C. Prosencephalon.—The prosencephalon or primitive forebrain includes the diencephalon or ’tween-brain and the telencephalon or cerebral hemispheres.

4. Diencephalon.

—The diencephalon or ’tween-brain includes the thalami and the other parts bounding the third ventricle. The diencephalon is seen in entire brains only in ventral view (Fig. 138, c, d, e, etc.)

The diencephalon may be considered as forming almost or quite the most cranial portion of the median nervous tube,—the cranial wall of the third ventricle (the lamina terminalis) (Fig. 143, d), ending in the median line in the deep fissure between the hemispheres of the cerebrum. Parts of the brain which extend farther craniad than this are lateral portions, due to the forward growth of the lateral hemispheres. The two hemispheres of the cerebrum may be considered as lateral outgrowths of the central ’tween-brain; these outgrowths have extended dorsad, laterad, craniad, and caudad, so as to cover almost completely the ’tween-brain.

In early stages the cerebral hemispheres are projections from the cranial end of the ’tween-brain, so that the plane of junction was nearly transverse, the cranial end of the ’tween-brain joining the caudal end of the hemispheres. With the increasing size and backward growth of the latter, the attachment to the ’tween-brain has been shifted from a cranial to nearly a lateral position, and at the same time the originally lateral surface of the ’tween-brain has become nearly caudal. This is shown in Fig. 141, the deep fissure at 1 marking the line of attachment between the ’tween-brain and the hemispheres. The dorsoventral plane of junction of ’tween-brain and hemispheres is (as Fig. 141 shows) not wholly lateral, but oblique, passing from its cranial end near the middle line caudolaterad.

A second peculiarity of the ’tween-brain lies in the thinness of its roof. The roof is exceedingly thin and is so intimately connected with the pia mater that they cannot be removed separately. The ventral thick floor of the ’tween-brain is directly continuous with the similar floor of the cerebrum; but where the roof of the ’tween-brain joins the roof of the cerebrum along the oblique plane already indicated, the roof is very thin and is intimately connected to the pia mater, and is at the same time folded into the lateral ventricles to form, together with the pia mater, the choroid plexuses of the lateral ventricles (Fig. 148, e). When the pia mater is removed the thin roof of the brain along the line of junction of the ’tween-brain and the cerebrum is brought away and there appears to be a direct communication between lateral ventricles and the exterior.

We may now take up the parts of the diencephalon in detail.

a. Thalamus (Fig. 141, t).—The two thalami are large oblique rounded ridges, forming the sides of the ’tween-brain. They lie just craniad of the cranial corpora quadrigemina (q), but separated from them by a broad groove, and are completely covered by the caudally projecting part of the cerebral hemispheres. The medial larger end of each thalamus is near the middle line; thence the thalamus extends caudolaterad, and rises at its lateral extremity into a sharply rounded projection, the corpus geniculatum laterale (x). From this projection the thalamus is continued ventrad and then craniomediad as a large band of fibres, the optic tract (tractus opticus) (Fig. 142, d), which extends to the optic chiasma (e), where it passes into the optic nerves (f). On the caudal border of the thalamus, near the median line, is a very faint projection, the pulvinar (Fig. 141, w); this lies just craniad of the lateral border of the cranial corpus quadrigeminum (q). Just ventrad of the corpus geniculatum laterale (x) is the prominent rounded corpus geniculatum mediale (s); this is connected by a prominent ridge, the brachium quadrigeminum inferius (r), with the caudal corpus quadrigeminum (p). In a similar manner the brachium quadrigeminum superius passes from the cranial corpus quadrigeminum (q) into the thalamus itself.

Between the two thalami there exists a groove, the sulcus hypothalamicus. Over this groove lies the roof of the third ventricle, forming the choroid plexus of the third ventricle (Fig. 143, w). The medial surface of the two thalami are flat and extend directly ventrad, forming part of the lateral boundary of the third ventricle (Fig. 143, h). The two medial surfaces meet over a considerable area across the narrow cavity of the third ventricle, and unite, forming the massa intermedia (Fig. 143, f) or intermediate mass of the thalamus (“middle commissure”). This connection of the thalami of the two sides is thus not a primitive one, forming no part of the roof or floor of the central cavity, but is a secondary connection due to a growing together of a part of the two sides of the ventricle across its cavity. Along the dorsal edge of the medial border of each thalamus passes a distinct white strand, the stria medullaris (Fig. 141, u); the two striæ meet in an arch caudad, lying beneath the pineal body (2).

The thalamus is separated craniolaterad by a groove (1) from the corpus striatum (y), on the floor of the cerebral hemisphere.

b. The roof of the third ventricle is thin and united with the pia mater, as already stated. The pia mater bears many blood-vessels, and the two are folded into the groove between the optic thalami, forming the lamina chorioidea epithelialis, or choroid plexus of the third ventricle (Fig. 143, w). The roof is attached to the dorsomedial borders of the thalami and becomes continuous with the choroid plexus of the lateral ventricles (Fig. 148, e) at the craniolateral borders of the thalami.

The pineal body (corpus pineale) or epiphysis (Fig. 141, 2; Fig. 143, y) is a small conical body which is formed as an outgrowth of the caudal part of the roof of the third ventricle. It lies on the roof of the brain between the two cranial corpora quadrigemina (Fig. 141, q). It is hollow, containing an extension of the third ventricle. From its craniolateral angles two white strands, the habenulæ, extend into the striæ medullares (Fig. 141, u) of the thalami.

Just ventrad of the pineal body is a transverse band of white fibres, lying in the caudal part of the roof of the third ventricle. This interconnects the two thalami, and forms the posterior commissure (Fig. 143, z′) (commissura posterior). From this commissure a thin sheet of tissue extends to the pineal body.

c. The floor of the third ventricle appears in a ventral view of the brain (Fig. 138, Fig. 142) as a somewhat diamond-shaped space craniad of the pedunculi cerebri (Fig. 142, a) and bounded along its cranial margin by the optic tracts (Fig. 142, d). The optic tracts come from the thalami, as already described; they converge and unite to form the optic chiasma (Fig. 142, e), from which the optic nerves (f) diverge.

Immediately caudad of the optic chiasma lies a considerable rounded gray elevation, the tuber cinereum (Fig. 142, h). This bears on its ventral surface in the natural condition the infundibulum (Fig. 138, d) with the hypophysis (Fig. 138, e); in cases where the two latter structures have been removed (Fig. 142) the tuber cinereum (h) bears a small longitudinal opening (i) for attachment of the infundibulum. The infundibulum (Fig. 138, d) is a hollow extension of the floor of the third ventricle, and is attached to the middle of the ventral surface of the tuber cinereum. It bears at its ventral end the hypophysis (pituitary body) (Fig. 138, e), a vascular non-nervous body of unknown function. The hypophysis is lodged in the sella turcica of the sphenoid bone.

At its caudal border the tuber cinereum bears two white elevations, the mammillary bodies (corpora mammillaria) (Fig. 142, g).

The third ventricle (Fig. 143, h; Fig. 141, v) is a very narrow slit-like space, of considerable extent dorsoventrally, but less than a millimeter in width; it lies between the medial ends of the thalami (Fig. 141), and extends ventrad into the tuber cinereum, and thence into the infundibulum (Fig. 143, g′). Dorsad it extends into the corpus pineale (Fig. 143, y). The third ventricle communicates caudally near its dorsal border with the aqueductus cerebri (j); craniolaterally with the lateral ventricles, through the interventricular foramen (foramen of Monroe). Its cranial boundary forms in the middle line a thin plate, the lamina terminalis (d), which is, morphologically, the cranial termination of the cerebrospinal axis; it lies at the bottom of the deep fissure between the cerebral hemispheres. At the dorsal border of the lamina terminalis is a strong transverse band of fibres connecting the two sides of the brain; this is the anterior commissure (c). The cavity of the third ventricle is much encroached upon by the meeting and secondary union of the two thalami across the middle line, forming the massa intermedia (f).

The boundaries of the third ventricle, in order, are as follows, beginning dorsocraniad of the communication with the aqueductus cerebri: the posterior commissure (Fig. 143, z′), the pineal body (y), the choroid plexus of the third ventricle (w), the columns of the fornix (v), the anterior commissure (c), the lamina terminalis (d), the tuber cinereum with the infundibulum (g′), the substantia perforata posterior, and the midbrain. The lateral boundaries are formed by the thalami (Fig. 141, t).

5. Telencephalon.

—The telencephalon includes the two cerebral hemispheres. The name cerebrum is also applied to this portion of the brain; frequently, however, the name cerebrum is used as signifying the entire mass of the brain craniad of the rhombencephalon,—therefore including mesencephalon, diencephalon, and telencephalon. The term will here be used as synonymous with telencephalon.

The cerebral hemispheres arise as two lateral outgrowths from the cranial end of the primitive forebrain. They have undergone great increase in size in the course of evolution, so as to form the larger part of the brain; at the same time important modifications of structure have taken place. In the original condition the medial faces of the two hemispheres are not connected, the two being separate outgrowths of the ’tween-brain, and connected only through the latter. But secondary connections have been formed across the fissure between the two hemispheres, resulting in the production of the corpus callosum (Fig. 143, p; Fig. 147; Figs. 149-152, a), a broad transverse band of white fibres connecting the two hemispheres dorsad of the ’tween-brain. Ventrad of the corpus callosum another secondary union has resulted in the production of the fornix (Fig. 143, u).

External Features.—With increase in size the mass of the cerebrum shows externally a tendency to divide into three lobes, one craniad, the frontal (Fig. 145, A); one caudoventrad, the temporal (B); and one caudodorsad, the occipital (C). The two latter are not distinctly marked off from one another. The limit between the temporal and frontal lobes is marked by a short deep fissure, the lateral fissure (fissura cerebri lateralis), or fissure of Sylvius (Fig. 144, 9; Fig. 145, a). Each lobe is thrown up into elevations or gyri, which are separated by grooves or sulci; these are described below. The homology of the cerebral gyri and sulci of the cat with those of man is in most cases uncertain.

The sulci and gyri of the cerebral hemispheres may be described briefly in their main features as follows: On the lateral surface of the hemisphere (Fig. 145) the lateral fissure (fissura cerebri lateralis), or fissure of Sylvius, separating temporal and frontal lobes, forms the most convenient point of departure for an understanding of the fissures. The fissure of Sylvius is short but deep, and is formed early during development. In man it covers an area, the insula, at its bottom, which may be seen by separating the sides of the fissure. In the cat the insula is rudimentary and can be demonstrated only with difficulty.

The lateral surface of the hemisphere is marked by fissures which form three concentric arches (b-c, d, and e), irregular and incomplete, about the fissure of Sylvius. These arches all open ventrad. The first arch (b-c) (that next to the fissure of Sylvius) lacks the central part, the keystone, so that a complete arch is not formed, but only the two sides of the arch, as two separate sulci (b and c). The one of these craniad of the fissure of Sylvius is the sulcus anterior (b); the other is the sulcus posterior (c). In the dog this arch is usually complete and the sulcus forming it is frequently known as the ectosylvian sulcus (sulcus ectosylvius).

The gyri included between the fissure of Sylvius on the one hand and the anterior and posterior sulci on the other are known as the anterior (1) and posterior (2) Sylvian gyri.

The second arch is formed by the suprasylvian sulcus (sulcus suprasylvius) (d). The gyrus between the anterior and posterior sulci ventrad and the suprasylvian sulcus dorsad is the ectosylvian gyrus (3) (because dorsad of the ectosylvian sulcus).

The third arched sulcus, next to the medial margin of the hemisphere, is variable: it forms the lateral sulcus (e). The gyrus lying between the suprasylvian and lateral sulci is the suprasylvian gyrus (4) (because dorsad of the suprasylvian sulcus). The lateral sulcus is sometimes broken into two (as in Fig. 137, h), the two ends overlapping so as to leave a small gyrus between the two ends.

The gyrus dorsomediad of the lateral sulcus, extending onto the medial surface of the hemisphere as far as the splenial sulcus, is the marginal gyrus (5).

The four sets of gyri included between these three sets of arched sulci and the fissure of Sylvius unite caudad and craniad to form single gyri. The caudal one is the gyrus compositus posterior (6). The cranial one (less regular than the caudal one) is the sigmoid gyrus (7). The latter curves about the cruciate sulcus (f) (described below).

In the cranial part of the lateral surface of the hemisphere are certain sulci and gyri which do not belong to the system above described. The cruciate sulcus (f) is a short transverse sulcus passing from the lateral surface of the hemisphere onto its medial surface, where it extends caudad about a centimeter. Curving around the lateral end of this, separated from it by part of the sigmoid gyrus, is the short coronal sulcus (g). Connected usually with the end of the lateral sulcus (e) and running nearly parallel with the cruciate sulcus is the short sulcus ansatus (h).

At its ventral end the fissure of Sylvius (a) joins a longitudinal groove passing craniad and caudad from the point of junction. That part which passes craniad is the sulcus rhinalis (i); it forms the dorsolateral boundary of the olfactory tract (8). Passing dorsocraniad from the sulcus rhinalis is a sulcus which separates a very small cranial lobe from the rest of the hemisphere; this is the supraorbital or præsylvian sulcus (j). The portion of the hemisphere craniad of this sulcus is the orbital gyrus (9).

The sulcus rhinalis (i) is continuous caudad with the sulcus rhinalis posterior (or sulcus postrhinalis) (k). This extends caudad onto that portion of the hemisphere which faces the cerebellum. It forms the lateral boundary of a large elongated oval lobe lying at the side of the ventral floor of the midbrain and ’tween-brain. This lobe has been called tractus postrhinalis or lobus pyriformis (Fig. 145, 10; Fig. 138, f).

In addition to the sulci and gyri above mentioned, any given specimen will usually show a number of small inconstant sulci and gyri in various regions; these inconstant structures will not be here described.

On the medial surface of the hemisphere (Fig. 146), the following arrangement of sulci and gyri seems to be typical. Some distance from the dorsal margin a long sulcus runs parallel with the margin; this is the sulcus splenialis (a). The marginal gyrus (1) is dorsad of the splenial sulcus, passing onto the lateral surface of the hemisphere, where it is bounded by the lateral sulcus. The splenial sulcus extends onto the caudal surface of the hemisphere. A shallow sulcus marginalis (b) occurs frequently between the sulcus splenialis and the dorsal margin of the hemisphere. Cranioventrad or the cranial end of the splenial sulcus is that portion of the sulcus cruciatus (c) that lies on the medial surface of the hemisphere. Ventrad of this is a short shallow furrow which has been called the sulcus falcialis (d).

Immediately dorsad of the corpus callosum (h), separating its dorsal surface from a part of the hemisphere, is the supracallosal or callosal sulcus (e). Between the supracallosal (e) and splenial (a) sulci is the gyrus fornicatus (2).

On the caudal surface of the hemisphere (that facing the cerebellum) appear the ends of the sulcus splenialis (a) and the sulcus rhinalis posterior (f). Hidden in the natural condition by the thalami and optic tract is the broad hippocampal sulcus (g), marking externally the course of the hippocampus.

A further extension of the surface of the cerebrum has taken place in connection with the sense of smell. A mass of gray matter, the olfactory bulb (Fig. 144, a), reckoned as a part of the cerebrum, is separated from the latter and lies against the cribriform plate of the ethmoid bone. From it the olfactory fibres pass through the perforations of the plate to the olfactory mucous membrane. The olfactory bulb contains a cavity, a part of the lateral ventricle.

The bulb lies against the ventral surface of the frontal lobe and projects craniad of it. It is connected to the cerebrum by a tract of fibres, the olfactory tract (Fig. 138, a), which is divisible into two roots, medial and lateral. The medial root comes from the medial surface of the frontal lobe, where it is continuous with a tract extending to the cranial end of the corpus callosum. The lateral root is traceable from an elevated gyrus-like portion of the cerebrum which lies at the side of the infundibulum and is known as the lobus pyriformis or tractus postrhinalis (Fig. 138, f). The lateral root is divisible into a medial white strand and a lateral gray strand.

That part of the brain comprising the olfactory bulb and the parts intimately related to it are frequently included under the term rhinencephalon.

In the triangular area between the two olfactory tracts and craniad of the optic chiasma appears a mass of gray matter, subdivided by a longitudinal fissure. This possesses numerous openings through which blood-vessels pass to the brain substance, and is thence known as the anterior perforated substance (substantia perforata anterior) (Fig. 138, b).

Internal Structures of the Cerebrum.—The cavity of each of the cerebral hemispheres is known as a lateral ventricle. The two lateral ventricles constitute the first and second of the ventricles of the brain, whence the application of the names third and fourth ventricles to the cavities of the ’tween-brain and hindbrain. The lateral ventricles do not grow at the same rate as the walls of the hemispheres, so that they remain comparatively small. The cavity of each ventricle is further reduced in size by the development on its floor of a large ridge-like thickening, the corpus striatum (Fig. 148, f).

The dorsal wall or roof of the lateral ventricle joins the thin roof of the third ventricle on each side along an oblique curved line (Fig. 141, 1) which follows the cranial or lateral border of the thalamus (Fig. 141, t). Along this line the thin roof of the brain is folded in together with the pia mater to form the choroid plexus of the lateral ventricles (lamina chorioidea epithelialis) (Fig. 148, e). When this is pulled out there is left a fissure, the “great transverse fissure of the cerebrum” (Fig. 141, 1). Just dorsad of the groove between the thalami there runs a tract of white fibres known as the fornix (Fig. 148, a). The two halves of the fornix separate at the cranial ends of the thalami and pass ventrad, forming thus the pillars of the fornix (Fig. 148, b; Fig. 143, v). Dorsad of the cranial end of the fornix the corpus callosum (Fig. 143, p) passes from one hemisphere to the other. Caudad of the pillars of the fornix, the lateral ventricles communicate with the third ventricle by way of the interventricular foramen (foramen of Monroe).

The parts of the cerebrum may now be taken up in detail.

The corpus callosum (Fig. 147; Fig. 143, p; Figs. 149-152, a) is a broad transverse band of fibres forming a secondary connection between the medial walls of the two hemispheres, dorsad of the roof of the third ventricle. Its outer surface (Fig. 147) is exposed at the bottom of the fissure which separates the hemispheres. On each side it passes laterad, forming the roof of the lateral ventricle. Its cranial part lies dorsad of the corpus striatum, and its caudal part dorsad of the thalamus. Laterally its fibres radiate into the substance of the hemispheres. At its cranial end the corpus callosum bends ventrad and then caudad (Fig. 143). The part which turns to pass ventrad is the genu (Fig. 143, q) or knee, while the part which projects caudad is the rostrum (r). The caudal border of the corpus callosum is also thickened and turned ventrad and is called the splenium (Fig. 143, s); it lies dorsad of the cranial corpora quadrigemina (z). The caudal half of the ventral surface of the corpus callosum is united with the fornix (Fig. 143, u).

The fornix (Fig. 148, a; Fig. 143, u, v; Figs. 150-152, b) consists of an arched tract of longitudinal fibres near the medial border of each hemisphere, ventrad of the corpus callosum. Each tract begins in the mammillary bodies, and passes dorsad, the two converging until they run side by side, forming a cylindrical dorsoventral bundle known as the columns or pillars of the fornix (Fig. 143, v; Fig. 150, e), which cross the anterior commissure (Fig. 143, c; Fig. 150, f) caudad of the latter. Caudad of the pillars of the fornix, between these and the thalamus, lies on each side the interventricular foramen or foramen of Monroe, a small opening which connects the lateral ventricles with the third ventricle. Dorsad of the foramen the fornix turns caudad, the two fibre-tracts of each hemisphere lying side by side and closely connected (Fig. 148, a), forming thus another secondary union between the medial surfaces of the two hemispheres. This portion of the fornix is the corpus or body (Fig. 148, a); it lies dorsad of the roof of the third ventricle and passes to the splenium (Fig. 143, s) of the corpus callosum, and its dorsal surface unites with the ventral surface of the latter (Fig. 143). Caudad the two halves of the fornix diverge, forming the crura of the fornix (Fig. 148, c); these and the body are continuous laterally with the hippocampus (Fig. 148, d) and the fimbria (Fig. 148, beneath e).

The anterior commissure (Fig. 143, c; Fig. 150, f) is a transverse band of white fibres which stretches from one hemisphere to the other about half way between the interventricular foramen or foramen of Monroe and the floor of the third ventricle, and just craniad of the pillars of the fornix. This tract of fibres is developed in the original wall of the third ventricle, so that it does not form a secondary connection between the halves of the cerebrum, as do the fornix and corpus callosum. It lies dorsad of the lamina terminalis (Fig. 143, d) and is continuous with it.

The septum pellucidum (Fig. 143, t) is a vertical partition which separates the lateral ventricles and fills the interval between the corpus callosum dorsad and the fornix ventrad. It is triangular and translucent. It is formed from the medial walls of the two hemispheres and therefore is made up of two laminæ which embrace between them a space which originally was a part of the fissure separating the hemispheres. This space has been called the fifth ventricle.

The hippocampus (Fig. 148, d; Fig. 152, d) is an elongated rounded elevation of the floor of the lateral ventricle. It is continuous mediad with the fornix (Fig. 148, a) and extends thence along the inferior horn of the lateral ventricle to its end in the temporal lobe. It is somewhat narrower at its lateral end. It is thus curved into a semicircle in conformity with the inferior horn of the ventricle. Its dorsal surface is convex and looks into the lateral ventricle; its ventral surface is concave and rests upon the thalamus and the optic tract. Along the craniolateral edge of the hippocampus is a broad, clearly marked fibre-tract, the fimbria; this runs parallel to the choroid plexus of the lateral ventricle and beneath it, and is continuous at its medial end with the fornix.

The corpus striatum (Fig. 148, f; Fig. 141, y; Fig. 150, i) is a fusiform or ovoid elevation of the floor of the lateral ventricle. One of its narrow ends lies about opposite the middle of the thalamus (Fig. 141, t) and it extends thence obliquely mediad nearly parallel with the thalamus. In the groove between it and the thalamus is the choroid plexus of the lateral ventricles (Fig. 148, e). Internally the corpus striatum is made up of several layers of different texture (Fig. 150, i), whence its name.