512 



COMPAEATI\E PHYSIOLOGY. 



It is evident, from an inspection of the cranial cavities of 

 those enormous fossil forms that preceded the higher verte- 

 brates, that their brains, in proportion to their bodies, were 

 very small, so that any variation in the direction of increase 

 in the encephalon — especially the cerebrum — must have given 

 the creatures, the subject of such variation, a decided advan- 

 tage in the struggle for existence, and one which may partly 

 account, perhaps, for the extinction of those animals of vast 

 proportions but limited intelligence. That the size of the brain 



Fig. 368.— a, brain of a chelonian; B, of a fcetal calf; C, of a cat. (AH after Gegen- 

 baur.) /, indicates cerebral hemispheres ; //, thalamus ; ///, corpora quadri- 

 gemina; 7F, cerebellum; T^ medulla; e(, corpus striatum; /, fornix; A, hippocam- 

 pus; sr, fourth ventricle^ gr, geniculate body; ol, olfactory lobe. It will be observed 

 (1) how the fcetal brain in a higher animal form resembles the developed brain in 

 a lower form, and (3) how certain parts become crowded together and covered 

 over by more prominent regions, e.g., the cerebrum, as we ascend the animal scale. 



as well as its quality can be increased by use, seems to have 

 been established by the measurements, at different periods of 

 development, of the heads of those engaged in intellectual pur- 

 suits, and comparing the results with those obtained by similar 

 measurement of the heads of those not thus specially employed. 

 Of course, it must be assumed that the head measurement is a 

 gauge of the size of the brain, which is approximately true, if 

 not entirely so. 



Recent investigations seem to show that the development 

 of the ganglion cells of the brain takes place first in the me- 

 dulla, next in the cerebellum, after that in the mid-brain, and 

 finally in the cerebral cortex. Animals most helpless at birth 

 are those with the least development of such cells. The me- 



