426 HISTORY OF THE HUMAN BODY 



change may be found in some reptiles, where the develop- 

 ment of a pair of small subordinate lobes posterior to 

 the main ones makes it clear that the four-lobed form in 

 mammals is due to the development of a new pair of lobes 

 posterior to the others, and not merely to the formation 

 of a cross-furrow. Subordinate lobes like those of reptiles are 

 found also in birds. 



The floor of the mesencephalon is thickened in all cases and 

 is of considerable functional importance. Through this region 

 pass the fibers of connection between the cerebral lobes and the 

 medulla, and as the hemispheres increase in size, these bundles 

 become greater and form the pedunculi cerebri [crura 

 cerebri}, especially conspicuous in mammals, as would be 

 expected. 



During the process of phylogenetic development the roof 

 and sides of the metencephalon become selected as a region 

 where a large part of the work of the central nervous system 

 is accomplished. This part, the cerebellum, is thus almost al- 

 ways large and voluminous, and often, even in fishes, becomes 

 folded up into several plicae, thus emphasizing its great func- 

 tional importance. 



It has already been shown how both the corpora striata and 

 the lobi optici, although of supreme importance in some fishes, 

 eventually become, except perhaps in birds, entirely subordi- 

 nated to the cerebral hemispheres ; the cerebellum, on the other 

 hand, has retained from the first an office of great importance, 

 and in mammals becomes subordinated to the hemispheres 

 alone. There are occasional exceptions to the general impor- 

 tance of this part, as in the case of the singularly small cere- 

 bellum of the frog, but such cases are very few. The floor of 

 the metacephalon is utilized in part for the location of com- 

 missural fibers between the two lateral halves of the cerebellum, 

 and in mammals, corresponding to the increase in size of this 

 organ, this commissural bundle becomes large and conspicu- 

 ous, forming a broad loop around the base of the medulla, the 

 pons [ Varoli\\ . 



Although this region of the myelencephalon is perhaps the 

 most complex of any part of the brain, this complexity lies in 



