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).
Fig. 145.—Diagram of the Sulci and Gyri on the Lateral Surface of the Hemisphere.
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).