Lesson No 21
Diencephalon comprises the thalamencephalon and hypothalamus. Thalamencephalon consists of thalamus opticus, epithalamus and metathalamus. Hypothalamus formed by front optic part and back (olfactory) part.
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
Coronal section of Thalamus, lateral and third ventricles
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.
Hind- and mid-brains; postero-lateral view.
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.
situated in middle place on midsagittal dissection of the brain
Thalamus situated in middle place on midsagittal dissection of the brain
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.
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 hypophysis cerebri in position. Shown in sagittal section.
Median sagittal through the hypophysis of an adult monkey. Semidiagrammatic.
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)
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
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.
Coronal section of brain immediately in front of pons.
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
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.
Hind- and mid-brains; postero-lateral view.
The Metathalamus 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
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 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.
Structures of the hypothalamus
25-26 Structures of the hypothalamus
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
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) 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.
Scheme showing central connections of the optic nerves and optic tracts
The roof 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
The telencephalon includes the cerebral hemispheres with their cavities, the lateral ventricles.
Each cerebral hemisphere may be divided into three fundamental parts: the pallium, the rhinencephalon, and basal nuclei, also fornix and corpus callosum.
Cortex of the Cerebrum
A hemisphere is covered by cortex and has inferior, dorsolateral and medial surfaces. 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. The right and left hemispheres are separated medially by a deep cleft, named the longitudinal cerebral fissure. The surfaces of the hemispheres are molded into a number of irregular eminencies, named gyri or convolutions, and separated by furrows termed fissures and sulci. The hemispheres consist of 5 lobes: frontal, perietal, occipital, temporal and insula (located in depth of lateral sulcus).
On the dorsolateral surface of the hemisphere can be finding the Central sulcus [Rolandi] that separates frontal and parietal lobes. The Lateral sulcus [Sylvii] that separates temporal lobe from the frontal and parietal lobes. Parietooccipital sulcus passes between parietal and occipital lobes on the medial surface.
There are some sulci on the dorsolateral surface of the frontal lobe: precentral sulcus, superior frontal sulcus, and inferior frontal sulcus. They separate: precentral gyrus, superior frontal gyrus, middle frontal gyrus, and inferior frontal gyrus. The inferior frontal gyrus is divided into opercular, triangular and orbital parts by anterior and ascending rami.
The lateral surface of the parietal lobe is cleft by a well-marked furrow, the intraparietal sulcus and the postcentral sulcus. There are postcentral gyrus, superior parietal lobule and inferior parietal lobule. The last contains the supramarginal and angular gyri.
The temporal lobe is divided into superior, middle, and inferior gyri by the superior and middle temporal sulci. Three or four gyri will be seen springing from the depth of the hinder end of the lateral sulcus, these are named the transverse temporal gyri (Heschl).
The occipital lobe is small and pyramidal in shape. It is traversed by the transverse occipital sulci that border occipital gyri.
On the insula (it is located in depth of lateral sulcus) they distinguish circular and central sulci which border longi and breve gyri.
There are some sulci on the medial surface of the hemisphere: sulcus corporis callosi, cingulate sulcus, hippocampal sulcus, parietooccipital and calcarine sulcus. They separte corporus callosum from gyrus cinguli, superior frontal gyri, gyrus parahippocampalis, dentate gyrus, paracentral lobule (of Bets), precuneus and cuneus. Gyrus cinguli and gyrus parahippocampalis with isthmus form fornicate gyrus that it is the central part of the rhinencephalon.
On the inferior surface of the hemisphere can be finding: collateral sulcus, occipitotemporal sulcus, rhinal sulcus and orbital sulcus. They separate the lateral occipitotemporal gyrus, medial occipitotemporal gyrus, lingual gyrus, gyrus rectus and orbital gyri.
We distinguish some specific cortical fields that control motor, sensory, language and others functions. They can be divided into more general motor area, sensory area and also some specific sensory centres.
Motor area involved with the control of voluntary muscles. It is located in precentral gyrus and paracentral lobule (motor homunculus).
Centre of conjugate deviaton of the eyes to the opposite side is located in posterior part of the middle frontal gyrus.
Sensory area responsible for cutaneous and muscular sensations (temperature, pain and tactile), located in postcentral gyrus and paracentral lobule (sensory homunculus).
Centre of stereognosia is located in superior parietal lobule closely to intraparietal sulcus. It responsible for the body schema: the right can be differentiated from the left.
Auditory centre located in the transverse temporal gyri (Heschl). Function is interpretation of auditory sensations.
Visual cortical centre situated in calcarine sulcus. Function of the occipital lobe is connscious perception of vision.
Centre of praxia is located supramarginal gyrus.
Smell and tasting centre located in part of limbic system – uncus (#
1. Motor speech area (Broca's centre). Damage in the region of the lower frontal convolution produces motor aphasia.
2. Writing (graphic) area located in middle frontal gyrus.
3. Auditory language centre (Wernicke`s area) located in superior temporal gyrus. It responsible for understanding of spoken language.
4. Reading centre situated in angular gyrus (responsible also for reading, writing, counting and calculating).
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 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
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 gyri and their intervening fissures and the sulci are fairly constant in their arrangement; at the same time they vary within certain limits, not only in different individuals, but on the two hemispheres of the same brain. The convoluted condition of the surface permits of a great increase of the gray matter without the sacrifice of much additional space. The number and extent of the gyri, as well as the depth of the intervening furrows, appear to bear a direct relation to the intellectual powers of the individual.
Certain of the fissures and sulci are utilized for the purpose of dividing the hemisphere into lobes, and are therefore termed interlobular; included under this category are the lateral cerebral, parietoöccipital, calcarine, and collateral fissures, the central and cingulate sulci, and the sulcus circularis.
Lateral surface of left cerebral hemisphere, viewed from the side
The Lateral Cerebral Fissure (fissura
cerebri lateralis [Sylvii]; fissure of Sylvius) is a
well-marked cleft on the inferior and lateral surfaces of the hemisphere, and
consists of a short stem which divides into three rami. The stem is
situated on the base of the brain, and commences in a depression at the lateral
angle of the anterior perforated substance. From this point it extends between
the anterior part of the temporal lobe and the orbital surface of the frontal
lobe, and reaches the lateral surface of the hemisphere. Here it divides into
three rami: an anterior horizontal, an anterior ascending, and a posterior. The
anterior horizontal ramus passes foward for about
The Central Sulcus (sulcus
centralis [Rolandi]; fissure of Rolando; central fissure)is
situated about the middle of the lateral surface of the hemisphere, and begins
in or near the longitudinal cerebral fissure, a little behind its mid-point. It
runs sinuously downward and forward, and ends a little above the posterior
ramus of the lateral fissure, and about
The Parietoöccipital Fissure (fissura parietoöccipitalis).—Only a small part of this fissure is seen on the lateral surface of the hemisphere, its chief part being on the medial surface.
The lateral part of the
parietoöccipital fissure is situated about
The Sulcus Circularis (circuminsular fissure) is on the lower and lateral surfaces of the hemisphere: it surrounds the insula and separates it from the frontal, parietal, and temporal lobes.
Lobes of the Hemispheres.—By means of these fissures and sulci, assisted by certain arbitrary lines, each hemisphere is divided into the following lobes: the frontal, the parietal, the temporal, the occipital, the limbic, and the insula.
Frontal Lobe (lobus frontalis).—On the lateral surface of the hemisphere this lobe extends from the frontal pole to the central sulcus, the latter separating it from the parietal lobe. Below, it is limited by the posterior ramus of the lateral fissure, which intervenes between it and the central lobe. On the medial surface, it is separated from the cingulate gyrus by the cingulate sulcus; and on the inferior surface, it is bounded behind by the stem of the lateral fissure.
Principal fissures and lobes of the cerebrum viewed laterally
The lateral surface of the frontal lobe is tranversed by three sulci which divide it into four gyri: the sulci are named the precentral, and the superior and inferior frontal; the gyri are the anterior central, and the superior, middle, and inferior frontal. The precentral sulcus runs parallel to the central sulcus, and is usually divided into an upper and a lower part; between it and the central sulcus is the anterior central gyrus. From the precentral sulcus, the superior and inferior frontal sulci run forward and downward, and divide the remainder of the lateral surface of the lobe into three parallel gyri, named, respectively the superior, middle, and inferior frontal gyri.
The anterior central gyrus (gyrus centralis anterior; ascending frontal convolution; precentral gyre) is bounded in front by the precentral sulcus, behind by the central sulcus; it extends from the supero-medial border of the hemisphere to the posterior ramus of the lateral fissure.
The superior frontal gyrus (gyrus frontalis superior; superfrontal gyre) is situated above the superior frontal sulcus and is continued on to the medial surface of the hemisphere. The portion on the lateral surface of the hemisphere is usually more or less completely subdivided into an upper and a lower part by an antero-posterior sulcus, the paramedial sulcus, which, however, is frequently interrupted by bridging gyri.
surface of the forebrain cortex
Supero-Lateral surface of the forebrain cortex
The inferior frontal gyrus (gyrus frontalis inferior; subfrontal gyre) lies below the inferior frontal sulcus, and extends forward from the lower part of the precentral sulcus; it is continuous with the lateral and posterior orbital gyri on the under surface of the lobe. It is subdivided by the anterior horizontal and ascending rami of the lateral fissure into three parts, viz., (1) the orbital part, below the anterior horizontal ramus of the fissure; (2) the triangular part (cap of Broca), between the ascending and horizontal rami; and (3) the basilar part, behind the anterior ascending ramus. The left inferior frontal gyrus is, as a rule, more highly developed than the right, and is named the gyrus of Broca, from the fact that Broca described it as the center for articulate speech.
The inferior or orbital surface of the frontal lobe is concave, and rests on the orbital plate of the frontal bone. It is divided into four orbital gyri by a well-marked H-shaped orbital sulcus. These are named, from their position, the medial, anterior, lateral, and posterior orbital gyri. The medial orbital gyrus presents a well-marked antero-posterior sulcus, the olfactory sulcus, for the olfactory tract; the portion medial to this is named the straight gyrus, and is continuous with the superior frontal gyrus on the medial surface.
Temporal Lobe (lobus temporalis).—The temporal lobe presents superior, lateral, and inferior surfaces.
The superior surface forms the lower limit of the lateral fissure and overlaps the insula. On opening out the lateral fissure, three or four gyri will be seen springing from the depth of the hinder end of the fissure, and running obliquely forward and outward on the posterior part of the upper surface of the superior temporal gyrus; these are named the transverse temporal gyri (Heschl).
The lateral surface is bounded above by the posterior ramus of the lateral fissure, and by the imaginary line continued backward from it; below, it is limited by the infero-lateral border of the hemisphere. It is divided into superior, middle, and inferior gyri by the superior and middle temporal sulci. The superior temporal sulcus runs from before backward across the temporal lobe, some little distance below, but parallel with, the posterior ramus of the lateral fissure; and hence it is often termed the parallel sulcus. The middle temporal sulcus takes the same direction as the superior, but is situated at a lower level, and is usually subdivided into two or more parts. The superior temporal gyrus lies between the posterior ramus of the lateral fissure and the superior temporal sulcus, and is continuous behind with the supramarginal and angular gyri. The middle temporal gyrus is placed between the superior and middle temporal sulci, and is joined posteriorly with the angular gyrus. The inferior temporal gyrus is placed below the middle temporal sulcus, and is connected behind with the inferior occipital gyrus; it also extends around the infero-lateral border on to the inferior surface of the temporal lobe, where it is limited by the inferior sulcus.
Section of brain showing upper surface of temporal lobe.
The inferior surface is concave, and is continuous posteriorly with the tentorial surface of the occipital lobe. It is traversed by the inferior temporal sulcus, which extends from near the occipital pole behind, to within a short distance of the temporal pole in front, but is frequently subdivided by bridging gyri. Lateral to this fissure is the narrow tentorial part of the inferior temporal gyrus, and medial to it the fusiform gyrus, which extends from the occipital to the temporal pole; this gyrus is limited medially by the collateral fissure, which separates it from the lingual gyrus behind and from the hippocampal gyrus in front.
The Insula (
The insula of the left side, exposed by removing the opercula.
The term limbic lobe was introduced by Broca, and under it he included the cingulate and hippocampal gyri, which together arch around the corpus callosum and the hippocampal fissure. These he separated on the morphological ground that they are well-developed in animals possessing a keen sense of smell (osmatic animals), such as the dog and fox. They were thus regarded as a part of the rhinencephalon, but it is now recognized that they belong to the neopallium; the cingulate gyrus is therefore sometimes described as a part of the frontal lobe, and the hippocampal as a part of the temporal lobe.
On the medial surface of the brain students should find:
· Cerebral hemispheres communicate each other by corpus callosum. Corpus callosum anteriorly carries a genu that passes into rostrum. Last continue as a lamina rostralis and lamina terminalis. Back part of the corpus callosum called splemium
Lateral surface of left cerebral hemisphere, viewed from above.
· Above the corpus callosum there is fornix cerebri posteriorly passes into crura fornicis, anteriorly continue as a columna fornicis
· Lamina septi pellucidi is tightened between corpus callosum and columna fornicis
Thalamus positioned under fornix
· Backward from the thalamus there is quadrigeminal plate with two upper and two lower hillocks, the superior and inferior colliculi
Medial surface of left cerebral hemisphere.
The medial part of the parietoöccipital fissure runs downward and forward as a deep cleft on the medial surface of the hemisphere, and joins the calcarine fissure below and behind the posterior end of the corpus callosum. In most cases it contains a submerged gyrus.
The Calcarine Fissure (fissura calcarina) is on the medial surface of the hemisphere. It begins near the occipital pole in two converging rami, and runs forward to a point a little below the splenium of the corpus callosum, where it is joined at an acute angle by the medial part of the parietoöccipital fissure. The anterior part of this fissure gives rise to the prominence of the calcar avis in the posterior cornu of the lateral ventricle.
The Cingulate Sulcus (sulcus cinguli; callosomarginal fissure) is on the medial surface of the hemisphere; it begins below the anterior end of the corpus callosum and runs upward and forward nearly parallel to the rostrum of this body and, curving in front of the genu, is continued backward above the corpus callosum, and finally ascends to the supero-medial border of the hemisphere a short distance behind the upper end of the central sulcus. It separates the superior frontal from the cingulate gyrus.
The Collateral Fissure (fissura collateralis) is on the tentorial surface of the hemisphere and extends from near the occipital pole to within a short distance of the temporal pole. Behind, it lies below and lateral to the calcarine fissure, from which it is separated by the lingual gyrus; in front, it is situated between the hippocampal gyrus and the anterior part of the fusiform gyrus.
· Cerebral aqueduct [Silvii] passes under mesencephalic tectum. It communicates front with the 3d ventricle, back with the 4th ventricle
· The 4th ventricle is bordered upper and lower by the superior medullary velum and inferior medullary velum
· The floor of the IVth ventricle, the rhomboid fossa, occupies the dorsal surface of the medulla oblongata and the pons
· Cerebellum consists of two hemispheres which connected by vermis
Cerebral hemispheres are separated by median longitudinal fissura that passes to the corpus callosum. Cerebrum is separated from the cerebellum by transverse fissura.
The medial surface of the frontal lobe is occupied by the medial part of the superior frontal gyrus (marginal gyrus) (727). It lies between the cingulate sulcus and the supero-medial margin of the hemisphere. The posterior part of this gyrus is sometimes marked off by a vertical sulcus, and is distinguished as the paracentral lobule, because it is continuous with the anterior and posterior central gyri.
Parietal Lobe (lobus parietalis).—The parietal lobe is separated from the frontal lobe by the central sulcus, but its boundaries below and behind are not so definite. Posteriorly, it is limited by the parietoöccipital fissure, and by a line carried across the hemisphere from the end of this fissure toward the preoccipital notch. Below, it is separated from the temporal lobe by the posterior ramus of the lateral fissure, and by a line carried backward from it to meet the line passing downward to the preoccipital notch.
Orbital surface of left frontal lobe.
The lateral surface of the parietal lobe (726) is cleft by a well-marked furrow, the intraparietal sulcus of Turner, which consists of an oblique and a horizontal portion. The oblique part is named the postcentral sulcus, and commences below, about midway between the lower end of the central sulcus and the upturned end of the lateral fissure. It runs upward and backward, parallel to the central sulcus, and is sometimes divided into an upper and a lower ramus. It forms the hinder limit of the posterior central gyrus.
From about the middle of the postcentral sulcus, or from the upper end of its inferior ramus, the horizontal portion of the intraparietal sulcus is carried backward and slightly upward on the parietal lobe, and is prolonged, under the name of the occipital ramus, on to the occipital lobe, where it divides into two parts, which form nearly a right angle with the main stem and constitute the transverse occipital sulcus. The part of the parietal lobe above the horizontal portion of the intraparietal sulcus is named the superior parietal lobule; the part below, the inferior parietal lobule.
The posterior central gyrus (gyrus centralis posterior; ascending parietal convolution; postcentral gyre) extends from the longitudinal fissure above to the posterior ramus of the lateral fissure below. It lies parallel with the anterior central gyrus, with which it is connected below, and also, sometimes, above, the central sulcus.
The superior parietal lobule (lobulus parietalis superior) is bounded in front by the upper part of the postcentral sulcus, but is usually connected with the posterior central gyrus above the end of the sulcus; behind it is the lateral part of the parietoöccipital fissure, around the end of which it is joined to the occipital lobe by a curved gyrus, the arcus parietoöccipitalis; below, it is separated from the inferior parietal lobule by the horizontal portion of the intraparietal sulcus.
The inferior parietal lobule (lobulus parietalis inferior; subparietal district or lobule) lies below the horizontal portion of the intraparietal sulcus, and behind the lower part of the postcentral sulcus. It is divided from before backward into two gyri. One, the supramarginal, arches over the upturned end of the lateral fissure; it is continuous in front with the postcentral gyrus, and behind with the superior temporal gyrus. The second, the angular, arches over the posterior end of the superior temporal sulcus, behind which it is continuous with the middle temporal gyrus.
The medial surface of the parietal lobe is bounded behind by the medial part of the parietoöccipital fissure; in front, by the posterior end of the cingulate sulcus; and below, it is separated from the cingulate gyrus by the subparietal sulcus. It is of small size, and consists of a square-shaped convolution, which is termed the precuneus or quadrate lobe.
Occipital Lobe (lobus occipitalis).—The occipital lobe is small and pyramidal in shape; it presents three surfaces: lateral, medial, and tentorial.
The lateral surface is limited in front by the lateral part of the parietoöccipital fissure, and by a line carried from the end of this fissure to the preoccipital notch; it is traversed by the transverse occipital and the lateral occipital sulci. The transverse occipital sulcus is continuous with the posterior end of the occipital ramus of the intraparietal sulcus, and runs across the upper part of the lobe, a short distance behind the parietoöccipital fissure. The lateral occipital sulcus extends from behind forward, and divides the lateral surface of the occipital lobe into a superior and an inferior gyrus, which are continuous in front with the parietal and temporal lobes.
The medial surface of the occipital lobe is bounded in front by the medial part of the parietoöccipital fissure, and is traversed by the calcarine fissure, which subdivides it into the cuneus and the lingual gyrus. The cuneus is a wedge-shaped area between the calcarine fissure and the medial part of the parietoöccipital fissure. The lingual gyrus lies between the calcarine fissure and the posterior part of the collateral fissure; behind, it reaches the occipital pole; in front, it is continued on to the tentorial surface of the temporal lobe, and joins the hippocampal gyrus.
The tentorial surface of the occipital lobe is limited in front by an imaginary transverse line through the preoccipital notch, and consists of the posterior part of the fusiform gyrus (occipitotemporal convolution) and the lower part of the lingual gyrus, which are separated from each other by the posterior segment of the collateral fissure.
The cingulate gyrus (gyrus cinguli; callosal convolution) is an arch-shaped convolution, lying in close relation to the superficial surface of the corpus callosum, from which it is separated by a slit-like fissure, the callosal fissure. It commences below the rostrum of the corpus callosum, curves around in front of the genu, extends along the upper surface of the body, and finally turns downward behind the splenium, where it is connected by a narrow isthmus with the hippocampal gyrus. It is separated from the medial part of the superior frontal gyrus by the cingulate sulcus, and from the precuneus by the subparietal sulcus.
The hippocampal gyrus (gyrus hippocampi) is bounded above by the hippocampal fissure, and below by the anterior part of the collateral fissure. Behind, it is continuous superiorly, through the isthmus, with the cingulate gyrus and inferiorly with the lingual gyrus. Running in the substance of the cingulate and hippocampal gyri, and connecting them together, is a tract of arched fibers, named the cingulum (page 843). The anterior extremity of the hippocampal gyrus is recurved in the form of a hook (uncus), which is separated from the apex of the temporal lobe by a slight fissure, the incisura temporalis. Although superficially continuous with the hippocampal gyrus, the uncus forms morphologically a part of the rhinencephalon.
Scheme of rhinencephalon.
The Hippocampal sulcus begins immediately behind the splenium of the corpus callosum, and runs forward between the hippocampal and dentate gyri to end in the uncus. It is a complete fissure (page 819), and gives rise to the prominence of the hippocampus in the inferior cornu of the lateral ventricle.