HEMISPHERES OF THE BRAIN. 



645 



The number and extent of the convolutions, as well as the depth of the inter- 

 vening sulci, appear to bear a close relation to the intellectual power of the individ- 

 ual, as is shown in their increasing complexity of arrangement as one ascends from 

 the lowest mammalia up to man, where they present a most complex arrangement. 

 Again, in the child, at birth, before the intellectual faculties are exercised, the 

 convolutions are simpler, and the sulci between them shallower, than in the adult. 

 In old age, when the mental faculties have diminished in activity, they become 

 less prominently marked. By their arrangement the convolutions are adapted to 

 increase the amount of gray matter without occupying much additional space, while 

 they also afford a greater extent of surface for the termination of white fibres in 

 gray matter. 



It will be convenient, in the first instance, to describe the fissure which sepa- 

 rates the two hemispheres from each other, and those which divide each hemisphere 

 into its larger divisions. 



The Longitudinal Fissure (Fig. 343). This great fissure separates the cerebrum 

 into two hemispheres, and reaches from the front to the back of the organ : it contains 

 a vertical process of the dura mater, the falx cerebri (page 640). In front and be- 

 hind it extends from the top to the bottom of the cerebrum, and completely sepa- 



^Parieto- 

 'occipital 

 Fissure 



FIG. 344. Fissures and lobes on the external surface of the cerebral hemispheres. 



rates the two hemispheres, but its middle portion only separates the hemispheres 

 for about half their vertical extent, the floor of this part of the fissure being 

 formed by the great central white commissure, the corpus callosum, which connects 

 the two hemispheres together. 



The remaining fissures are situated in one or other of the two hemispheres, with 

 the exception of the transverse fissure, one-half of which is contained in each 

 hemisphere. 



Sylvian Fissure (Fig. 344). This fissure is a well-marked cleft on the base and 

 side of the hemisphere. Starting at the base of the brain in a depression, the 

 vallecula Sylvii, in which is situated the anterior perforated spaces it passes out- 

 ward to the external surface of the hemisphere. It here gives off a short anterior 



