SUMMARY OF STRUCTURES 277 



have gradually been deprived of their morphological prominence. In the 

 primates they appear as inconspicuous elements covered by the greatly 

 expanded endbrain. 



In viewing the mesencephalon and its two sets of colliculi, the tact ot 

 their primordial eminence in auditory and visual functions must not be 

 overlooked. Nor must it be considered a sudden departure from an old 

 architectural design that these two areas of the brain have passed into rela- 

 tive insignificance. It was by the gradual delegation of visual and auditory 

 functions to other regions that the midbrain became progressively reduced. 

 Apparently the object of this delegation of former power to other areas was 

 to enlist the activities of a more promising region. There was need of suffi- 

 cient expansion to accommodate the rapidly growing demand for more 

 complex associations in the realms of sight and hearing. The portion of the 

 nervous system which has shown the chief propensity in this direction is the 

 telencephalon. It is the last of all the brain constituents to make its appear- 

 ance in the process of individual as well as phyletic development. In the end- 

 brain there are special regions known respectively as the auditory cjr temporal 

 area and the visual or occipital area, which have largely taken over the 

 functions of hearing and of vision. 



COMPARISON OF THE INFERIOR COLLICULI OF LOWER PRIMATES 



Yet in spite of this assumption of functional activity on the part of the 

 cerebral hemispheres, the midbrain has not altogether given over its original 

 autonomy. A comparison of the lower primates, such as lemur, with man 

 clearly reveals this fact. Even a comparison of closely allied lower primates 

 indicates that to some extent at least the progressive delegation of function 

 from the colliculi to the endbrain is in process. The inferior colliculus, con- 

 nected with hearing, shows in its planimetric coefficients a definite decline 

 almost as striking in its longitudinal coefficients. 



