Part 78

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).

a, olfactory bulb; b, cerebral hemisphere; c, cerebellum; d, medulla oblongata; e, tractus olfactorius; f, lobus pyriformis or tractus postrhinalis; g, pons; h, trapezium; i, pyramid; j, area elliptica; k, area ovalis. II, N. opticus; V, N. trigeminus; VII, N. facialis; VIII, N. acusticus; IX, N. glossopharyngeus; X, N. vagus; XI, N. accessorius. 1. sulcus præsylvius; 2, sulcus cruciatus; 3, sulcus ansatus; 4, sulcus lateralis; 5, sulcus suprasylvius; 6, sulcus anterior; 7, sulcus posterior; 8, sulcus rhinalis; 9. fissura Sylvii; 10. sulcus rhinalis posterior.]

=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=).

A, frontal lobe; B, temporal lobe; C, occipital lobe. a, lateral fissure or fissure of Sylvius; b, sulcus anterior; c, sulcus posterior (the broken line connecting the ends of a and b serves to show how the first arch is completed to form the ectosylvian sulcus, as in the dog); d, sulcus suprasylvius; e, sulcus lateralis; f, sulcus cruciatus; g, sulcus coronalis; h, sulcus ansatus; i, sulcus rhinalis; j, sulcus præsylvius; k, sulcus rhinalis posterior. 1, anterior Sylvian gyrus; 2, posterior Sylvian gyrus; 3, gyrus ectosylvius; 4, gyrus suprasylvius; 5, gyrus marginalis; 6, gyrus compositus posterior; 7, sigmoid gyrus; 8, olfactory tract; 9, orbital gyrus; 10, lobus pyriformis; 11, olfactory bulb.]

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.

a, sulcus splenialis; b, sulcus marginalis; c, sulcus cruciatus; d, sulcus falcialis; e, sulcus supracallosalis; f, sulcus rhinalis posterior; g, hippocampal sulcus; h, corpus callosum. 1, gyrus marginalis; 2, gyrus fornicatus; 3, gyrus compositus posterior.]

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).