BASE OF THE BRAIN. 



477 



lodged. Anteriorly to the optic commissures, this fissure reaches 

 quite to the base, but on the superior surface it is interrupted 

 by a broad lamina of white fibres, the corpus callosum, or great 

 commissure of the hemispheres; and, at the upper part of its 

 posterior extremity', it is crossed by a species of bridge, under 

 which the venaa Galeni pass to the interior. Deep, irregular 

 sulci map out the surface of the hemispheres into numerous con- 

 volutions, the number of which, and the depth of the sulci, 

 indicate the range of intelligence possessed by the animal, as the 

 grey matter, which is believed to be the seat of the intellectual 

 faculties, is found on the surface of the convolutions and on the 

 sides and bottom of the sulci, so 

 that the deeper and more numer- 

 ous these are, the greater is the 

 amount of grey matter in the brain. 

 In the domesticated animals, 

 each hemisphere is divided into 

 an anterior and a greater lobe, 

 the posterior lobe which is found 

 in man being wanting. The 

 anterior lobe, which forms the 

 bulk of the organ, is separated 

 from the greater lobe by the 

 fissure of Sylvius, not very dis- 

 tinct in the lower animals. This 

 fissure, which is on a level with 

 the optic tracts, and contains the 

 middle cerebral artery, divides 

 into two branches, one going 

 upwards and forwards, the 

 other downwards and inwards. 

 Between the bifurcations, a cluster of convolutions, forming the 

 'island of Reil or central lobe, converge towards the median line. 

 These convolutions, from their being concealed in the fissure, are 

 also called the covered convolutions. 



Fig. 17 



Superior aspect of the brain. «, Left, and 

 a. Right cerebral hemisphere ; a", Anterior 

 lobe ; a'", Greater lobe ; 6, Corpus cal- 

 losum ; f. Opening of veuae Galeni ; d, 

 Venniforni process ; d' d", Lateral lobes of 

 cerebellum. 



BASE OF THE BRAIN. 



At the base of the brain are the crura, or 'peduncles of the 

 cerebrum, two large bundles Avhich, commencing in front of the 

 pons Varolii, proceed forwards in a divergent manner, entering 



