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 corf/us 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 venae 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 

 anttrior lobe, which forms the 

 bulk of the organ, is separated 

 from the greater lobe by the 

 lissure 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 centred lobe, converge towards the median line. 

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

 also called the covered convolutions. 



FIG. 17s. 



Superior aspect of the brain, o, Left, and 

 a', Eight cerebral hemisphere ; a", Anterior 

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

 losum ; c , Opening of venae Galeni ; d, 

 Vermiform process ; d' d", Lateral lobes of 

 cerebellum. 



BASE OF THE BRAIX. 



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

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

 pons Varolii, 'proceed forwards in a divergent manner, entering 



