358 NERVOUS SYSTEM. 
hemispheres dorsad of the ’tween-brain. Ventrad of the corpus 
callosum another secondary union has resulted in the produc- 
tion of the fornix (Fig. 143, z). 
54 ¥ of 
41! uy Wa 
Fic. 144.—LATERAL SURFACE OF THE BRAIN. 
a, olfactory bulb; 4, cerebral hemisphere; ¢, cerebellum; d@, medulla oblongata; 
, tractus olfactorius; /, lobus pyriformis or tractus postrhinalis; g, pons; 4, trape- 
zium; 2, pyramid; 7, area elliptica; 4, area ovalis. //, N. opticus; V, N. trigemi- 
nus; V//, N, facialis; VZ/7, N. acusticus; ZX, N. glossopharyngeus; X, N. vagus; 
AZ, N. accessorius. 1, sulcus prasylvius; 2, sulcus cruciatus; 3, sulcus ansatus; 
4, sulcus lateralis; 5, sulcus suprasylvius; 6, sulcus anterior; 7, sulcus posterior; 8, 
sulcus rhinalis; 9, fissura Sylvii; ro, 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 caudoven- 
trad, the temporal (2); and one caudodorsad, the occipi- 
tal (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 
= oe V ¢/ 
