FUNCTIONS OF THE BRAIN. 809 



heavier than in man ; thus, in the whale its weight is about five pounds, 

 while in the elephant it varies from eight to ten pounds. 



Compared with the weight of the body, the brain diminishes in 

 weight in the following order : 



In mammals, 1 to 186; in birds, 1 to 212; in reptiles, 1 to 1321; and 

 in fishes, 1 to 5668. 



In mammals the relative proportion of the brain to the bod.y is 

 smaller in the larger species. Thus, in the ox it is as 1 to 860 ; the 

 elephant, 1 to 500 ; the horse, 1 to 400 ; sheep, 1 to 350 ; dog, 1 to 305 ; 

 the cat, 1 to 156; the rabbit, 1 to 140; the rat, 1 to 76; field-mouse, 1 to 

 31 ; man, 1 to 36. It is thus seen that in few animals is the brain 

 heavier compared to the body than it is in man, though in a few singing- 

 birds it may amount to as much as 1 to 12. 



It must not be forgotten that the estimates of the weight of the 

 brain include the sensory and motor ganglia at the base of the cerebrum, 

 the optic thalami, the corpora striati, and the cerebellum, and in the lower 

 mammals these portions of the brain constitute by far the greater part ; 

 hence the size of the cerebral lobes as an index of the power of 

 intelligence is not disturbed by these figures. 



To recapitulate, the principal difference noticed in the brain in 

 passing from the highest to the lowest vertebrata is not only in its 

 relative decrease in size but also in its gradual simplification in form and 

 structure, more especially in the cerebral hemispheres and cerebellum. 

 These organs, indeed, gradually become smaller in proportion to the 

 sensory and motor ganglia at the base of the cerebrum ; or, in other 

 words, the ganglia exhibit a greater proportionate size as compared with 

 the cerebral hemispheres and cerebellum. 



In the highest mammals the cerebral hemispheres completely cover 

 the olfactory lobes in front and the corpora quadrigemina behind, and 

 in man even overlap the cerebellum ; but in the carnivora, ruminants, 

 and lower mammals the cerebral hemispheres no longer overlap, but 

 even fail to cover any part of the cerebellum, while in the ruminants 

 the anterior part of the hemispheres is so diminished as to permit 

 the projection of the olfactory lobes beyond them. 



In rodents the cerebral lobes have become still more retracted 

 and now a portion of the corpora quadrigemina becomes visible. 

 In birds, while the olfactory lobes are covered, the optic lobes are 

 exposed, and in reptiles, amphibia, and fishes the cerebral hemispheres 

 become so much further reduced in size as to merely cover the corpora 

 striata with a thin layer of cerebral substance. 



In the lowest vertebrata the parts of the encephalon thus appear to 

 be arranged in a double symmetrical row, one behind the other. The 

 most anterior in this row are the ganglionic masses which form the 



