276 THE LOWER PRIMATES 



most commonly seen in those animals which have gamed comparative free- 

 dom of the upper extremities from the oflices of locomotion, is obviously 

 true without further proof. As a corollary to the supposition that the pontile 

 nuclei increase in proportion as the animal becomes more capable in its 

 skilled performances, it is true that these nuclei also expand as greater 

 manual differentiation is attained. It may therefore be held that the progres- 

 sive increment in this important group of nuclei bears definite relation to the 

 increasing capacities acquired as the forelimbs become more precisely cap- 

 able of manual function. 



VII. The Colliculi of the Midbrain in Their Relation to 

 Sight and Hearing 



their primordial eminence in auditory and visual functions 



The history of the mesencephalon indicates that this portion of the 

 brain has presided over many important functions during the development 

 of the phylum. It quite as clearly demonstrates the waning predominance of 

 this part of the brain as evolution proceeded. Not the least important among 

 the earlier functions of the midbrain are those connected with the special 

 senses of sight and hearing. One portion of the midbrain has become so 

 highly specialized for visual function that it has earned for itself in many of 

 the lower forms of vertebrates the title of optic lobe. Thus in the fish, in the 

 amphibia, in the reptiles and in the birds the optic lobe is one of the most 

 conspicuous parts of the entire brain. Similarly, a more caudal portion ot the 

 roofplate of the midbrain became highly differentiated in connection with 

 the function of hearing. But one of the most striking marks of progress as the 

 brain found opportunity to provide for more ample syntheses in the ditTerent 

 sensory spheres is the manner in which the midbrain has lost its ancient 

 prestige. Those extensive areas developed in relation to sight and hearing 



