830 MAN 



cerebellar peclunck' (Spx) as they approach tlu'ir k'\ el of decHissalioii. 

 Lateral to the converging peduncle arc (ibcrs ot the lateral fillet ( Lf ) entering 

 the inferior coiliculus and thus completing tlie lirst stage in thepathway of 

 liearing. The mesial lillet (Mf) establishes the boundary between the basis 

 and tegmentum in which iormer may still be discerned the characteristics 

 of the pons Varolii. 



The special feature at this level in all primates is the appearance of the 

 inferior collieuli. That they show some demonstrable diminution in man is 

 indicative of the process of telencephalization which has proceeded through- 

 out the primate series. In man it reaches its termination in that the chief 

 responsibilitx of the function of hearing has at length been delegated to the 

 cerebral cortex in the region of the temporal lobe. 



LEVEL OF THE SUPERIOR COLLICULUS (FIG. 354) 



Here the alterations invariably' associated w ith this region ot the brain 

 are all apparent. The essential changes are typified by the appearance of 

 two dorsal elevations in the roofplate of the mesencephalon, the superior 

 collieuli (SC). These structures have ()b\ lously become much reduced 

 in size as compared with the other primates. They retain but a vestige of 

 their former importance, representing as they do the much reduced optic 

 lobes of the lower \ertebrates. Nothing could demonstrate more eonelusively 

 the evolutional process which has steadily acKanced m the brain than the 

 changes affecting this portion of the stem. In man the proct^ss of delegating 

 function from a lower to a higher, more expansible area ot the brain is carried 

 to its fullest expression. Although the superior eollieulus doe^s manitest a 

 certain degree of stratilication in the human brain, although several layers of 

 cells and libers may \k- detected in this structure, these are so ruchmeiitary 

 as to lea\e no doubt that the funetion of this lormerix important region has 



