TROGLODYTES GORILLA 679 



tlu' iiKTcast'd expansion in the tenijjoral region especially in the auditory 

 area of tlie heniis|)liere. Thus, in gorilla, as also in the other higher anthro- 

 poids, the funetion of hiaring has undergone a marked degree of telenee])h- 

 alization. The tenij^oral lobe is not only larger, but also more complexly 

 con\()luted. The central gray matter (Cen) surrounds the much reduced 

 ventricular space lormmg tlu' Sylvian aqueduct. In the roofjjiate above the 

 aciucduct are seen decussating libers ol the interior collicular commissure. 

 In the ventral portion ot the central gray matter near the midline is a well- 

 defined aggregation ol nerve cells constituting the nucleus trochlearis 

 (Ntr) from which enurgcnt libers of the fourth nerve take origin. On 

 the lateral boundary of the central gray matter is a thin zone of heavily 

 myclinized axons, the tractus mesenccphalici trigemini. Lateral to this 

 tract is the extensive mass constituting the inferior colliculus which 

 appears to retain some of its primiti\e stratified arrangement. The stratum 

 suj)erliciale griseum is clearix defined, but because of its deficiency in his- 

 tological detail, has a definitely vestigeal appearance. Four other layers 

 may be discerned. The j)rimordial tendenex' of this portion of the brain to 

 present delinite stratification is still retained e\'en in this iiigh representative 

 of the primate ordt'r. 



The boundar\ between the tegmentum and basis in this jjortion of the 

 axis is, as in prect'ding sections, determined by the mesial fillet (Mf). 

 The tegmentum itself contains a large and somewhat diiluse reticular ior- 

 mation (Ref'i in the center of which, in process of complete decussation, 

 arc the fibers of the suj)erior cerebellar ixtluncle (Spx). The basis has 

 many of the a|)()earances typical of the j)ons \'arolii. The scattered bundles of 

 the j)yramidal s\stem (Py) are showing a tendency to reassemble as a 

 single fasciculus. The basilar groove along the ventral surface of the pons has 

 become deeper as it approaches the point of divergence of the cerebral 

 peduncles. 



