[Illustration: FIG. 144.--LATERAL SURFACE OF THE BRAIN. _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=). [Illustration: FIG. 145.--DIAGRAM OF THE SULCI AND GYRI ON THE LATERAL SURFACE OF THE HEMISPHERE.