as yet not fully determined.

The corpora mammillaria (corpus albicantia) are two round white masses, each about the size of a small pea, placed side by side below the gray substance of the floor of the third ventricle in front of the posterior perforated substance. They consist of white substance externally and of gray substance internally, the cells of the latter forming two nuclei, a medial of smaller and a lateral of larger cells. The white substance is mainly formed by the fibers of the columns of the fornix, which descend to the base of the brain and end partly in the corpora mammillaria. From the cells of the gray substance of each mammillary body two fasciculi arise: one, the thalamomammillary fasciculus (bundle of Vicq d’Azyr), passes upward into the anterior nucleus of the thalamus; the other is directed downward into the tegmentum. Afferent fibers are believed to reach the corpus mammillare from the medial lemniscus and from the tegmentum.

The tuber cinereum is a hollow eminence of gray substance situated between the corpora mammillaria behind, and the optic chiasma in front. Laterally it is continuous with the anterior perforated substances and anteriorly with a thin lamina, the lamina terminalis. From the under surface of the tuber cinereum a hollow conical process, the infundibulum, projects downward and forward and is attached to the posterior lobe of the hypophysis. In the lateral part of the tuber cinereum is a nucleus of nerve cells, the basal optic nucleus of Meynert, while close to the cavity of the third ventricle are three additional nuclei. Between the tuber cinereum and the corpora mammillaria a small elevation, with a corresponding depression in the third ventricle, is sometimes seen. Retzius has named it the eminentia saccularis, and regards it as a representative of the saccus vasculosus found in this situation in some of the lower vertebrates. The hypophysis (pituitary body) is a reddish-gray, somewhat oval mass, measuring about 12.5 mm. in its transverse, and about 8 mm. in its antero-posterior diameter. It is attached to the end of the infundibulum, and is situated in the fossa hypophyseos of the sphenoidal bone, where it is retained by a circular fold of dura mater, the diaphragma sella; this fold almost completely roofs in the fossa, leaving only a small central aperture through which the infundibulum passes.

Chiasma (chiasma opticum; optic commissure).—The optic chiasma is a flattened, somewhat quadrilateral band of fibers, situated at the junction of the floor and anterior wall of the third ventricle. Most of its fibers have their origins in the retina, and reach the chiasma through the optic nerves, which are continuous with its antero-lateral angles. In the chiasma, they undergo a partial decussation the fibers from the nasal half of the retina decussate and enter the optic tract of the opposite side, while the fibers from the temporal half of the retina do not undergo decussation, but pass back into the optic tract of the same side. Occupying the posterior part of the commissure, however, is a strand of fibers, the commissure of Gudden, which is not derived from the optic nerves; it forms a connecting link between the medial geniculate bodies. Optic Tracts.—The optic tracts are continued backward and lateralward from the postero-lateral angles of the optic chiasma. Each passes between the anterior perforated substance and the tuber cinereum, and, winding around the ventrolateral aspect of the cerebral peduncle, divides into a medial and a lateral root. The former comprises the fibers of Gudden’s commissure. The lateral root consists mainly of afferent fibers which arise in the retina and undergo partial decussation in the optic chiasma, as described; but it also contains a few fine efferent fibers which have their origins in the brain and their terminations in the retina. When traced backward, the afferent fibers of the lateral root are found to end in the lateral geniculate body and pulvinar of the thalamus, and in the superior colliculus; and these three structures constitute the lower visual centers. Fibers arise from the nerve cells in these centers and pass through the occipital part of the internal capsule, under the name of the optic radiations, to the cortex of the occipital lobe of the cerebrum, where the higher or cortical visual center is situated. Some of the fibers of the optic radiations take an opposite course, arising from the cells of the occipital cortex and passing to the lower visual centers. Some fibers are detached from the optic tract, and pass through the cerebral peduncle to the nucleus of the oculomotor nerve. These may be regarded as the afferent branches for the Sphincter pupillæ and Ciliaris muscles. Other fibers have been described as reaching the cerebellum through the superior peduncle; while others, again, are lost in the pons.

PARASYMPATHETIC (Craniosacral) division

1.   Central part of parasympathetic division consists of the cranial part and pelvic part. Cranial part located in midbrain and rhomboid fossa. Mesencephalic portion contains accessory oculomotor (Yakubovych-Edinger-Westphal) nucleus, and bulbar portion contains superior salivary nucleus and inferior salivary nucleus and dorsal nucleus of vagus nerve. Pelvic part carries the parasympathetic nuclei, which lie in gray matter of sacral segments SII - SIV of spinal cord.

2.   Peripheral part consists of the ganglia, nerves and fibers.

Ciliary ganglion is formed by bodies of second /postganglionic/ neurons. It positioned in orbite near the optic nerve. The preganglionic fibres start from accessory oculomotor (Yakubovych-Edinger-Westphal) nucleus in composition of oculomotor nerve and separate from inferior branch as a radix oculomotorius, terminate by synapse with cells in ciliary ganglion. The postganglionic nervous fibres in composition of short ciliary nerves (nervi ciliares breves) pass to sphincter muscle of pupil and ciliary muscle. Sensory branches of nasociliary nerve and sympathetic postganglionic fibres from cavernous plexus pass through the ganglion.

Pterygopalatine ganglion lies in pterygopalatine fossa. It receives parasympathetic preganglionic fibres from superior salivary nucleus (with greater petrosal nerve). Last forms n. canalis pterygoidei (Vidian nerve) with sympathetic rootlet (from internal carotid plexus), sympathetic neurons innervate blood vessels. The postganglionic fibres join the ganglionic (sensory) nerves from maxillary nerve. Zygomatic nerve carries parasympathetic fibres to the lacrimal nerve (through the communicating branch) for innervating lacrimal gland. Nasal and palatine nerves provide complete innervating of the mucous membrane (and glands) in nasal cavity and palate.

Submandibular ganglion lies on medial surface same name salivary gland. Preganglionic parasympathetic fibres start from superior salivary nucleus in composition of chorda tympani. Last joins the lingual nerve that gives off sensory twigs for ganglion. Postganglionic³ fibres from this ganglion together with sensory and sympathetic fibres (from facial plexus) innervate submandibular salivary gland (often also sublingual gland).

Sublingual ganglion /inconstant/ located on external surface of sublingual salivary glands. It receives and gives off the same branches as submandibular parasympathetic ganglion.

Otic ganglion adjoins with mandibular of nerve under ovale foramen. This ganglion obtains parasympathetic innervation from inferior salivary nucleus (lesser petrosal nrerve). Postganglionic fibres in composition of auriculotemporal nerve innervate parotid salivary gland. Postganglionic sympathetic fibres (from middle meningeal plexus) which innervate the vessels of parotid salivary glands.

Parasympathetic part of X vagus nerve commences in dorsal nucleus of vagus nerve and contains a numerous of intramural ganglia. These ganglia enter to composition of cardiac, esophageal, pulmonary, gastric, intestinal, and others splanchnic plexus. Postganglionic neurons supply smooth muscles, glands and vessels of internal rogans in neck, thoracic and abdominal regions.

Parasympathetic nerves of the pelvis origin from nuclei in sacral segments SII - SIV of spinal cord. Preganglionic fibers [pelvic splanchnic nerves] pass to the terminal ganglia near organs. Postganglionic fibers innervate urinary bladder, reproductive organs, Lower half of descending colon, sygmoid colon and rectum.

The Parasympathetic dorsal nucleus of vagus nerves, is probably a mixed nucleus and contains not only the terminations of the sympathetic afferent or sensory fibers and the cells connected with them but contains also cells which give rise to sympathetic efferent or preganglionic fibers. These preganglionic fibers terminate in sympathetic ganglia from which the impulses are carried by other neurons. The cells of the dorsal nucleus are spindle-shaped, like those of the posterior column of the spinal cord, and the nucleus is usually considered as representing the base of the posterior column. It measures about 2 cm. in length, and in the lower, closed part of the medulla oblongata is situated behind the hypoglossal nucleus; whereas in the upper, open part it lies lateral to that nucleus, and corresponds to an eminence, named the ala cinerea (trigonum vagi), in the rhomboid fossa.

The Parasympathetic part of Oculomotor Nerve

The oculomotor nerve supplies somatic motor fibers to all the ocular muscles, except the Obliquus superior and Rectus lateralis; it also supplies through its connections with the ciliary ganglion, parasympathetic motor fibers to the Sphincter pupillæ and the Ciliaris muscles.   

The fibers of the oculomotor nerve arise from a nucleus which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle. From this nucleus the fibers pass forward through the tegmentum, the red nucleus, and the medial part of the substantia nigra, forming a series of curves with a lateral convexity, and emerge from the oculomotor sulcus on the medial side of the cerebral peduncle.   

  The nucleus of the oculomotor nerve does not consist of a continuous column of cells, but is broken up into a number of smaller nuclei, which are arranged in two groups, anterior and posterior. Those of the posterior group are six in number, five of which are symmetrical on the two sides of the middle line, while the sixth is centrally placed and is common to the nerves of both sides. The anterior group consists of two nuclei, an antero-medial and an antero-lateral.

  The nucleus of the oculomotor nerve, considered from a physiological standpoint, can be subdivided into several smaller groups of cells, each group controlling a particular muscle.

  On emerging from the brain, the nerve is invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid. It passes between the superior cerebellar and posterior cerebral arteries, and then pierces the dura mater in front of and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli. It runs along the lateral wall of the cavernous sinus, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus of the sympathetic, and a communicating branch from the ophthalmic division of the trigeminal. It then divides into two branches, which enter the orbit through the superior orbital fissure, between the two heads of the Rectus lateralis. Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami.

  The superior ramus, the smaller, passes medialward over the optic nerve, and supplies the Rectus superior and Levator palpebræ superioris. The inferior ramus, the larger, divides into three branches. One passes beneath the optic nerve to the Rectus medialis; another, to the Rectus inferior; the third and longest runs forward between the Recti inferior and lateralis to the Obliquus inferior. From the last a short thick branch is given off to the lower part of the ciliary ganglion, and forms its short root. All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the Obliquus inferior, which enters the muscle at its posterior border.

The The Parasympathetic part of facial nerve locatedat the nervus intermedius (pars intermedii of Wrisberg). The two parts emerge at the lower border of the pons in the recess between the olive and the inferior peduncle, the motor part being the more medial, immediately to the lateral side of the sensory part is the acoustic nerve.

These are preganglionic fibers of the parasympathetic system and terminate in the submandibular ganglion and small ganglia in the hilus of the submandibular gland. From these ganglia postganglionic fibers are conveyed to these glands. The sensory part contains the fibers of taste for the anterior two-thirds of the tongue and a few somatic sensory fibers from the middle ear region. A few splanchnic sensory fibers are also present.

The motor root arises from a nucleus which lies deeply in the reticular formation of the lower part of the pons.

The sensory root arises from the genicular ganglion, which is situated on the geniculum of the facial nerve in the facial canal, behind the hiatus of the canal. The cells of this ganglion are unipolar, and the single process divides in a T-shaped manner into central and peripheral branches. The central branches leave the trunk of the facial nerve in the internal acoustic meatus, and form the sensory root; the peripheral branches are continued into the chorda tympani and greater superficial petrosal nerves.

The greater superficial petrosal nerve (large superficial petrosal nerve) arises from the genicular ganglion, and consists chiefly of sensory branches which are distributed to the mucous membrane of the soft palate; but it probably contains a few motor fibers which form the motor root of the sphenopalatine ganglion. It passes forward through the hiatus of the facial canal, and runs in a sulcus on the anterior surface of the petrous portion of the temporal bone beneath the semilunar ganglion, to the foramen lacerum. It receives a twig from the tympanic plexus, and in the foramen is joined by the deep petrosal, from the sympathetic plexus on the internal carotid artery, to form the nerve of the pterygoid canal which passes forward through the pterygoid canal and ends in the sphenopalatine ganglion. The genicular ganglion is connected with the otic ganglion by a branch which joins the lesser superficial petrosal nerve, and also with the sympathetic filaments accompanying the middle meningeal artery. According to Arnold, a twig passes back from the ganglion to the acoustic nerve. Just before the facial nerve emerges from the stylomastoid foramen, it generally receives a twig from the auricular branch of the vagus.

After its exit from the stylomastoid foramen, the facial nerve sends a twig to the glossopharyngeal, and communicates with the auricular branch of the vagus, with the great auricular nerve of the cervical plexus, with the auriculotemporal nerve in the parotid gland, and with the lesser occipital behind the ear; on the face with the terminal branches of the trigeminal, and in the neck with the cutaneous cervical nerve.

The Chorda Tympani Nerve is given off from the facial as it passes downward behind the tympanic cavity, about 6 mm. from the stylomastoid foramen. It runs upward and forward in a canal, and enters the tympanic cavity, through an aperture (iter chordæ posterius) on its posterior wall, close to the medial surface of the posterior border of the tympanic membrane and on a level with the upper end of the manubrium of the malleus. It traverses the tympanic cavity, between the fibrous and mucous layers of the tympanic membrane, crosses the manubrium of the malleus, and emerges from the cavity through a foramen situated at the inner end of the petrotympanic fissure, and named the iter chordæ anterius (canal of Huguier). It then descends between the Pterygoideus externus and internus on the medial surface of the spina angularis of the sphenoid, which it sometimes grooves, and joins, at an acute angle, the posterior border of the lingual nerve. It receives a few efferent fibers from the motor root; these enter the submandibular ganglion, and through it are distributed to the submandibular and sublingual glands; the majority of its fibers are afferent, and are continued onward through the muscular substance of the tongue to the mucous membrane covering its anterior two-thirds; they constitute the nerve of taste for this portion of the tongue. Before uniting with the lingual nerve the chorda tympani is joined by a small branch from the otic ganglion.

The Parasympathetic part of Glossopharyngeal nerve. The parasympathetic efferent fibers from the inferior salivary nucleus beneath, are both preganglionic motor fibers and preganglionic secretory fibers of the parasympathetic system. The secretory fibers pass to the otic ganglion and from it secondary neurons are distributed to the parotid gland.

The Tympanic Nerve (n. tympanicus; nerve of Jacobson) arises from the petrous ganglion, and ascends to the tympanic cavity through a small canal on the under surface of the petrous portion of the temporal bone on the ridge which separates the carotid canal from the jugular fossa. In the tympanic cavity it divides into branches which form the tympanic plexus and are contained in grooves upon the surface of the promontory. This plexus gives off: (1) the lesser superficial petrosal nerve; (2) a branch to join the greater superficial petrosal nerve; and (3) branches to the tympanic cavity, all of which will be described in connection with the anatomy of the middle ear.

The Pharyngeal Branches (rami pharyngei) are three or four filaments which unite, opposite the Constrictor pharyngis medius, with the pharyngeal branches of the vagus and parasympathetic, to form the pharyngeal plexus; branches from this plexus perforate the muscular coat of the pharynx and supply its muscles and mucous membrane. The Tonsillar Branches (rami tonsillares) supply the palatine tonsil, forming around it a plexus from which filaments are distributed to the soft palate and fauces, where they communicate with the palatine nerves.

The Lingual Branches (rami linguales) are two in number; one supplies the papillæ vallatæ and the mucous membrane covering the base of the tongue; the other supplies the mucous membrane and follicular glands of the posterior part of the tongue, and communicates with the lingual nerve.

The Parasympathetic part of Vagus Nerve

The vagus nerve is composed of both motor and sensory fibers, and has a more extensive course and distribution than any of the other cranial nerves, since it passes through the neck and thorax to the abdomen.    

  The vagus is attached by eight or ten filaments to the medulla oblongata in the groove between the olive and the inferior peduncle, below the glossopharyngeal. The sensory fibers arise from the cells of the jugular ganglion and ganglion nodosum of the nerve, and, when traced into the medulla oblongata mostly end by arborizing around the cells of the inferior part of a nucleus which lies beneath the ala cinerea in the lower part of the rhomboid fossa. These are the sympathetic afferent fibers. Some of the sensory fibers of the glossopharyngeal nerve have been seen to end in the upper part of this nucleus. A few of the sensory fibers of the vagus, probably taste fibers, descend in the fasciculus solitarius and end around its cells. The somatic sensory fibers, few in number, from the posterior part of the external auditory meatus and the back of the ear, probably join the spinal tract of the trigeminal as it descends in the medulla. The somatic motor fibers arise from the cells of the nucleus ambiguus, already referred to in connection with the motor root of the glossopharyngeal nerve.

  The Parasympathetic efferent fibers, distributed probably as preganglionic fibers to the thoracic and abdominal viscera, i. e., as motor fibers to the bronchial tree, inhibitory fibers to the heart, motor fibers to the esophagus, stomach, small intestine and gall passages, and as secretory fibers to the stomach and pancreas, arise from the dorsal nucleus of the vagus.   

  The filaments of the nerve unite, and form a flat cord, which passes beneath the flocculus to the jugular foramen, through which it leaves the cranium. In emerging through this opening, the vagus is accompanied by and contained in the same sheath of dura mater with the accessory nerve, a septum separating them from the glossopharyngeal which lies in front. In this situation the vagus presents a well-marked ganglionic enlargement, which is called the jugular ganglion (ganglion of the root); to it the accessory nerve is connected by one or two filaments. After its exit from the jugular foramen the vagus is joined by the cranial portion of the accessory nerve, and enlarges into a second gangliform swelling, called the ganglion nodosum (ganglion of the trunk); through this the fibers of the cranial portion of the accessory pass without interruption, being principally distributed to the pharyngeal and superior laryngeal branches of the vagus, but some of its fibers descend in the trunk of the vagus, to be distributed with the recurrent nerve and probably also with the cardiac nerves.   

  The vagus nerve passes vertically down the neck within the carotid sheath, lying between the internal jugular vein and internal carotid artery as far as the upper border of the thyroid cartilage, and then between the same vein and the common carotid artery to the root of the neck. The further course of the nerve differs on the two sides of the body.

  On the right side, the nerve passes across the subclavian artery between it and the right innominate vein, and descends by the side of the trachea to the back of the root of the lung, where it spreads out in the posterior pulmonary plexus. From the lower part of this plexus two cords descend on the esophagus, and divide to form, with branches from the opposite nerve, the esophageal plexus. Below, these branches are collected into a single cord, which runs along the back of the esophagus enters the abdomen, and is distributed to the postero-inferior surface of the stomach, joining the left side of the celiac plexus, and sending filaments to the lienal plexus.

  On the left side, the vagus enters the thorax between the left carotid and subclavian arteries, behind the left innominate vein. It crosses the left side of the arch of the aorta, and descends behind the root of the left lung, forming there the posterior pulmonary plexus. From this it runs along the anterior surface of the esophagus, where it unites with the nerve of the right side in the esophageal plexus, and is continued to the stomach, distributing branches over its anterosuperior surface; some of these extend over the fundus, and others along the lesser curvature. Filaments from these branches enter the lesser omentum, and join the hepatic plexus.

  The Jugular Ganglion (ganglion jugulare; ganglion of the root) is of a grayish color, spherical in form, about 4 mm. in diameter.

  Branches of Communication.—This ganglion is connected by several delicate filaments to the cranial portion of the accessory nerve; it also communicates by a twig with the petrous ganglion of the glossopharyngeal, with the facial nerve by means of its auricular branch, and with the sympathetic by means of an ascending filament from the superior cervical ganglion.

  The Ganglion Nodosum (ganglion of the trunk; inferior ganglion) is cylindrical in form, of a reddish color, and 2.5 cm. in length. Passing through it is the cranial portion of the accessory nerve, which blends with the vagus below the ganglion.  

Branches of Communication.—This ganglion is connected with the hypoglossal, the superior cervical ganglion of the sympathetic, and the loop between the first and second cervical nerves.

Branches of Distribution.—The branches of distribution of the vagus are:  

In the Jugular Fossa… Meningeal.

Auricular.

In the Neck Pharyngeal.

Superior laryngeal.

Recurrent.

Superior cardiac.

In the Thorax………. Inferior cardiac. Anterior bronchial. Posterior bronchial. Esophageal.

In the Abdomen……. Gastric. Celiac. Hepatic.

The Pharyngeal Branch (ramus pharyngeus), the principal motor nerve of the pharynx, arises from the upper part of the ganglion nodosum, and consists principally of filaments from the cranial portion of the accessory nerve. It passes across the internal carotid artery to the upper border of the Constrictor pharyngis medius, where it divides into numerous filaments, which join with branches from the glossopharyngeal, sympathetic, and external laryngeal to form the pharyngeal plexus. From the plexus, branches are distributed to the muscles and mucous membrane of the pharynx and the muscles of the soft palate, except the Tensor veli palatini. A minute filament descends and joins the hypoglossal nerve as it winds around the occipital artery.

  The Superior Laryngeal Nerve (n. laryngeus superior) larger than the preceding, arises from the middle of the ganglion nodosum and in its course receives a branch from the superior cervical ganglion of the sympathetic. It descends, by the side of the pharynx, behind the internal carotid artery, and divides into two branches, external and internal.

  The external branch (ramus externus), the smaller, descends on the larynx, beneath the Sternothyreoideus, to supply the Cricothyreoideus. It gives branches to the pharyngeal plexus and the Constrictor pharyngis inferior, and communicates with the superior cardiac nerve, behind the common carotid artery.

  The internal branch (ramus internus) descends to the hyothyroid membrane, pierces it in company with the superior laryngeal artery, and is distributed to the mucous membrane of the larynx. Of these branches some are distributed to the epiglottis, the base of the tongue, and the epiglottic glands; others pass backward, in the aryepiglottic fold, to supply the mucous membrane surrounding the entrance of the larynx, and that lining the cavity of the larynx as low down as the vocal folds. A filament descends beneath the mucous membrane on the inner surface of the thyroid cartilage and joins the recurrent nerve.

  The Recurrent Nerve (n. recurrens; inferior or recurrent laryngeal nerve) arises, on the right side, in front of the subclavian artery; winds from before backward around that vessel, and ascends obliquely to the side of the trachea behind the common carotid artery, and either in front of or behind the inferior thyroid artery. On the left side, it arises on the left of the arch of the aorta, and winds below the aorta at the point where the ligamentum arteriosum is attached, and then ascends to the side of the trachea. The nerve on either side ascends in the groove between the trachea and esophagus, passes under the lower border of the Constrictor pharyngis inferior, and enters the larynx behind the articulation of the inferior cornu of the thyroid cartilage with the cricoid; it is distributed to all the muscles of the larynx, excepting the Cricothyreoideus. It communicates with the internal branch of the superior laryngeal nerve, and gives off a few filaments to the mucous membrane of the lower part of the larynx.

  As the recurrent nerve hooks around the subclavian artery or aorta, it gives off several cardiac filaments to the deep part of the cardiac plexus. As it ascends in the neck it gives off branches, more numerous on the left than on the right side, to the mucous membrane and muscular coat of the esophagus; branches to the mucous membrane and muscular fibers of the trachea; and some pharyngeal filaments to the Constrictor pharyngis inferior.

  The Superior Cardiac Branches (rami cardiaci superiores; cervical cardiac branches), two or three in number, arise from the vagus, at the upper and lower parts of the neck.

  The upper branches are small, and communicate with the cardiac branches of the sympathetic. They can be traced to the deep part of the cardiac plexus.

  The lower branch arises at the root of the neck, just above the first rib. That from the right vagus passes in front or by the side of the innominate artery, and proceeds to the deep part of the cardiac plexus; that from the left runs down across the left side of the arch of the aorta, and joins the superficial part of the cardiac plexus.    23

  The Inferior Cardiac Branches (rami cardiaci inferiores; thoracic cardiac branches), on the right side, arise from the trunk of the vagus as it lies by the side of the trachea, and from its recurrent nerve; on the left side from the recurrent nerve only; passing inward, they end in the deep part of the cardiac plexus.

  The Anterior Bronchial Branches (rami bronchiales anteriores; anterior or ventral pulmonary branches), two or three in number, and of small size, are distributed on the anterior surface of the root of the lung. They join with filaments from the sympathetic, and form the anterior pulmonary plexus.

  The Posterior Bronchial Branches (rami bronchiales posteriores; posterior or dorsal pulmonary branches), more numerous and larger than the anterior, are distributed on the posterior surface of the root of the lung; they are joined by filaments from the third and fourth (sometimes also from the first and second) thoracic ganglia of the sympathetic trunk, and form the posterior pulmonary plexus. Branches from this plexus accompany the ramifications of the bronchi through the substance of the lung.

  The Esophageal Branches (rami æsophagei) are given off both above and below the bronchial branches; the lower are numerous and larger than the upper. They form, together with the branches from the opposite nerve, the esophageal plexus. From this plexus filaments are distributed to the back of the pericardium.

  The Gastric Branches (rami gastrici) are distributed to the stomach. The right vagus forms the posterior gastric plexus on the postero-inferior surface of the stomach and the left the anterior gastric plexus on the antero-superior surface.

  The Celiac Branches (rami cæliaci) are mainly derived from the right vagus: they join the celiac plexus and through it supply branches to the pancreas, spleen, kidneys, suprarenal bodies, and intestine.

  The Hepatic Branches (rami hepatici) arise from the left vagus: they join the hepatic plexus and through it are conveyed to the liver.

X Vagus nerve (mixed) contains motor fibers which start from nucleus ambiguus, parasympathetic (preganglionic) fibers form dorsal nucleus and sensory fibers from superior and inferior ganglia in jugular foramen.

*          Cranial part of vagus nerve gives off the following branches:

Meningeal branch which starts from superior ganglion and passes to cranial dura mater in posterior cranial fossa;

Auricular branch, which starts from superior ganglion

, passes over mastoid canalicule of temporal bone and innervates the skin of external surface of auricle and posterior wall of external acoustic meatus.

The vagus nerve is composed of both motor and sensory fibers, and has a more extensive course and distribution than any of the other cranial nerves, since it passes through the neck and thorax to the abdomen.


Plan of upper portions of glossopharyngeal, vagus, and accessory nerves.

  The vagus is attached by eight or ten filaments to the medulla oblongata in the groove between the olive and the inferior peduncle, below the glossopharyngeal. The sensory fibers arise from the cells of the jugular ganglion and ganglion nodosum of the nerve, and, when traced into the medulla oblongata mostly end by arborizing around the cells of the inferior part of a nucleus which lies beneath the ala cinerea in the lower part of the rhomboid fossa. These are the sympathetic afferent fibers. Some of the sensory fibers of the glossopharyngeal nerve have been seen to end in the upper part of this nucleus. A few of the sensory fibers of the vagus, probably taste fibers, descend in the fasciculus solitarius and end around its cells. The somatic sensory fibers, few in number, from the posterior part of the external auditory meatus and the back of the ear, probably join the spinal tract of the trigeminal as it descends in the medulla. The somatic motor fibers arise from the cells of the nucleus ambiguus, already referred to in connection with the motor root of the glossopharyngeal nerve.

Sympathetic part of the vegetative nervous system.

  The sympathetic efferent fibers, distributed probably as preganglionic fibers to the thoracic and abdominal viscera, i. e., as motor fibers to the bronchial tree, inhibitory fibers to the heart, motor fibers to the esophagus, stomach, small intestine and gall passages, and as secretory fibers to the stomach and pancreas, arise from the dorsal nucleus of the vagus.

  The filaments of the nerve unite, and form a flat cord, which passes beneath the flocculus to the jugular foramen, through which it leaves the cranium. In emerging through this opening, the vagus is accompanied by and contained in the same sheath of dura mater with the accessory nerve, a septum separating them from the glossopharyngeal which lies in front (792). In this situation the vagus presents a well-marked ganglionic enlargement, which is called the jugular ganglion (ganglion of the root); to it the accessory nerve is connected by one or two filaments. After its exit from the jugular foramen the vagus is joined by the cranial portion of the accessory nerve, and enlarges into a second gangliform swelling, called the ganglion nodosum (ganglion of the trunk); through this the fibers of the cranial portion of the accessory pass without interruption, being principally distributed to the pharyngeal and superior laryngeal branches of the vagus, but some of its fibers descend in the trunk of the vagus, to be distributed with the recurrent nerve and probably also with the cardiac nerves.


Upper part of medulla spinalis and hind- and mid-brains; posterior aspect, exposed in situ.

  The vagus nerve passes vertically down the neck within the carotid sheath, lying between the internal jugular vein and internal carotid artery as far as the upper border of the thyroid cartilage, and then between the same vein and the common carotid artery to the root of the neck. The further course of the nerve differs on the two sides of the body.

  On the right side, the nerve passes across the subclavian artery between it and the right innominate vein, and descends by the side of the trachea to the back of the root of the lung, where it spreads out in the posterior pulmonary plexus. From the lower part of this plexus two cords descend on the esophagus, and divide to form, with branches from the opposite nerve, the esophageal plexus. Below, these branches are collected into a single cord, which runs along the back of the esophagus enters the abdomen, and is distributed to the postero-inferior surface of the stomach, joining the left side of the celiac plexus, and sending filaments to the lienal plexus.

  On the left side, the vagus enters the thorax between the left carotid and subclavian arteries, behind the left innominate vein. It crosses the left side of the arch of the aorta, and descends behind the root of the left lung, forming there the posterior pulmonary plexus. From this it runs along the anterior surface of the esophagus, where it unites with the nerve of the right side in the esophageal plexus, and is continued to the stomach, distributing branches over its anterosuperior surface; some of these extend over the fundus, and others along the lesser curvature. Filaments from these branches enter the lesser omentum, and join the hepatic plexus.

  The Jugular Ganglion (ganglion jugulare; ganglion of the root) is of a grayish color, spherical in form, about 4 mm. in diameter.

 Branches of Communication.—This ganglion is connected by several delicate filaments to the cranial portion of the accessory nerve; it also communicates by a twig with the petrous ganglion of the glossopharyngeal, with the facial nerve by means of its auricular branch, and with the sympathetic by means of an ascending filament from the superior cervical ganglion.

  The Ganglion Nodosum (ganglion of the trunk; inferior ganglion) is cylindrical in form, of a reddish color, and 2.5 cm. in length. Passing through it is the cranial portion of the accessory nerve, which blends with the vagus below the ganglion.

 

Branches of Communication.—This ganglion is connected with the hypoglossal, the superior cervical ganglion of the sympathetic, and the loop between the first and second cervical nerves.

 

Branches of Distribution.—The branches of distribution of the vagus are:

  The Meningeal Branch (ramus meningeus; dural branch) is a recurrent filament given off from the jugular ganglion; it is distributed to the dura mater in the posterior fossa of the base of the skull.

  The Auricular Branch (ramus auricularis; nerve of Arnold) arises from the jugular ganglion, and is joined soon after its origin by a filament from the petrous ganglion of the glossopharyngeal; it passes behind the internal jugular vein, and enters the mastoid canaliculus on the lateral wall of the jugular fossa. Traversing the substance of the temporal bone, it crosses the facial canal about 4 mm. above the stylomastoid foramen, and here it gives off an ascending branch which joins the facial nerve. The nerve reaches the surface by passing through the tympanomastoid fissure between the mastoid process and the tympanic part of the temporal bone, and divides into two branches: one joins the posterior auricular nerve, the other is distributed to the skin of the back of the auricula and to the posterior part of the external acoustic meatus.

  The Pharyngeal Branch (ramus pharyngeus), the principal motor nerve of the pharynx, arises from the upper part of the ganglion nodosum, and consists principally of filaments from the cranial portion of the accessory nerve. It passes across the internal carotid artery to the upper border of the Constrictor pharyngis medius, where it divides into numerous filaments, which join with branches from the glossopharyngeal, sympathetic, and external laryngeal to form the pharyngeal plexus. From the plexus, branches are distributed to the muscles and mucous membrane of the pharynx and the muscles of the soft palate, except the Tensor veli palatini. A minute filament descends and joins the hypoglossal nerve as it winds around the occipital artery.


Course and distribution of the glossopharyngeal, vagus, and accessory nerves.

  The Superior Laryngeal Nerve (n. laryngeus superior) larger than the preceding, arises from the middle of the ganglion nodosum and in its course receives a branch from the superior cervical ganglion of the sympathetic. It descends, by the side of the pharynx, behind the internal carotid artery, and divides into two branches, external and internal.

  The external branch (ramus externus), the smaller, descends on the larynx, beneath the Sternothyreoideus, to supply the Cricothyreoideus. It gives branches to the pharyngeal plexus and the Constrictor pharyngis inferior, and communicates with the superior cardiac nerve, behind the common carotid artery.

  The internal branch (ramus internus) descends to the hyothyroid membrane, pierces it in company with the superior laryngeal artery, and is distributed to the mucous membrane of the larynx. Of these branches some are distributed to the epiglottis, the base of the tongue, and the epiglottic glands; others pass backward, in the aryepiglottic fold, to supply the mucous membrane surrounding the entrance of the larynx, and that lining the cavity of the larynx as low down as the vocal folds. A filament descends beneath the mucous membrane on the inner surface of the thyroid cartilage and joins the recurrent nerve.

  The Recurrent Nerve (n. recurrens; inferior or recurrent laryngeal nerve) arises, on the right side, in front of the subclavian artery; winds from before backward around that vessel, and ascends obliquely to the side of the trachea behind the common carotid artery, and either in front of or behind the inferior thyroid artery. On the left side, it arises on the left of the arch of the aorta, and winds below the aorta at the point where the ligamentum arteriosum is attached, and then ascends to the side of the trachea. The nerve on either side ascends in the groove between the trachea and esophagus, passes under the lower border of the Constrictor pharyngis inferior, and enters the larynx behind the articulation of the inferior cornu of the thyroid cartilage with the cricoid; it is distributed to all the muscles of the larynx, excepting the Cricothyreoideus. It communicates with the internal branch of the superior laryngeal nerve, and gives off a few filaments to the mucous membrane of the lower part of the larynx.

  As the recurrent nerve hooks around the subclavian artery or aorta, it gives off several cardiac filaments to the deep part of the cardiac plexus. As it ascends in the neck it gives off branches, more numerous on the left than on the right side, to the mucous membrane and muscular coat of the esophagus; branches to the mucous membrane and muscular fibers of the trachea; and some pharyngeal filaments to the Constrictor pharyngis inferior.

  The Superior Cardiac Branches (rami cardiaci superiores; cervical cardiac branches), two or three in number, arise from the vagus, at the upper and lower parts of the neck.

  The upper branches are small, and communicate with the cardiac branches of the sympathetic. They can be traced to the deep part of the cardiac plexus.

  The lower branch arises at the root of the neck, just above the first rib. That from the right vagus passes in front or by the side of the innominate artery, and proceeds to the deep part of the cardiac plexus; that from the left runs down across the left side of the arch of the aorta, and joins the superficial part of the cardiac plexus.

*                   Cervical part of vagus nerve gives off:

Pharyngeal branches with branches of Glossopharyngeal nerve and sympathetic trunk form pharyngeal plexus, that innervates mucous membrane and muscles of the throat (superior and middle constrictors; levator veli palatini, palatopharyngeus and palatoglossus, uvulae muscles).

Superior cervical cardiac branches pass downward along common carotid artery and  communicate with sympathetic nerves, enter into cardiac plexus and supply the heart (sensory and parasympathetic innervating).

Superior laryngeal nerve originate from inferior ganglion and carry sensory, motor and parasympathetic preganglionic fibers. Motor fibers of the external branch innervate cricothyroid and inferior constrictor muscles, sensory fibers (internal branch) supply mucous membrane of the larynx over vocal fold, mucous membrane of the epiglottis and tongue root.

Recurrent laryngeal nerve passes upward between esophagus and trachea and sends a numerous twigs. Inferior laryngeal nerve supplies mucous membrane of the larynx below vocal fold and the rest of muscles (thyroarytenoid, lateral and posterior cricoarytenoid, transverse and oblique arytenoid, vocalis). Tracheal, esophageal and inferior cervical cardiac branches supply internal organs.

*                   Thoracic part of vagus nerve gives off:

Thoracic cardiac branches which pass to cardiac plexus;

Bronchial branches with sympathetic nerves form pulmonary plexus. Last enters in lungs with bronchi.

Esophageal branches form esophageal plexus round this organ.

Abdominal part of vagus nerve is represented by anterior and posterior vagal trunks, which originate from esophageal plexus. Anterior vagal trunk located on front surface of the stomach and gives branches gives off the anterior gastric and hepatic branches. Posterior vagal trunk supplies back gastric wall, and gives off coeliac branches to reach coeliac plexus. Then fibers of vagus nerve with sympathetic fibers supply the liver, spleen, pancreas, kidneys, small and large intestine (including a upper department of descending colon).

Sympathetic centres are located in lateral intermediate nuclei of lateral horns (spinal cord segments C8-Th1-Th12-L1-L2).

Peripheral sympathetic division includes right and left sympathetic trunks, communicating branches, prevertebral sympathetic ganglia, plexuses and fibbers, which pass to organs and tissues.


The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses.

Sympathetic trunk is a paired formation, which is found on sides from vertebral column and consists of 20-25 paravertebral sympathetic ganglia, joint between each other by interganglionic rami. The white communicating branches from spinal nerves approach to sympathetic trunk They are preganglionic fiber (passes from lateral intermediate nucleus in composition of anterior rootlets and spinal nerve to nearest paravertebral sympathetic ganglion). The white communicating branches pass to eighth cervical, all thoracic and two top lumbar spinal ganglion.

Sympathetic trunk consists of 3 cervical, 10-12 thoracic, 4-5 lumbar, 4-5 sacral and one unpaired coccigeal ganglia. White communicating branches (preganglionic fibers) approach to upper cervical, lower sacral and coccigeal ganglia by the  interganglionic branches. Ganglia of sympathetic trunk give off the gray communicating branches, which direct to nearest spinal nerve and contain postganglionic fibers.

Superior cervical ganglion is a largest ganglion of sympathetic trunk, is located in the level of transversal processes of 2-3 cervical vertebrae. Superior cervical ganglion gives off the following branches:

-         gray communicating rami for I-IV cervical spinal nerves;

-         internal carotid nerve passes to carotid artery and forms internal carotid  plexus, which passes into cranial cavity. Sympathetic rootlet for pterygopalatine ganglion (radix sympathica - deep petrosal nerve) separates from internal carotid plexus. Deep petrosal nerve passes through the pterygopalatine canal get the pterygopalatine fossa, transitory passing  through the ganglion and realize innervation of the vessels and glands of mucous membrane of the nose cavity and mouth, conjuctive and face skin;

-         jugular nerve is passes on wall of internal jugular vein, where divides into branches passing to the 9th, 10th and 11th cranial nerves;

-         laryngо-pharyngeі nerves take hand in formation laryngо-pharyngeus plexus, innervating mucous membrane and vessels of the pharynx and larynx;

-         superior cervical cardiac nerve passes down parallelly with sympathetic trunk, to deep part of cardiac plexus.

Middle cervical ganglion, inconstant, located anteriorly from transversal process of 6th cervical vertebra. This ganglion is connected with superior and inferior ganglia by interganglionic rami. They form subclavian loop around subclavian artery. Middle cervical ganglion gives off such branches:

-         gray communicating branches to V-VI cervical spinal nerves;

-         common carotid nerves, which take hand in formation of external carotid plexus and plexus of inferior thyroid artery;

-         middle cervical cardiac nerve passes alongside of superior cervical cardiac nerve and enters into deep part of cardiac plexus.

Inferior cervical ganglion frequently flows together with first thoracic ganglion and forms a cervicothoracic ganglion (stellate ganglion). It lies on neck of first rib, behind subclavian artery. Ganglion gives off the following branches:

-         gray communicating branches to VI-VIII cervical spinal nerves;

-         subclavian nerves, which form subclavian plexus, that ramifies on branches of this artery;

-         branches to vagus and phrenic nerves;

-         vertebral nerve, which forms vertebral plexus round vertebral artery. This plexus spreads with branches of vertebral artery to spinal cord and brain meninges;

inferior cervical cardiac nerve is passes to deep part of cardiac plexus.

The cephalic portion of the sympathetic system begins as the internal carotid nerve, which appears to be a direct prolongation of the superior cervical ganglion. It is soft in texture, and of a reddish color. It ascends by the side of the internal carotid artery, and, entering the carotid canal in the temporal bone, divides into two branches, which lie one on the lateral and the other on the medial side of that vessel.

  The lateral branch, the larger of the two, distributes filaments to the internal carotid artery, and forms the internal carotid plexus.

  The medial branch also distributes filaments to the internal carotid artery, and, continuing onward, forms the cavernous plexus.

  The internal carotid plexus (plexus caroticus internus; carotid plexus) is situated on the lateral side of the internal carotid artery, and in the plexus there occasionally exists a small gangliform swelling, the carotid ganglion, on the under surface of the artery. The internal carotid plexus communicates with the semilunar ganglion, the abducent nerve, and the sphenopalatine ganglion; it distributes filaments to the wall of the carotid artery, and also communicates with the tympanic branch of the glossopharyngeal nerve.

  The communicating branches with the abducent nerve consist of one or two filaments which join that nerve as it lies upon the lateral side of the internal carotid artery. The communication with the sphenopalatine ganglion is effected by a branch, the deep petrosal, given off from the plexus on the lateral side of the artery; this branch passes through the cartilage filling up the foramen lacerum, and joins the greater superficial petrosal to form the nerve of the pterygoid canal (Vidian nerve), which passes through the pterygoid canal to the sphenopalatine ganglion. The communication with the tympanic branch of the glossopharyngeal nerve is effected by the caroticotympanic, which may consist of two or three delicate filaments.

  The cavernous plexus (plexus cavernosus) is situated below and medial to that part of the internal carotid artery which is placed by the side of the sella turcica in the cavernous sinus, and is formed chiefly by the medial division of the internal carotid nerve. It communicates with the oculomotor, the trochlear, the ophthalmic and the abducent nerves, and with the ciliary ganglion, and distributes filaments to the wall of the internal carotid artery. The branch of communication with the oculomotor nerve joins that nerve at its point of division; the branch to the trochlear nerve joins it as it lies on the lateral wall of the cavernous sinus; other filaments are connected with the under surface of the ophthalmic nerve; and a second filament joins the abducent nerve.

  The filaments of connection with the ciliary ganglion arise from the anterior part of the cavernous plexus and enter the orbit through the superior orbital fissure; they may join the nasociliary branch of the ophthalmic nerve, or be continued forward as a separate branch.

  The terminal filaments from the internal carotid and cavernous plexuses are prolonged as plexuses around the anterior and middle cerebral arteries and the ophthalmic artery; along the former vessels, they may be traced to the pia mater; along the latter, into the orbit, where they accompany each of the branches of the vessel. The filaments prolonged on to the anterior communicating artery connect the sympathetic nerves of the right and left sides.

The cervical portion of the sympathetic trunk consists of three ganglia, distinguished, according to their positions, as the superior, middle, and inferior ganglia, connected by intervening cords. This portion receives no white rami communicantes from the cervical spinal nerves; its spinal fibers are derived from the white rami of the upper thoracic nerves, and enter the corresponding thoracic ganglia of the sympathetic trunk, through which they ascend into the neck.

  The superior cervical ganglion (ganglion cervicale superius), the largest of the three, is placed opposite the second and third cervical vertebræ. It is of a reddishgray color, and usually fusiform in shape; sometimes broad and flattened, and occasionally constricted at intervals; it is believed to be formed by the coalescence of four ganglia, corresponding to the upper four cervical nerves. It is in relation, in front, with the sheath of the internal carotid artery and internal jugular vein; behind, with the Longus capitis muscle.

  Its branches may be divided into inferior, lateral, medial, and anterior.

  The Inferior Branch communicates with the middle cervical ganglion.

  The Lateral Branches (external branches) consist of gray rami communicantes to the upper four cervical nerves and to certain of the cranial nerves. Sometimes the branch to the fourth cervical nerve may come from the trunk connecting the upper and middle cervical ganglia. The branches to the cranial nerves consist of delicate filaments, which run to the ganglion nodosum of the vagus, and to the hypoglossal nerve. A filament, the jugular nerve, passes upward to the base of the skull, and divides to join the petrous ganglion of the glossopharyngeal, and the jugular ganglion of the vagus.

  The Medial Branches (internal branches) are peripheral, and are the larnygopharyngeal branches and the superior cardiac nerve.

  The laryngopharyngeal branches (rami laryngopharyngei) pass to the side of the pharynx, where they join with branches from the glossopharyngeal, vagus, and external laryngeal nerves to form the pharyngeal plexus.

  The superior cardiac nerve (n. cardiacus superior) arises by two or more branches from the superior cervical ganglion, and occasionally receives a filament from the trunk between the first and second cervical ganglia. It runs down the neck behind the common carotid artery, and in front of the Longus colli muscle; and crosses in front of the inferior thyroid artery, and recurrent nerve. The course of the nerves on the two sides then differ. The right nerve, at the root of the neck, passes either in front of or behind the subclavian artery, and along the innominate artery to the back of the arch of the aorta, where it joins the deep part of the cardiac plexus. It is connected with other branches of the sympathetic; about the middle of the neck it receives filaments from the external laryngeal nerve; lower down, one or two twigs from the vagus; and as it enters the thorax it is joined by a filament from the recurrent nerve. Filaments from the nerve communicate with the thyroid branches from the middle cervical ganglion. The left nerve, in the thorax, runs in front of the left common carotid artery and across the left side of the arch of the aorta, to the superficial part of the cardiac plexus.


Diagram of the cervical sympathetic.

 

  The Anterior Branches (nn. carotici externi) ramify upon the common carotid artery and upon the external carotid artery and its branches, forming around each a delicate plexus, on the nerves composing which small ganglia are occasionally found. The plexuses accompanying some of these arteries have important communications with other nerves. That surrounding the external maxillary artery communicates with the submaxillary ganglion by a filament; and that accompanying the middle meningeal artery sends an offset to the otic ganglion, and a second, the external petrosal nerve, to the genicular ganglion of the facial nerve.

  The middle cervical ganglion (ganglion cervicale medium) is the smallest of the three cervical ganglia, and is occasionally wanting. It is placed opposite the sixth cervical vertebra, usually in front of, or close to, the inferior thyroid artery. It is probably formed by the coalescence of two ganglia corresponding to the fifth and sixth cervical nerves.

  It sends gray rami communicantes to the fifth and sixth cervical nerves, and gives off the middle cardiac nerve.

  The Middle Cardiac Nerve (n. cardiacus medius; great cardiac nerve), the largest of the three cardiac nerves, arises from the middle cervical ganglion, or from the trunk between the middle and inferior ganglia. On the right side it descends behind the common carotid artery, and at the root of the neck runs either in front of or behind the subclavian artery; it then descends on the trachea, receives a few filaments from the recurrent nerve, and joins the right half of the deep part of the cardiac plexus. In the neck, it communicates with the superior cardiac and recurrent nerves. On the left side, the middle cardiac nerve enters the chest between the left carotid and subclavian arteries, and joins the left half of the deep part of the cardiac plexus.


Plan of right sympathetic cord and splanchnic nerves.

  

The inferior cervical ganglion (ganglion cervicale inferius) is situated between the base of the transverse process of the last cervical vertebra and the neck of the first rib, on the medial side of the costocervical artery. Its form is irregular; it is larger in size than the preceding, and is frequently fused with the first thoracic ganglion. It is probably formed by the coalescence of two ganglia which correspond to the seventh and eighth cervical nerves. It is connected to the middle cervical ganglion by two or more cords, one of which forms a loop around the subclavian artery and supplies offsets to it. This loop is named the ansa subclavia (Vieussenii).

  The ganglion sends gray rami communicantes to the seventh and eighth cervical nerves.

  It gives off the inferior cardiac nerve, and offsets to bloodvessels.

  The inferior cardiac nerve (n. cardiacus inferior) arises from either the inferior cervical or the first thoracic ganglion. It descends behind the subclavian artery and along the front of the trachea, to join the deep part of the cardiac plexus. It communicates freely behind the subclavian artery with the recurrent nerve and the middle cardiac nerve.

  The offsets to blood vessels form plexuses on the subclavian artery and its branches. The plexus on the vertebral artery is continued on to the basilar, posterior cerebral, and cerebellar arteries. The plexus on the inferior thyroid artery accompanies the artery to the thyroid gland, and communicates with the recurrent and external laryngeal nerves, with the superior cardiac nerve, and with the plexus on the common carotid artery.

Prepared by

Galytska-Harhalis O.Ya.