158 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



it really contains a part of the third ventricle as well as the lateral 

 ventricles in the hemispheres. Each hemisphere consists of a gan- 

 glionic mass, the corpus striatum, in or near the floor, and a part, the 

 palUum, covering the ventricle. This latter part undergoes the 

 greatest modification in the different groups. In front the cerebrum 

 proper passes into or connects with the olfactory lobe, into which a 

 part of the ventricle may extend. 



In most vertebrates the pallium is relatively thin, compared with 

 the corpora striata, and in the teleosts it is reduced to a very thin, 

 non-nervous wall to the ventricle. In the mammals it is greatly in- 

 creased, forming in the higher groups by far the greater part of the 

 whole brain, a development which is correlated with the greater men- 

 tal development. The increase is also accompanied by structural 

 alterations in the palhum. In the ichthyopsida the gray matter 

 (nerve cells) is confined to the ventricular surface, the outer surface 

 being white matter (fibres). In the reptiles there is the beginning of 

 a second layer of cells at some distance from the ventricular surface. 

 This is increased in birds while in the mammals it forms a marked 

 layer (cerebral cortex) over almost the whole of the surface of the 

 pallium. 



Fig. 164. — ^Lateral view of left cerebral hemisphere of dog, showing lobes, gyri and 

 sulci. /, frontal lobe; 0, occipital lobe; ol, olfactory lobe; p, parietal lobe; s, fissure of 

 Sylvius; t, temporal lobe. 



This increase in cerebral cortex is accomodated in part by lon- 

 gitudinal growth of the cerebrum, but there is a limit to such growth, 

 so in most mammals the posterior ends of the hemispheres bend down- 

 ward and then grow forward, so that the (morphologically) posterior 

 end of each hemisphere is brought in close spatial relations with the ol- 

 factory region (fig. 164), covering up a portion of the side wall of the 

 hemisphere (the insula), the bending being indicated by a deep 

 lateral cerebral fissure (fissure of Sylvius) (fig. 164, s), found in most 

 mammalian, brains at the bottom of which is the insula (of Riel) . In 

 the lower mammals the general surface of the cerebrum is smooth, but 

 in the higher it is thrown into a number of folds (fig. 164) or convo- 



