426 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



more arid more extend backwards, and at last com- 

 pletely overlie the cerebellum. As they increase in 

 size they become broken up into distinct lobes, 

 frontal, occipital, and temporal. The cerebellum 

 diminishes in proportionate size, and the flocculi 

 cease to be conspicuous at its sides. (Compare ol, in 

 Fig. 182; A, B, and c.) 



Transverse commissures are always richly deve- 

 loped, the corpus callosum connecting the two 

 cerebral hemispheres, and the pons varolii, which 

 bridges over the hind-brain, being parts which are 

 developed in mammals only. The optic lobes are 

 divided transversely, so that the " corpora bigemina " 

 of the lower vertebrates are now the " corpora quacl- 

 rigemma " ; this mid-brain is proportionately small. 



A very complete series of gradations of all these 

 differential characters is to be observed as we pass up 

 the scale of the Mammalia. This is to be seen, first 

 of all, in the proportionate increase in the weight of 

 the brain, as compared with the rest of the body, for, 

 while that of the rabbit is about y^th part, that of 

 man is ^th. 



In the Prototheria the corpus callosum is always 

 small, and the cerebral hemispheres, which are smooth, 

 do not cover the cerebellum. The Metatheria 

 differ a good deal among themselves. Among the 

 Euttieria, the Insectivora exhibit a brain of very 

 low character; the cerebral hemispheres are often 

 quite smooth, the olfactory lobes are large, and project 

 in front of the hemispheres, which only just, if at 

 all, overlap the cerebellum behind (as in Tupaia; 

 Fig. 183). This latter has a large vermis. The 

 corpus callosum is thin and nearly straight, while 

 the corpora quadrigemina are proportionately large. 

 The pons varolii is very small. In the hedgehog 

 there is a single simple groove (sulcus) on either 

 hemisphere. 



