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A MANUAL OF PHYSIOLOGY 



Relation of Size of Brain to Intelligence. While it is the 

 case that some men of great ability have had remarkably heavy 

 and richly convoluted brains, it would seem that in general 

 neither great size nor any other obvious anatomical peculiarity 

 of the cerebrum is constantly associated with exceptional 

 intellectual power. In the animal kingdom, as a whole, there is 

 undoubtedly some relation between the status of a group and 

 the average brain development within the group. But that 

 this is a relation which is complicated by other circumstances 

 than the mere degree of intelligence is sufficiently shown by the 

 fact that a mouse has more brain, in proportion to its size, 

 than a man, and thirteen times more than a horse ; while both 

 in the rabbit and sheep the ratio of brain- weight to body- 

 weight is nearly twice as great as in the horse, in the dog only 

 half as great as in the cat, and not very much more than in 

 the donkey. The following tables, too, which illustrate the 

 weight of the brain in man at different ages, show that, although 

 we might give ' the infant phenomenon ' an anatomical basis, 

 we should greatly overrate the intellectual acuteness of the 

 average baby if we were to measure it by the ratio of brain to 

 body-weight alone. 



-HUSCHKE. 



In some small birds the ratio is as high as I : 12, in large 

 birds as low as I : 1,200 ; in certain fishes a gramme of brain 

 has to serve for over 5 kilos of body. As a rule, especially 

 within a given species, the brain is proportionally of greater 

 size in small than in large animals. It is to be supposed that 

 quality as well as quantity of brain substance is a potent factor 

 in determining the degree of mental capacity. 



The Cerebral Circulation. The arrangement of the cerebral 

 bloodvessels has certain peculiarities which it is of importance to 

 remember in connection with the study of the diseases of the brain, 

 many of which are caused by lesions in the vascular system -haemor- 

 rhage or embolism. Four great arterial trunks carry blood to the 



