THE CEREBRUM. 785 



slightly depresses it, and from which it is separated by the transverse septum of 

 the dura mater (tentorium). 



The anterior extremity — or late — is lodged in the fossa formed on each side of 

 the crista galli by the frontal and sphenoid bones. 



The most important peculiarities to be noted on the surface of the hemi- 

 spheres, are the convolutions, which we will now describe. 



Cerebral Convolutions.^ — These are the portions of the cerebral cortex 

 which are disposed in the form of more or less sinuous ridges, separated from 

 each other by deep grooves or fissures {sulci). 



A convolution (or gyrus) may include several plaits or folds — that is, several 

 parts separated or merely indicated by shallow grooves ; so that convolutions 

 should not be confounded with plaits. 



The convolutions are not independent, strictly speaking, but are often united 

 to those adjoining by more or less concealed reliefs, called passage plaits (annec- 

 tent or bridging gyri) ; sometimes two or more are very ostensibly connected in 

 this way. 



The convolutions in Solipeds are very sinuous, and limited by deep sulci ; 

 some of these latter — more important than the others by their depth and constancy 

 — are named ^sswres. Notwithstanding an apparent great irregularity, the cere- 

 bral convolutions are somewhat constant in their aiTangement ; so that it is 

 possible to describe them one by one. 



a. Fissure and grooves. — On the inferior face of the brain is seen a transverse 

 depression at the optic commissure (Fig. 424, 10). This depression — named the 

 fissure or vcdley of Sylvius — passes from within to without, and, on arriving at the 

 side of the brain, divides into two or three branches, of which one — the principal 

 — is inflected backwards, reaches the posterior extremity of the hemisphere, and 

 forms the superior limit of a lobe named the temporal or sphenoidal lobe (Fig. 433, 

 7). The other one or two branches remain slightly buried in the middle part of 

 the hemisphere ; they lodge the divisions of the middle cerebral artery. 



In glancing over the superior surface, there will be perceived a deep groov^e 

 coming from the great longitudinal fissure, which it intersects at nearly a right 

 angle. This groove — which Leuret has named the crucial fissure (Fig. 423, 12) 

 — is surrounded by a more or less apparent convolution, designated the sigmoid 

 gyrus (Fig. 423, 11). The crucial fissure divides the superior face of the hemi- 

 sphere into two parts — the anterior belonging to the frontal lobe, the posterior to 

 the parieto-occipitcd lobe. 



On the external face of the hemisphere is seen a deep groove, which, com- 

 mencing between the middle and posterior branch of the fissure of Sylvius, is 

 directed upwards in a curve, the concavity of which is inferior, and ends in the 

 vicinity of the crucial fissure (Fig. 433, 8, 8). This curved groove resembles the 

 fissure of Rolando. If looked at in the other direction — from before to behind 

 — it appears to bifurcate in its middle part, and the upper branch {interparietal 



' Leuret believes that the cerebral convolutions of animals are arranged after a certain 

 type (longitudinal type) essentially different from the human type. We think the difference 

 is not so great as has been imagined ; and it appears possible to discover in the brain of brutes 

 if not the longitudino-transverse type, at least the tendency to this type observed in the human 

 brain. In the description given above, the value of this assertion can be judged. We ought 

 to state that our conclusions are based on the comparative examination of the brains of adults 

 and foetuses, on tlie distribution of the blood-vessels, and on the physiological experiments 

 which enabled us to fix the position of the excitable zones of the cerebral cortex in the larger 

 Quadrupeds. 



