SUMMOARY OF STRUCTURES 279 



action in which the stimuli arc first submitted to a process of deliberation 

 and selection. 



The decline of the superior or visual colliculus is more conspicuous in the 

 planimetric coefficients than that of the inferior colliculus; and furthermore 

 the longitudinal coefficients of the superior colliculus show a definite decrease 

 in mycetes if compared either with the lemur or tarsius. The inference seems 

 fair that the superior colliculus in tarsius is vested with more visual function 

 than is the case with the other lower primates here discussed. The decrease, 

 therefore, in prominence of both the inferior and superior colliculus, although 

 it is not strikingly shown in the coefficients, must be accepted as marking 

 the inception of that gradual process whose final consummation is seen in the 

 conditions observed in man. These two midbrain structures, even within the 

 circumscribed limits of the primate order, disclose a most illuminating record 

 regarding the evolutional process which has taken place in the brain stem. 

 Unlike other structures heretofore considered, their evidence speaks in favor 

 of waning function, of a steady delegation of this function to other expanding 

 portions of the brain. For those who may wish to realize more clearly how this 

 part of the midbrain reveals the course of evolution, a review of the conditions 

 in the mesencephalon of the lower forms of vertebrates is especially recom- 

 mended. Here may be seen the huge optic lobes of the fish, the bird or the 

 reptile. Their size compared with the relatively insignificant colliculi of 

 primates speaks strongly in favor of an evolutionary defiorescence which 

 reaches its final stages in the human brain. 



VIII. The Oculomotor Decussation in Relation 

 TO Binocular Vision 



Another notable feature in the mesencephalic portion of the brain stem 

 is the internuclear connection between the two main cell groups in the oculo- 

 motor nucleus. This nucleus, involved as it is in the regulation of oculomotor 



