692 



NERVOUS SYSTEM. 



to greatly increase the extent of the gray substance. While these convolutions are not 

 exactly the same in all human brains, or even in both sides of the brain, their arrange- 

 ment and relations may be described in a general way with sufficient accuracy to enable 

 us to recognize easily the most important physiological points in the descriptive anatomy 

 of the cerebral surface. The diagrammatic figure 226*, taken from Dalton, gives a gen- 

 eral view of the fissures and of the most important convolutions. 



FIG. 226*. Diagrammatic figure showing the cerebral convolutions. (Dalton.) 



Aside from the great longitudinal fissure which divides the hemispheres in the median 

 line, the diagram shows three deep fissures, marked by heavy, dark lines, and five fissures 

 of less importance indicated by lighter dark lines. Each cerebral hemisphere is divided, 

 according to Sappey, into two lobes. The anterior lobe includes that portion lying in 

 front of S, the fissure of Sylvius, and the posterior lobe, all that portion lying behind the 

 fissure of Sylvius. English anatomists, however, generally describe three lobes : the 

 anterior lobe, lying in front of the fissure of Sylvius ; a middle lobe, occupying the middle 

 fossa of the skull; and a posterior lobe, lying just above the cerebellum ; but there is no 

 distinct line of demarkation between the middle and the posterior lobe. 



S, in Fig. 226*, represents the fissure of Sylvius, with its branches a and 5, 5, I ; R 

 represents the fissure of Rolando, and P represents the parietal fissure. Above and in 

 front of the anterior portion of the fissure of Sylvius, is a short, curved fissure, bounding 

 anteriorly the third frontal convolution (3, 3, 3) which, in the left hemisphere, is sup- 

 posed to be the seat of the faculty of articulate language. 



The first frontal convolution (1, 1, 1) is bounded internally by the great median fissure 

 and externally by a shallow fissure nearly parallel to the median fissure. The second 

 frontal convolution (2, 2, 2, 2) lies next the first frontal convolution, and is bounded ex- 

 ternally by two shallow fissures lying in front of the fissure of Sylvius and the fissure of 

 Rolando. The third frontal convolution (3, 3, 3) curves around the short branch (a) of 

 the fissure of Sylvius. On either side of the fissure of Rolando, we have the anterior 

 central convolution (4, 4, 4) and the posterior central convolution (5, 5, 5). Curving 



