There are twelve pairs of cranial nerves; they
are attached to the brain and are transmitted through foramina in the base of
the cranium. The different pairs are named from before backward as follows:
1st. Olfactory, 2d. Optic, 3d.
Oculomotor, 4th.
Trochlear, 5th.
Trigeminal, 6th.
Abducent, 7th.
Facial with (13th
Intermediate), 8th. Vestibulocochlear, 9th.
Glossopharyngeal, 10th.
Vagus, 11th.
Accessory, 12th.
Hypoglossal
The area of attachment of a cranial nerve to the
surface of the brain is termed its superficial or apparent origin.
The fibers of the nerve can be traced into the substance of the brain to a
special nucleus of gray substance. The motor or efferent cranial nerves
arise within the brain from groups of nerve cells which constitute their nuclei
of origin.
The
sensory or afferent cranial nerves arise from groups of nerve cells outside the
brain; these nerve cells may be grouped to form ganglia on the trunks of the
nerves or may be situated in peripheral sensory organs such as the nose and
eye. The central processes of these cells run into the brain, and there end by
arborizing around nerve cells, which are grouped to form nuclei of
termination. The nuclei of origin of the motor nerves and the nuclei of
termination of the sensory nerves are brought into relationship with the
cerebral cortex, the former through the geniculate fibers of the internal
capsule, the latter through the lemniscus. The geniculate fibers arise from the
cells of the motor area of the cortex, and, after crossing the middle line, end
by arborizing around the cells of the nuclei of origin of the motor cranial
nerves. On the other hand, fibers arise from the cells of the nuclei of
termination of the sensory nerves, and after crossing to the opposite side,
join the lemniscus, and thus connect these nuclei, directly or indirectly, with
the cerebral cortex.
The Olfactory
Nerves
The olfactory
nerves or nerves of smell are distributed to the mucous membrane of
the olfactory region of the nasal cavity: this region comprises the superior
nasal concha, and the corresponding part of the nasal septum.
The nerves
originate from the central or deep processes of the olfactory cells of the
nasal mucous membrane. They form a plexiform net-work in the mucous membrane,
and are then collected into about twenty branches, which pierce the cribriform
plate of the ethmoid bone in two groups, a lateral and a medial
group, and end in the glomeruli of the olfactory bulb. Each branch receives
tubular sheaths from the dura mater and pia mater, the former being lost in the
periosteum of the nose, the latter in the neurolemma of the nerve.
The olfactory
nerves are non-medullated, and consist of axis-cylinders surrounded by nucleated
sheaths, in which, however, there are fewer nuclei than are found in the
sheaths of ordinary non-medullated nerve fibers.
The olfactory
center in the cortex is generally associated with the rhinencephalon.
The olfactary
nerves are developed from the cells of the ectoderm which lines the olfactory
pits; these cells undergo proliferation and give rise to what are termed the olfactory
cells of the nose. The axons of the olfactory cells grow into the overlying
olfactory bulb and form the olfactory nerves.
The
Optic Nerve (N. Opticus) The optic nerve, or nerve
of sight consists mainly of fibers derived
from the ganglionic cells of the retina. These axons terminate in arborizations
around the cells in the lateral geniculate body, pulvinar, and superior colliculus
which constitute the lower or primary visual centers. From the cells of the
lateral geniculate body and the pulvinar fibers pass to the cortical visual
center, situated in the cuneus and in the neighborhood of the calcarine
fissure. A few fibers of the optic nerve, of small caliber, pass from the
primary centers to the retina and are supposed to govern chemical changes in
the retina and also the movements of some of its elements (pigment cells and
cones). There are also a few fine fibers, afferent fibers, extending from the
retina to the brain, that are supposed to be concerned in pupillary reflexes.
The optic nerve is peculiar in
that its fibers and ganglion cells are probably third in the series of neurons
from the receptors to the brain. Consequently the optic nerve corresponds rather to a tract of fibers within the brain
than to the other cranial nerves. Its fibers pass backward and medialward
through the orbit and optic foramen to the optic commissure where they
partially decussate. The mixed fibers from the two nerves are continued in the
optic tracts, the primary visual centers of the brain.
The orbital portion of the
optic nerve is from
The Optic Chiasma (chiasma
opticum), somewhat quadrilateral in form, rests upon the tuberculum sellж and on the
anterior part of the diaphragma sellж. It is in relation, above, with the
lamina terminalis; behind, with the tuber cinereum; on either side,
with the anterior perforated substance. Within the chiasma, the optic nerves undergo
a partial decussation. The fibers forming the medial part of each tract and
posterior part of the chiasma have no connection with the optic nerves. They
simply cross in the chiasma, and connect the medial geniculate bodies of the
two sides; they form the commissure of Gudden. The remaining and
principal part of the chiasma consists of two sets of fibers, crossed and
uncrossed. The crossed fibers which are the more numerous, occupy the
central part of the chiasma, and pass from the optic nerve of one side to the
optic tract of the other, decussating in the chiasma with similar fibers of the
opposite optic nerve. The uncrossed fibers occupy the lateral part of
the chiasma, and pass from the nerve of one side into the tract of the same
side.
The crossed fibers of the
optic nerve tend to occupy the medial side of the nerve and the uncrossed
fibers the lateral side. In the optic tract, however, the fibers are much more
intermingled.
The Optic Tract passes
backward and outward from the optic chiasma over the tuber cinereum and
anterior perforated space to the cerebral peduncle and winds obliquely across
its under surface. Its fibers terminate in the lateral geniculate body, the
pulvinar and the superior colliculus. It is adherent to the tuber cinereum and
the cerebral peduncle as it passes over them. In the region of the lateral
geniculate body it splits into two bands. The medial and smaller one is a part
of the commissure of Gudden and ends in the medial geniculate body.
From its mode of development,
and from its structure, the optic nerve must be regarded as a prolongation of
the brain substance, rather than as an ordinary cerebrospinal nerve. As it
passes from the brain it receives sheaths from the three cerebral membranes, a
perineural sheath from the pia mater, an intermediate sheath from the
arachnoid, and an outer sheath from the dura mater, which is also connected
with the periosteum as it passes through the optic foramen. These sheaths are
separated from each other by cavities which communicate with the subdural and
subarachnoid cavities respectively. The innermost or perineural sheath sends a
process around the arteria centralis retinж into the interior of the nerve, and enters
intimately into its structure.
The
Oculomotor Nerve (N.
Oculomotorius)
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, sympathetic motor fibers to the Sphincter pupillж 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 palpebrж 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
Trochlear Nerve (N. Trochlearis; Fourth Nerve)
The trochlear nerve the
smallest of the cranial nerves, supplies the Obliquus superior oculi.
It arises from a
nucleus situated in the floor of the cerebral aqueduct, opposite the upper part
of the inferior colliculus. From its origin it runs downward through the
tegmentum, and then turns backward into the upper part of the anterior
medullary velum. Here it decussates with its fellow of the opposite side and
emerges from the surface of the velum at the side of the frenulum veli,
immediately behind the inferior colliculus.
The nerve is directed across
the superior cerebellar peduncle, and then winds forward around the cerebral
peduncle, immediately above the pons, pierces the dura mater in the free border
of the tentorium cerebelli, just behind, and lateral to, the posterior clinoid
process, and passes forward in the lateral wall of the cavernous sinus, between
the oculomotor nerve and the ophthalmic division of the trigeminal. It crosses
the oculomotor nerve, and enters the orbit through the superior orbital
fissure. It now becomes the highest of all the nerves, and lies medial to the
frontal nerve. In the orbit it passes medialward, above the origin of the
Levator palpebrж
superioris, and finally enters the orbital surface of the Obliquus superior.
In the lateral wall of the
cavernous sinus the trochlear nerve forms communications with the ophthalmic
division of the trigeminal and with the cavernous plexus of the sympathetic. In
the superior orbital fissure it occasionally gives off a branch to the lacrimal
nerve. It gives off a recurrent branch which passes backward between the layers
of the tentorium cerebelli and divides into two or three filaments which may be
traced as far as the wall of the transverse sinus.
Innervating of the Extrinsic ocular muscles
subdivide into recti (straight) and oblique:
The Trigeminal Nerve
(N. Trigeminus)
Trigeminal nerve, the 5th cranial
nerve, has motor nucleus and mesencephalic, pontine, spinal sensory
nuclei. They localised in pons (rhomboid fossa), mesencephalon and spinal cord.
Nerve starts from brain by sensory rootlet and motor rootlet
between pons and middle cerebellar pedunculi. Sensory rootlet represents by
central process of sensory cells, which lie in trigeminal ganglion
(Gasser`s) on top of pyramide of temporal bone. This ganglion is contained in
trigeminal cavity (Меckel`s) which is formed by dura mater. Motor rootlet
represents by axons from motor cells (motor nucleus).
Trigeminal nerve passes from skull by three
divisions:
1.
Ophthalmic nerve, sensory, exits from skull through the superior orbital
fissura;
2.
Maxillary nerve, sensory, exits from skull through the rotundum foramen;
3.
Mandibular nerve, mixed, exits from
skull through the ovale foramen.
5.1 Ophthalmic
nerve fossa subdivides in orbital into frontal nerve, lacrimal nerve and
nasociliary nerve. Nasociliary nerve passes on superomedial wall of
orbite and under block innervating medial eye corner and nose. Sensory long
ciliary nerves pass to the coats of the eyeball, short ciliary
nerves pass through the ciliary ganglion and carry parasympathetic and
sympathetic fibres for eyeball with all its
tissues and muscles (sphincter pupillae, dilator pupillae and ciliary
muscle). Posterior and anterior ethmoidal nerves pass through the
posterior and anterior ethmoidal foramen. They innervate mucous membrane of
ethmoidal cells (air sinus) and anterior part of nose cavity.
Frontal nerve passes
on middle part of upper wall of orbite and passes on forehead skin over supraorbital
and frontal notches by three branches, innervating forehead skin and top
eyelid. Lacrimal nerve passes on upper-lateral wall of orbite,
transfixes a lacrimal gland, innervating it. Passing out from gland, it
innervates the skin of lateral eye corner. Postganglionic and parasympathetic
fibres from pterygopalatine ganglion (which pass with zygomatic nerve) pass to
lacrimal nerve and provide a secretory innervation of the lacrimal gland.
5.2 Maxillary
nerve passes through the rotundum foramen and reaches the pterygopalatine
fossa, where ramifies on three branches: infraorbital nerve, zygomatic nerve
and ganglionic branches to pterygopalatine ganglion.
Infraorbital
nerve passes through the inferior orbital fissura enters into orbite, where
lies on its lower wall, passes in infraorbital fissura and infraorbital sulcus
and canal. Nerve passes into canine fossa, forming ‘pes anserinus minor’.
There are inferior palpebral, external nasal and superior labial
nerves that innervate skin from medial eye corner to mouth corner. Superior
alveolar nerves (posterior, middle and inferior) start from infraorbital
nerve in maxilla. They innervate mucous membrane of the maxillary (Haymori)
sinus and form superior dental plexus. The last gives off the superior
dental nerves and superior gingival branches of the upper jaw.
Zygomatic nerve passes through the inferior
orbital fissura entering orbite. Then it passes into zygomaticoorbital foramen
and divided into zygomaticofacial and zygomaticotemporal sensory
branches for skin of face and temporal region. Zygomatic nerve carries
postganglionic parasympathetic fibres from pterygopalatine ganglion and gives
off them to lacrimal nerve. Parasympathetic fibres provide secretory
innervation of the lacrimal gland.
The ganglionic branches start from maxillary
nerve and pass to pterygopalatine ganglion. Postganglionic branches include greater
palatine nerve and lesser palatine nerve that pass through the
greater palatine canal and lesser palatine foramens, innervating mucous
membrane of the hard and soft palatine. The posterior nasal (medial and
lateral) nerves pass through the sphenopalatine foramen pass into nasal cavity,
where innervate mucous membrane of the nasal cavity. Nasopalatine nerve
(Scarp’) start from the nasal branches and reach the mucous membrane of the
hard palatine through the incisive canal. Postganglionic parasympathetic fibres
from pterygopalatine ganglion are in composition of these nerves.
5.3 Mandibular nerve
carries both the motor and sensory fibres. After passing out from ovale foramen
mandibular nerve gives off the motor branches that innervate all 4 masticatory
muscles, also velli palatine tensor muscle and nerve tensor of
tympanic membrane muscle. Sensory branches of the mandibular nerve: buccal
nerve, auriculotemporal nerve, lingual nerve infeior alveolar nerve and
meningeal branches.
Buccal nerve transfixes a buccinator muscle and
innervates mucous membrane of the cheek and also mouth corner skin.
Auriculotemporal nerve begins by two rootlets
that envelop a middle meningeal artery, and then unite into one trunk, which
transfixes parotid gland, innervating it and skin of temporal area, also, the
auricle. Postganglionic parasympathetic fibres from otic ganglion pass
in composition of this nerve, which provide a secretory innervation of parotid
gland.
Lingual nerve passes on internal surface of lower
jaw under mucous membrane of the mouth cavity and enters into lower part of
tongue, providing a general sensory innervation of the anterior 2/3 part and
sensory innervation sublingual and submandibular salivary glands. Chorda
tympani (from 7th cranial nerve), which contains the gustatory (tasting) and
secretory (parasympathetic) fibres. Gustatory fibres innervate of tasting buds
on mucous membrane of the anterior 2/3 part of the tongue, and secretory
(parasympathetic) enter in to submandibular and sublingual
parasympatheticих ganglia. The postganglionic fibres from these ganglia
provide a secretory innervation the same name - submandibular and sublingual
salivary glands.
Inferior alveolar nerve (mixed) has a motor
branches that supply mylo-hyoid muscle and anterior belly of the digastric
muscle. The sensory fibres enter into mandibular channel, where form
inferior dental plexus, branches innervate the teeth and gums of lower
jaw. From canal these fibres are passing out from bone as a mental nerve,
which terminates in skin of lower lip and chin.
Trigeminal Nerve Reflexes.—Pains
referred to various branches of the trigeminal nerve are of very frequent
occurrence, and should always lead to a careful examination in order to
discover a local cause. As a general rule the diffusion of pain over the
various branches of the nerve is at first confined to one only of the main
divisions, and the search for the causative lesion should always commence with
a thorough examination of all those parts which are supplied by that division;
although in severe cases pain may radiate over the branches of the other main
divisions. The commonest example of this condition is the neuralgia which is so
often associated with dental caries—here, although the tooth itself may not
appear to be painful, the most distressing referred pains may be experienced,
and these are at once relieved by treatment directed to the affected tooth.
Many other examples of trigeminal reflexes could
be quoted, but it will be sufficient to mention the more common ones. Dealing
with the ophthalmic nerve, severe supraorbital pain is commonly associated with
acute glaucoma or with disease of the frontal or ethmoidal air cells. Malignant
growths or empyema of the maxillary antrum, or unhealthy conditions about the
inferior conchж
or the septum of the nose, are often found giving rise to “second division”
neuralgia, and should be always looked for in the absence of dental disease in
the maxilla.
It is on the mandibular nerve, however, that some
of the most striking reflexes are seen. It is quite common to meet with
patients who complain of pain in the ear, in whom there is no sign of aural
disease, and the cause is usually to be found in a carious tooth in the
mandible. Moreover, with an ulcer or cancer of the tongue, often the first pain
to be experienced is one which radiates to the ear and temporal fossa, over the
distribution of the auriculotemporal nerve.
The Abducent Nerve (N. Abducens;
Sixth Nerve)
The abducent nerve supplies the Rectus lateralis
oculi. Its fibers arise from a small nucleus situated in the upper part of
the rhomboid fossa, close to the middle line and beneath the colliculus
facialis. They pass downward and forward through the pons, and emerge in the
furrow between the lower border of the pons and the upper end of the pyramid of
the medulla oblongata. From the nucleus of the sixth nerve, fibers are
said to pass through the medial longitudinal fasciculus to the oculomotor nerve
of the opposite side, along which they are carried to the Rectus medialis. The
Rectus lateralis of one eye and the Rectus medialis of the other may therefore
be said to receive their nerves from the same nucleus.
The nerve pierces the dura mater on the dorsum
sellae of the sphenoid, runs through a notch in the bone below the posterior
clinoid process, and passes forward through the cavernous sinus, on the lateral
side of the internal carotid artery. It enters the orbit through the superior
orbital fissure, above the ophthalmic vein, from which it is separated by a
lamina of dura mater. It then passes between the two heads of the Rectus
lateralis, and enters the ocular surface of that muscle. The abducent nerve is
joined by several filaments from the carotid and cavernous plexuses, and by one
from the ophthalmic nerve. The oculomotor, trochlear, ophthalmic, and abducent
nerves bear certain relations to each other in the cavernous sinus, at the
superior orbital fissure, and in the cavity of the orbit, as follows: In
the cavernous sinus, the oculomotor, trochlear, and ophthalmic nerves are
placed in the lateral wall of the sinus, in the order given, from above
downward. The abducent nerve lies at the lateral side of the internal carotid
artery. As these nerves pass forward to the superior orbital fissure, the
oculomotor and ophthalmic divide into branches, and the abducent nerve
approaches the others; so that their relative positions are considerably
changed.
In the superior orbital fissure, the trochlear
nerve and the frontal and lacrimal divisions of the ophthalmic lie in this
order from the medial to the lateral side upon the same plane; they enter the
cavity of the orbit above the muscles. The remaining nerves enter the orbit
between the two heads of the Rectus lateralis. The superior division of the
oculomotor is the highest of these; beneath this lies the nasociliary branch of
the ophthalmic; then the inferior division of the oculomotor; and the abducent
lowest of all.
In the orbit, the trochlear, frontal, and
lacrimal nerves lie immediately beneath the periosteum, the trochlear nerve
resting on the Obliquus superior, the frontal on the Levator palpebrae
superioris, and the lacrimal on the Rectus lateralis. The superior division of
the oculomotor nerve lies immediately beneath the Rectus superior, while the
nasociliary nerve crosses the optic nerve to reach the medial wall of the
orbit. Beneath these is the optic nerve, surrounded in front by the ciliary
nerves, and having the ciliary ganglion on its lateral side, between it and the
Rectus lateralis. Below the optic nerve are the inferior division of the
oculomotor, and the abducent, the latter lying on the medial surface of the
Rectus lateralis.
VII Facial nerve consists
of 1- proper facial nerve, which has fibers starting from motor
nucleus in depth of facial colliculus in rhomboid fossa, and 2- intermediate
nerve, which has a fibers of sensory nucleus of solitarius tract and
parasympathetic - superior salivatory nucleus.
Facial nerve passes through facial canal, gives
off motor branch for stapedius muscle, then exits from skull through the
stylomastoid foramen and innervates posterior auricular, stylohyoid
muscles and posterior belly of digastric muscle. Facial nerve enters
into thickness of parotid salivary glands (but does not innervate it!) and
forms there parotid plexus. Last gives off the motor branches which innervate facial
expression (mimetic) muscles. There are: temporal
branches (supply auricular, orbicularis oculi and occipitofrontalis muscles),
zygomatic branches (they supply zygomatic major and orbicularis oculi muscles),
buccal branches (supply zygomatic major and minor, levator labii superioris,
buccinator, orbicularis oris, nasalis,
and risorius muscles), marginal mandibulae branch (supplies depressor
anguli oris and depressor labii inferioris, mentalis muscles) and cervical
branch (for the platisma).
Intermediate nerve
has two divisions: Nervus petrosus major and Chorda tympani.
Greater petrosal nerve
(preganglionic secretory fibers to the lacrimal glands, nasal glands and
palatal glands) separates off from the geniculate ganglion, extends through the
hiatus of the canal for the greater petrosal nerve, across the foramen lacerum
and finally through the pterygoid canal to the pterygopalatine ganglion.
Chorda tympani
carries preganglionic secretory fibers to the submandibular and sublingual
glands and taste fibers to the anterior two third of the tongue. Taste
(sensory) fibers contain peripheral axons of cells localized in geniculate
ganglion. The central process of neurocytes of this ganglion terminate in
nucleus of solitarius tract. Chorda tympani passes through the petrotympanic
fissura and joints the lingual nerve (from the Vth cranial
nerve), then its branches reach the tongue and parasympathetic
submandibular and sublingual ganglia for innervating the same names salivary
glands.
The Vestibulocochlear
Nerve (Eighth Nerve)
The vestibulocochlear nerve consists of two
distinct sets of fibers which differ in their peripheral endings, central
connections, functions, and time of medullation. It is soft in texture and
devoid of neurilemma. Cochlear Nerve, the nerve of hearing, arises from bipolar
cells in the spiral ganglion of the cochlea, situated near the inner edge of
the osseous spiral lamina. The peripheral fibers pass to the organ of Corti.
The central ones pass down the modiolus and then through the foramina of the
tractus spiralis foraminosus or through the foramen centrale into the lateral
or outer end of the internal auditory meatus. The nerve passes along the
internal auditory meatus with the vestibular nerve and across the subarachnoid
space, just above the flocculus, almost directly medialward toward the inferior
peduncle to terminate in the cochlear nucleus. The cochlear nerve is
placed lateral to the vestibular root. Its fibers end in two nuclei: one, the
accessory nucleus, lies immediately in front of the inferior peduncle; the
other, the tuberculum acusticum, somewhat lateral to it. The striae
medullares (striae acusticae) are the axons of the cells of the tuberculum
acusticum. They pass over the inferior peduncle, and across the rhomboid fossa
to the median sulcus. Here they dip into the substance of the pons, to end
around the cells of the superior olivary nuclei of both sides. There are,
however, other fibers, and these are both direct and crossed, which pass into
the lateral lemniscus. The cells of the accessory nucleus give origin to fibers
which run transversely in the pons and constitute the trapezium. Of the
trapezoid fibers some end around the cells of the superior olivary nucleus or
of the trapezoid nucleus of the same or opposite side, while others, crossed or
uncrossed, pass directly into the lateral lemniscus. If the further connections
of the cochlear nerve of one side, say the left, be considered, it is found
that they lie lateral to the main sensory tract, the lemniscus, and are
therefore termed the lateral lemniscus. The fibers comprising the left lateral
lemniscus arise in the superior olivary and trapezoid nuclei of the same or
opposite side, while others are the uninterrupted fibers already alluded to,
and these are either crossed or uncrossed, the former being the axons of the
cells of the right accessory nucleus or of the cells of the right tuberculum
acusticum, while the latter are derived from the cells of the left nuclei. In
the upper part of the lateral lemniscus there is a collection of nerve cells,
the nucleus of the lateral lemniscus, around the cells of which some of the
fibers arborize and from the cells of which axons originate to continue upward
the tract of the lateral lemniscus. The ultimate ending of the left lateral
lemniscus is partly in the opposite medial geniculate body, and partly in the
inferior colliculi. From the cells of these bodies new fibers arise and ascend
in the occipital part of the internal capsule to reach the posterior
three-fifths of the left superior temporal gyrus and the transverse temporal
gyri. Vestibular Nerve, the nerve of equilibration, arises from bipolar
cells in the vestibular ganglion, ganglion of Scarpa, which is situated in the
upper part of the outer end of the internal auditory meatus. The peripheral
fibers divide into three branches: the superior branch passes through the
foramina in the area vestibularis superior and ends in the utricle and in the
ampullae of the superior and lateral semicircular ducts; the fibers of the
inferior branch traverse the foramina in the area vestibularis inferior and end
in the saccule; the posterior branch runs through the foramen singulare and
supplies the ampulla of the posterior semicircular duct.
IX Glossopharyngeal nerve
is mixed nerve, which has a nucleus ambiguous (motor), nucleus of the tractus
solitarius (sensory) and inferior salivatory nucleus (parasympathetic) in
rhomboid fossa. Sensory fibers start from a superior and inferior ganglia
in region of the jugular foramen. Glossopharyngeal nerve reaches the tongue
root, where divides by final branches rr. linguales, that
innervating mucous membrane the posterior third of the tongue.
Glossopharyngeal nerve gives off the following
branches:
1.
Tympanic nerve
(mixed) passes through tympanic canalicule and enters into tympanic cavity. Here
its sensory branches together with caroticotympanic nerves (sympathetic) form tympanic
plexus which innervate mucous membrane of the tympanic cavity and auditory
tube (r. tubarius). The parasympathetic preganglionic fibers start from
inferior salivatory nucleus, exite from tympanic cavity through hyatus nervi
petrosi minoris as lesser petrosal nerve. Last passes in sulcus and
enters into otic ganglion. Postganglionic fibres provide a secretory
innervation of parotid salivary glands.
2.
Pharyngeal branches
pass to lateral wall of the pharynx, where together with branches of vagus
nerve and sympathetic trunk form pharyngeal plexus. Last provides
innervating of the muscles and mucous membrane of the throat.
3.
Tonsillar branches
pass to mucous membrane of the palatine tonsils and palatal arches.
4.
Branch of stylopharyngeal muscle.
5.
Branch of carotid sinus, innervating
the receptors of carotid glomus and sinus.
6.
Communicating branch with the
auricular branch of vagus nerve.
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.
*
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).
The Accessory Nerve (N.
Accessorius; Eleventh Nerve)
The accessory nerve consists of two parts: a
cranial and a spinal. 1) The Cranial Part (ramus internus; accessory
portion) is the smaller of the two. Its fibers arise from the cells of the
nucleus ambiguus and emerge as four or five delicate rootlets from the side of
the medulla oblongata, below the roots of the vagus. It runs lateralward to the
jugular foramen, where it interchanges fibers with the spinal portion or
becomes united to it for a short distance; here it is also connected by one or
two filaments with the jugular ganglion of the vagus. It then passes through the
jugular foramen, separates from the spinal portion and is continued over the
surface of the ganglion nodosum of the vagus, to the surface of which it is
adherent, and is distributed principally to the pharyngeal and superior
laryngeal branches of the vagus. Through the pharyngeal branch it probably
supplies the Musculus uvulae and Levator veli palatini. Some few filaments from
it are continued into the trunk of the vagus below the ganglion, to be
distributed with the recurrent nerve and probably also with the cardiac nerves.
2) The Spinal Part (ramus externus; spinal portion) is firm in
texture, and its fibers arise from the motor cells in the lateral part of the
anterior column of the gray substance of the medulla spinalis as low as the
fifth cervical nerve. Passing through the lateral funiculus of the medulla
spinalis, they emerge on its surface and unite to form a single trunk, which
ascends between the ligamentum denticulatum and the posterior roots of the
spinal nerves; enters the skull through the foramen magnum, and is then
directed to the jugular foramen, through which it passes, lying in the same
sheath of dura mater as the vagus, but separated from it by a fold of the
arachnoid. In the jugular foramen, it receives one or two filaments from the
cranial part of the nerve, or else joins it for a short distance and then
separates from it again. As its exit from the jugular foramen, it runs backward
in front of the internal jugular vein in 66.6 per cent. of cases, and behind in
it 33.3 per cent. (Tandler). The nerve then descends obliquely behind the
Digastricus and Stylohyoideus to the upper part of the Sternocleidomastoideus;
it pierces this muscle, and courses obliquely across the posterior triangle of
the neck, to end in the deep surface of the Trapezius. As it traverses the
Sternocleidomastoideus it gives several filaments to the muscle, and joins with
branches from the second cervical nerve. In the posterior triangle it unites
with the second and third cervical nerves, while beneath the Trapezius it forms
a plexus with the third and fourth cervical nerves, and from this plexus fibers
are distributed to the muscle.
The Hypoglossal Nerve (N.
Hypoglossus; Twelfth Nerve)
The hypoglossal nerve is the motor nerve of the
tongue. Its fibers arise from the cells of the hypoglossal nucleus, which
is an upward prolongation of the base of the anterior column of gray substance
of the medulla spinalis. This nucleus is about
The descending ramus (ramus descendens; descendens
hypoglossi), long and slender, quits the hypoglossal where it turns around the
occipital artery and descends in front of or in the sheath of the carotid
vessels; it gives a branch to the superior belly of the Omohyoideus, and then
joins the communicantes cervicales from the second and third cervical nerves;
just below the middle of the neck, to form a loop, the ansa cervicalis.
From the convexity of this loop branches pass to supply the Sternohyoideus, the
Sternothyreoideus, and the inferior belly of the Omohyoideus. According to