NERVOUS SYSTEM.

I. THE CENTRAL NERVOUS SYSTEM.

1. The Spinal Cord. Medulla spinalis

(Figs. 133-136).—The spinal cord is that portion of the nervous system which occupies the vertebral canal; it is continuous craniad with the brain. It has the form of a somewhat flattened cylinder and extends from the foramen magnum into the caudal region. It diminishes in diameter after entering the sacral region.

The cord has a cervical and a lumbar enlargement, the former (Fig. 133) marking the origin of the nerves which pass to the fore limb, and the latter (Fig. 136) the origin of those which pass to the hind limb.

The cervical enlargement (Fig. 133) lies in that part of the vertebral canal bounded by the fourth to seventh cervical and first thoracic vertebræ. Caudad of the first thoracic vertebra the cord continues of nearly uniform diameter to the lumbar enlargement (Fig. 136) which stretches from the third to the seventh lumbar vertebræ (inclusive). Caudad of the seventh lumbar vertebra it diminishes uniformly in diameter and ends in a slender cord, the filum terminale (Fig. 136, C), which may be traced into the caudal region.

The surface of the cord is marked by a number of longitudinal grooves or sulci, and fissures. The most prominent of these is the anterior median fissure (Fig. 134, c), along the ventral median line; into this the pia mater dips. The posterior median sulcus (Fig. 133, e; Fig. 134, a) is a shallow furrow along the dorsal median line. The anterior fissure and the posterior sulcus thus divide the cord into lateral halves. Each half is subdivided by the anterior and posterior lateral sulci.

The posterior lateral sulcus (Fig. 133, f; Fig. 134, b) lies at the side of the posterior median sulcus. It is broad and shallow and has the posterior roots of the spinal nerves emerging from its bottom.

The anterior lateral sulcus exists only after the forcible pulling out of the anterior (ventral) roots of the spinal nerves. It then marks the line along which they originate from the cord.

In cross-section (Fig. 134) the substance of the cord appears to the naked eye as composed of a darker central “gray matter” (g) and of an outer “white matter” (f). The gray matter has in section the form of the letter H. The cord is really tubular, having a central canal (h) which appears in section in the middle of the cross-bar of the H, while the two ends of each vertical bar extend toward the anterior and posterior lateral fissures.

The white matter is divided into funiculi (or columns) by the fissures and sulci, so that there is in each half of the cord an anterior, a posterior, and a lateral white funiculus.

In the cervical region (Fig. 134) there appears between the posterior lateral and posterior median sulci an intermediate sulcus which divides the posterior funiculus in this region into two. The median slenderer of these is the fasciculus gracilis (d) or column of Goll. The lateral and thicker is the fasciculus cuneatus (e) or column of Burdach.

The membranes of the spinal cord:

The dura mater is a tough fibrous membrane directly continuous with the dura mater of the brain. It differs from the dura mater of the brain in two particulars:

1. At the foramen magnum it splits into two layers, one of which lines the bony vertebral canal and forms its periosteum, while the other covers the cord.

2. It is separated from the cord by a considerable space. The dura mater is continuous with the fibrous sheaths of the spinal nerves at their points of exit. Along the sides of the cord it is connected to the pia mater by a delicate strand of connective tissue probably equivalent to the “ligamentum denticulatum” of man.

The arachnoid is a delicate cellular membrane lying beneath the dura mater, between it and the pia mater. It forms a continuous investment for the cord, is not vascular, and is said not to dip into the fissures of the cord.

The pia mater invests the cord closely and contains some blood-vessels. It is a delicate membrane which dips into the fissures and sulci of the cord and is connected to it by numerous strands of connective tissue that pass from it into the substance of the cord. The nerves pierce it.

Spinal Nerves.—From the spinal cord arise the spinal nerves. Of these there are about thirty-eight pairs in the cat. Eight are cervical, thirteen thoracic, seven lumbar, three sacral, and seven or eight caudal. Those leaving the cervical (Fig. 133, 5-8 and I) and lumbar (Fig. 136) enlargements are larger than the others. The first cervical nerve leaves the vertebral canal through the atlantal foramen, the second leaves between the arches of the atlas and axis, while all the others leave the vertebral canal by way of the intervertebral foramina. Each nerve arises from the cord by a dorsal and a ventral root. The dorsal root is chiefly sensory, the ventral motor in character. The dorsal root (radix posterior) (Figs. 133, 135, and 136, a) begins as a number (twelve or more) of separate nerve-bundles which emerge from the posterior lateral groove. These roots lie nearly in a single plane and pass laterad, converging to penetrate a ganglion, the spinal ganglion (b) (or ganglion of the posterior root). All the spinal ganglia except the first and second are situated in the intervertebral foramina or within the vertebral canal. The first and second are situated among the muscles surrounding the place of exit of the nerves.

The ventral root (radix anterior) (Fig. 135, e) arises as a larger number of small fibre-bundles which do not lie in a single plane, so that in a transverse section several rootlets may appear in a single section. The rootlets converge to form a single mass which joins the dorsal root just as it emerges from the spinal ganglion. The nerve formed by the junction of the ventral and dorsal roots is one of the spinal nerves.

The direction in which the nerves leave the cord varies. In the cervical region (Fig. 133) and cranial part of the lumbar region it is nearly laterad; at the cervical and lumbar enlargements it is laterocaudad. The nerves in the sacral and caudal regions pass almost directly caudad to reach the intervertebral foramina and form thus a brush which surrounds the filum terminale (Fig. 136, C) and is called the cauda equina (Fig. 136, B).

Each spinal nerve immediately after leaving the intervertebral foramen divides into two branches, a dorsal or posterior branch or ramus (c), and a ventral or anterior ramus (d). The dorsal ramus is in each case small (except in the first and second cervical nerves), and is distributed to the longitudinal muscles and integument of the back.

The ventral rami are larger and each is connected a short distance beyond its origin with the sympathetic system by a short ramus communicans or communicating branch. Each is then distributed to the integument and muscles of the ventral part of the body, including the limbs.

The ventral rami which pass to the limbs are much larger than the others. The ventral rami are further distinguished from the dorsal by the fact that they frequently unite with one another to form plexuses.

The peripheral distribution of the spinal nerves is described later.

2. The Brain. Encephalon.

—The brain is that portion of the central nervous system that is included within the cranial cavity. It is a direct continuation of the spinal cord, and presents many of the same essential characters as the latter, with great modifications in details.

The structure of the brain can best be understood if it be considered as a modified continuation of the spinal cord, and the relation of the parts to the essential parts of the cord noted. The spinal cord is a nearly straight tube, with a central cavity and thick walls. The brain is likewise tubular, with the cavities enlarged or subdivided in places, with the walls greatly thickened, and with a number of bends and constrictions in the tube. The relation of the structure of the brain to that of the spinal cord is most easily perceived by an examination of the brain of some lower vertebrate, as the frog or shark, and such an examination should be made before proceeding to the study of the more complicated brain of the cat.

In the following account of the brain of the cat all parts will be described as far as possible in relation with the tubular structure of the brain. The brain will be considered as a hollow structure, having central cavities, and the solid portions will be brought into relation as parts of the roof, sides, or floor of the cavities. The cavities of the brain, forming a direct continuation of the central canal of the spinal cord, are known as ventricles (ventriculi).

In a general view of the cat’s brain from the dorsal side (Fig. 137) four subdivisions are discernible. At the caudal end is a small stalk-like portion (IV) which is clearly a slightly modified continuation of the spinal cord; this is the myelencephalon or medulla oblongata. Just craniad of this, rising high above it, is an irregularly lobulated rounded portion, the cerebellum (III). This is produced as a thickening of the wall of the original tube; it forms the dorsal part of the metencephalon. These two portions of the brain have a common cavity, lying within the medulla oblongata and ventrad of the cerebellum, and formed by a widening of the cavity of the spinal cord; it is known as the fourth ventricle. Craniad of the cerebellum, separated from it by a deep transverse fissure, are two very large portions, the hemispheres of the cerebrum (II), separated from each other by a deep longitudinal fissure; they constitute together the telencephalon. The hemispheres are produced by a sort of forking of the original central tube;—by outgrowths on the sides of the tube, the central cavity extending into the outgrowths. The two outgrowths extend dorsad, as well as craniad and caudad from the original place of origin, forming the larger part of the brain; the extensions of the central cavity which they contain are known as the lateral ventricles. Finally, in front of the two hemispheres are seen the two small olfactory bulbs (I) which are mere extensions of the two hemispheres and contains cavities which are extensions of the lateral ventricles.

In a ventral view of the entire brain (Fig. 138) certain parts are visible which in the dorsal view are hidden by the large cerebral hemispheres. At the caudal end is seen, as before, the medulla oblongata, or myelencephalon, and craniad of this, on the ventral side of the cerebellum, a broad transverse tract, the pons (i); this, like the cerebellum, forms part of the metencephalon. All this part of the brain formed by the myelencephalon and metencephalon lies caudad of (behind) the rest of the brain and is marked off from it by a great fissure; it therefore receives as a whole the name hindbrain or rhombencephalon.

Just craniad of the pons are seen two short diverging arm-like bands of fibres (g), enclosing a small triangular space between them. These two arms with the space between them are all that is visible of a division of the brain which in dorsal view is completely covered by the backward projection of the cerebral hemispheres. This is the midbrain or mesencephalon. In this portion of the brain the continuation of the central cavity is a narrow canal which receives the name cerebral aqueduct (aqueductus cerebri) or aqueduct of Sylvius.

Just craniad of the midbrain is a small rather irregular area, bounded craniad by the band-like optic chiasma (c), from which arise the optic nerves (II), and showing caudad of this band a rounded irregular projection (e). This is a fourth division of the brain which is likewise covered dorsad by the hemispheres; it is the ’tween-brain or diencephalon. Its cavity is known as the third ventricle. Just craniad of the ’tween-brain, in the median line, is a deep fissure, showing that here in reality is the termination of the original tube, the further extension craniad being due to the pushing forward of the lateral outgrowths, or cerebral hemispheres, which extend in ventral view a considerable distance further craniad. The hemispheres lie also caudad, laterad, and dorsad of the midbrain and ’tween-brain, so that these parts are almost enveloped by the hemispheres. The hemispheres and ’tween-brain are sometimes taken together as forming a single portion, the forebrain or prosencephalon.

There are thus altogether five principal subdivisions of the brain, each enclosing a portion of the central cavity. These are the myelencephalon, the metencephalon (these two enclosing the fourth ventricle), the mesencephalon (enclosing the cerebral aqueduct), the diencephalon (enclosing the third ventricle), and the two cerebral hemispheres, constituting together the telencephalon, and enclosing the two lateral ventricles. A plan of the brain, considered as a tube enclosing cavities, is given in Figs. 139 and 140. Fig. 139 shows the plan considered as viewed from the dorsal side, Fig. 140 in a lateral view. These figures are of course pure diagrams, showing only what may be called the plan of structure, and omitting all details. In the actual conditions many important modifications even of the main features of the plan are met with.

In the following description the parts of the brain will be taken up in this order:

• A. Rhombencephalon (primitive hindbrain).
• 1. Myelencephalon.
• 2. Metencephalon.
• B. Mesencephalon (primitive midbrain).
• 3. Mesencephalon.
• C. Prosencephalon (primitive forebrain).
• 4. Diencephalon.
• 5. Telencephalon.

A. Rhombencephalon.—

1. Myelencephalon.

—The myelencephalon or medulla oblongata is transitional between the spinal cord and the brain. It has in general the structural peculiarities of the cord, but these undergo in it a gradual transformation into the arrangements characteristic of the brain.

It has the form of a flattened and truncated cone, which widens craniad and is limited at the cranial end by the pons (Fig. 138, i) ventrally and laterally; by the cerebellum (Fig. 137, III) dorsally. The cranial portion of its dorsal surface is overhung by the cerebellum. The points of origin of the roots of the first pair of cervical nerves (Fig. 138, s) may be taken as indicating the boundary between the spinal cord and the medulla oblongata; there is no other external marking to show the limits of the two.

The central canal of that part of the medulla which is overhung by the cerebellum is greatly widened (Fig. 141, h). The cavity thus formed is flattened and triangular, with the apex of the triangle caudad, and is the caudal part of the fourth ventricle (Fig. 141, h). The roof is very thin (Fig. 143, n) and is intimately connected with the pia mater, so that in removing the latter the thin roof of the fourth ventricle is removed with it and the cavity of the fourth ventricle is left exposed. The thin roof of the fourth ventricle is known as the velum medullare posterius (Fig. 143, n).

The ventral (anterior) fissure (Fig. 138, p) of the cord passes onto the medulla oblongata and ends at the pons (i), its end being slightly deeper and forming what is known as the foramen cæcum (q). The dorsal (posterior) sulcus (Fig. 141, a) is well marked; it ends at the caudal boundary of the fourth ventricle (h).

The anterior lateral sulcus (Fig. 138, r) may be traced craniad from the origin of the ventral roots of the first cervical nerves (s), along the lateral border of the area elliptica (n), then curving mediad to the lateral border of the pyramis (o), and finally reaching the pons (i). Its position is marked by the origin of the roots of the hypoglossal nerve (XII).

The posterior lateral sulcus (Fig. 141, b), marked on the spinal cord by the origin of the dorsal nerve-roots, curves laterad at the sides of the fourth ventricle (h) owing to the increasing width of the latter, and ends at an elevated area of oblique fibres, the area ovalis (f).

The columns or funiculi bounded by the longitudinal fissures present the following peculiarities:

The anterior white funiculus of the cord is replaced in the medulla by the pyramidal tracts (pyramides) (Fig. 138, o). The pyramidal tracts are formed by fibres which emerge from beneath the pons and pass caudad to disappear just craniad of the level of the first cervical nerve (s). The pyramidal tracts are bounded medially by the anterior median fissure (p), but laterally each is separated from the anterior lateral sulcus (r) over its caudal part by an elongated area elliptica (n), the human homologue of which is uncertain. It perhaps represents the oliva.

Laterad of the cranial portion of the pyramids is an irregular area known as the trapezium (l) which abuts caudad on the area elliptica (n) and the area ovalis (m). The area ovalis (Fig. 138, m; Fig. 141, f) (or zonula Arnoldi) is abroad band of oblique fibres which passes from the lateral side of the area elliptica craniodorsad to disappear under the cerebellum.

The posterior white funiculus was divided in the cervical region into two, the fasciculus gracilis (column of Goll) and the fasciculus cuneatus (column of Burdach). The fasciculus gracilis (Fig. 141, c) extends to the caudal end of the fourth ventricle (h) and ends there in an expansion, the clava (d), which forms the posterior boundary of the fourth ventricle. The fasciculus cuneatus (e) passes laterad on account of the width of the fourth ventricle and appears to end at the area ovalis (f), but it may be seen passing beneath the area ovalis, emerging at its cranial border and turning dorsad to enter the cerebellum. It forms the side walls of a part of the shallow fourth ventricle.

The lateral funiculi are divided by longitudinal furrows into three divisions. The dorsal one of these is the fasciculus cuneatus lateralis or column of Rolando (Fig. 141, g). It accompanies the fasciculus cuneatus into the cerebellum.

The entire mass formed by the fasciculus gracilis, the fasciculus cuneatus medialis, and fasciculus cuneatus lateralis is known as the corpus restiforme, and since its fibres pass into the cerebellum it is sometimes known as the pedunculus cerebelli (or crus cerebelli ad medullam).

The following cranial nerves arise from the medulla oblongata (Fig. 138).

The twelfth nerve (XII) (N. hypoglossus) arises by ten or fifteen rootlets from the ventral surface of the medulla oblongata in the anterior lateral fissure (r) opposite the caudal portion of the area elliptica (n).

The eleventh cranial nerve (XI) (N. accessorius) arises by numerous rootlets from the lateral surface of the medulla oblongata and of the spinal cord as far caudad as the sixth or seventh cervical nerve. These rootlets join to form a nerve which enters the cranium through the foramen magnum and is closely associated at its point of exit with the glossopharyngeal (IX) and vagus (X) nerves. The line of origin on the medulla passes between the dorsal and ventral roots of the cervical nerves and is along the middle of the lateral white funiculi.

The tenth cranial nerve (X) (N. vagus) arises by about eighteen very delicate rootlets from the surface of the area ovalis. The rootlets are divided into a dorsal and a ventral series. The dorsal series (about twelve) arise in the groove which separates the fasciculus cuneatus medialis from the fasciculus cuneatus lateralis. The ventral series arise somewhat ventrad of this groove. These rootlets are to be distinguished from those of the ninth nerve by their smaller size.

The ninth cranial nerve (IX) (N. glossopharyngeus) arises from the area ovalis from a line craniad of the dorsal line of origin of the vagus roots (X) and between these and those of the auditory (VIII). It arises by a number of rootlets which are larger than those of the vagus (X), with which this nerve is closely associated.

The eighth cranial nerve (VIII) (N. acusticus) appears at the lateral end of the trapezium (l). It arises from an elevation (Fig. 141, i) which is continued dorsomediad along the cranial border of the area ovalis.

The seventh cranial nerve (VII) (N. facialis) leaves the lateral border of the trapezium (l) near its cranial edge, between the fifth and eighth nerves. It is much smaller than the eighth nerve.

The sixth cranial nerve (VI) (N. abducens) arises by about six bundles from the groove between the pyramids and the trapezii and passes craniad.

2. Metencephalon.

—The metencephalon includes the pons and the cerebellum.

The pons (Fig. 138, i) is a mass of transverse fibres which forms the ventral and cranial part of the primitive hindbrain. It is a modification of the latter brought about by the development of the cerebellum, and the degree of its development is in direct ratio to that of the cerebellar and cerebral hemispheres. The pons forms a projecting mass of fibres which is marked by a median longitudinal groove, the sulcus basilaris (j), which indicates the course of the basilar artery (Fig. 121, c). Laterad the fibres of the pons converge somewhat and turning dorsad disappear in the cerebellum, forming the brachia pontis (Fig. 141, l).

The fifth cranial nerve (Fig. 138, V) (N. trigeminus) arises by two roots from the caudal border of the pons, near the lateral end. The ventral root (4) is small; the dorsal one is much larger and soon forms the large semilunar ganglion (k) from which three branches (1, 2, and 3) diverge. The ventral root (4) joins one of these branches (1).

The cerebellum (Fig. 137, III) is formed by an increase in size of the cranial portion of the primitive hindbrain. This increase has affected principally the surface of the roof, so that as the cerebellum has grown it has been thrown into many folds, the exact form of which varies in different specimens. The cerebellum has at the same time increased in size and has thus extended laterad as well as caudad and craniad. It thus touches the cerebrum in front (separated from it by the tentorium) and aids it in concealing the midbrain and ’tween-brain in dorsal view, while caudad in the same view it conceals the greater part of the medulla. The connections of the cerebellum with adjacent parts of the brain are also overhung and concealed.

The whole surface of the cerebellum is thrown up into numerous folds or gyri, separated from one another by deep fissures or sulci, which appear at first to render the surface wholly irregular. The entire mass is, however, divisible into a central portion, which from its resemblance to a segmented worm is called the vermis (j) (its cranial part is the superior vermis, and its caudal part the inferior vermis), and into lateral portions, the hemispheres (k). The vermis (j) occupies a median longitudinal position, and its gyri and sulci are in the main transverse. It is not directly connected with adjacent parts, and its ventral surface extends farther caudad and craniad than that of the hemispheres. The ventral part of the superior vermis is fitted against the posterior corpora quadrigemina.

The hemispheres (k) may again be subdivided into groups of gyri which have received special names. One of these, the so-called appendicular lobe, fits into the appendicular fossa of the petrous bone.

The cerebellum is connected to the adjacent parts by three tracts of fibres, sometimes known as crura cerebelli. The tract connecting it with the medulla oblongata is the corpus restiforme; that connecting it with the pons is the brachium pontis (Fig. 141, l); these have been described. A third tract passes craniad to the corpora quadrigemina (Fig. 141, p and q); this is the brachium conjunctivum (Fig. 141, k).

The cerebellum is composed of white and gray matter, the latter on the surface (Fig. 143, III). The folds of its surface present thus a contrivance for increasing the amount of gray matter. The white matter forms a central mass from which tracts extend into the folds. The whole mass of white matter has thus in section (more particularly in a longitudinal section of the vermis) the appearance of a tree, whence the name arbor vitæ (Fig. 143).

The fourth ventricle (Fig. 141, h; Fig. 143, m) is the cavity of the original hindbrain. It begins caudad at the clava (Fig. 141, d) as a widening and continuation of the central canal of the spinal cord and extends craniad, becoming wider and passing ventrad of the cerebellum. It becomes narrower craniad and ends at the posterior corpora quadrigemina (p), where it becomes continuous with the aqueductus cerebri (Fig. 141, o; Fig. 143, j). The cavity is shallow and is encroached upon dorsally by the vermis of the cerebellum (Fig. 143).

The floor of the cavity is known as the fossa rhomboidea (Fig. 141, h). It is formed by the continuation of the gray matter which surrounds the central canal of the cord. It is marked by a median longitudinal groove. At its widest part are seen two considerable tracts of white fibres (striæ medullares) which pass from near the median line laterad and extend into the auditory (eighth) nerves. The floor caudad and craniad of these striæ is marked by a number of elevations and depressions. Similarly situated elevations in the human brain differ from one another slightly in color, are made up of gray matter, and are the centres of origin of most of the cranial nerves. Their homologues in the cat appear not to have been determined.

The side walls of the fourth ventricle (Fig. 141) are formed by the following in order, beginning caudad: the clava (d), the corpus restiforme, brachium pontis (l), brachium conjunctivum (k), and caudal corpora quadrigemina (p) (colliculi inferiores).

The roof (Fig. 143) is formed caudad by a thin layer of non-nervous matter which is closely associated with the pia mater. This thin layer is known as the velum medullare posterius (n). It connects the dorsal surface of the medulla with the caudal border of the cerebellum. The pia mater covering this portion of the roof is vascular and is folded in toward the floor of the ventricle, forming the choroid plexus of the fourth ventricle. In the middle the roof of the fourth ventricle is the cerebellum, while craniad the roof is the velum medullare anterius (Fig. 143, l). This is a thin layer just craniad of the cerebellum, connecting it with the corpora quadrigemina, and attached laterally to the brachia conjunctiva. Here the fourth ventricle narrows craniad and becomes continuous with the slender aqueductus cerebri (Fig. 143, j) (aqueduct of Sylvius). The narrowed portion of the brain is frequently known as the isthmus rhombencephali.

The fourth cranial nerve (Fig. 141, n; Fig. 138, IV) (N. trochlearis) arises from the brain at the craniolateral angle of the velum medullare anterius.

B. Mesencephalon.—

3. Mesencephalon.

—The mesencephalon or midbrain includes the corpora quadrigemina (Fig. 141) and the pedunculi cerebri (Fig. 142). In the primary midbrain there is a pronounced thickening of the walls accompanied by a reduction of the central canal. The midbrain does not thus become very large and is concealed in the dorsal view by the cerebellum and cerebrum, though its floor appears in the ventral view just craniad of the pons (Fig. 138, g). Its narrow canal is the aqueductus cerebri (aqueduct of Sylvius) (Fig. 143, j). Its roof forms the corpora quadrigemina, and its floor the pedunculi cerebri.

In a dorsal view (Fig. 141) the roof is seen to be marked by two pairs of elevations, the corpora quadrigemina (p and q). The cranial pair (q) (known as the colliculi superiores) are circular in outline, surrounded on all sides except the cranial one by a deep groove. From the cranial side a tract of fibres (brachium quadrigeminum superius, or arm of the cranial corpus) extends craniad and disappears beneath the thalamus (t). Between the anterior or cranial corpora quadrigemina lies the pineal body or epiphysis (corpus pineale) (Fig. 143, y; Fig. 141, 2), a portion of the roof of the ’tween-brain. The caudal corpora (Fig. 141, p) are larger than the cranial ones, and ovoid in shape with the long axis vertical. They are united in the median line, and the velum medullare anterius (m) stretches between their caudal borders. The brachium of the caudal corpus quadrigeminum (brachium quadrigeminum inferius) (r) extends craniad and disappears beneath a considerable elevation, the corpus geniculatum mediale (s).

Crossing this brachium is seen a small tract of fibres which extends ventrad, crosses the pedunculus cerebri, and reaches the medial border of the latter. It is the tractus transversus peduncularis (Fig. 142, b). Ventrad of the caudal corpus quadrigeminum is seen a triangular area of oblique fibres which corresponds in position to the human lemniscus.

The pedunculi cerebri (peduncles of the cerebrum) form the ventral part of the midbrain. They appear in a ventral view of the entire brain (Fig. 138) as two broad tracts of fibres (g) emerging from beneath the pons and diverging from one another as they pass craniad, finally disappearing beneath the cerebral hemispheres. Each is made up of many fibre-bundles, which are apparent in surface view (Fig. 142). The peduncles are separated by a small triangular space, which is marked by a median longitudinal sulcus. In this space, just caudad of the mammillary bodies, is a small area through which a number of blood-vessels pass into the brain. This is known as the posterior perforated area (or substance) (Fig. 142, j). The cerebral peduncles (a) are crossed by the tractus transversus peduncularis (b) (see above).

The third cranial nerve (Fig. 138, III) (N. oculomotorius) leaves the brain at the medial border of the cerebral peduncle (g), just caudad of the tractus transversus peduncularis.

The aqueductus cerebri (Fig. 143, j; Fig. 153, d) (or aqueduct of Sylvius) is the continuation craniad of the fourth ventricle. It is a narrow passage, one or two millimeters in diameter, lying dorsad of the pedunculi cerebri and ventrad of the corpora quadrigemina.