STRUCTURAL CULMINATION 985 



primitive centers in the midlirain recede i)ut transfer their functional ofTices 

 to more expansive areas in the cortex of the cndbrain. This telencephalization 

 favors further enrichment in the realms of sight and hearing. The gradual 

 diminution in the size of the mesencephalic colliculi furnishes the structural 

 indication of this process. The transference of functions from an area 

 formerly prominent as the optic lobe of the midbrain entered upon its initial 

 stages in the earliest mammals. Even in the lowest of the primates, the 

 lemur, the marmoset and the tarsius, both sets of colliculi manifest a 

 degree of prominence which suggests the retention of some primordial func- 

 tion. Both collicular prominences appear to be still active to a large extent in 

 the functions of sight and hearing respectively. This is particularly true of 

 that range of auditory and A'isual activity which concerns itself with imme- 

 diate rellex reaction, thus oflering the animal a greater margin of safety. 

 However notably this may apply to the sense of hearing, it is also the case, 

 perhaps to a less marked degree, with reference to vision. From the lower 

 primates upward, a definite decrease in the prominence of the colliculi indi- 

 cates a larger measure of functional participation by the higher auditory and 

 visual centers. Evidence of their primitive importance may still be elicited 

 from collicular eminences even in man. This is recognized in the well-known 

 phenomenon when a sudden, unexpected noise may cause the individual to 

 start or to come to an abrupt halt. Similarly, a flash of light before the eyes 

 may produce all of the reactions of primitive defense. But responses such as 

 these represent vestiges of more extensi^■e reactions formerly administered 

 through the primordial centers for A'ision and hearing. Sounds and visual 

 stimuli are in the main treated with deliberation, as the result of cortical 

 elaboration. The progressive decrease in structural prominence of the 

 mesencephalic colliculi, taken in conjunction with the expansion in the 

 occipital portion of the cerebral cortex, supplies the evidence of an evolutional 



