TR0G1.0^^TES NIGER. TIIF CHIMPANZEE 599 



situatrd \ cut ronusial to the coinolut lones cjuinti, constituting the nucleus 

 niasticatonus (NM). The clillusc reticular formation occupies nearly the 

 entire lield ol the tegmentuni and is not (hllerentiated to form any speciah'zed 

 nuekar aggregation. This lev(.-l is signihcant as olU'ring some basis f(jr esti- 

 mating the degree to which tlu- hith cranial nerve contributes to the inlhix 

 of sensory impulses Irom the bod\, |3articularly as this inner\ation bears 

 upon the (hrt'ction ol the animal's motor control. 



LE\EL OF THE INFERIOR COLLICl LLS (^FIG. 267) 



At this le\el, changes incident to the appearance of the two caudal ele- 

 vations in the c|uadrigeminal phite ha\-e altered the contour of the section. 

 In this region the \i'ntricular chamber has been reduced in its dimensions to 

 those characteristic of the SyKian acjueduct w Inch is now entirel\ surroundi'd 

 by the central gra\ matter (Cen). In the ventromesial [portion of this 

 substance is a speciahzed region h)rming the nucleus Irochlearis 1 Ntr). A 

 (k'nse bundle of hbers passing Irom this nucleus constituti'S the lirst portion 

 ol the emergent course ol this motor ner\'e. At the lateral peripht-ry of 

 the central gray matter is a band-like collection ol libias, somewhat sepa- 

 rated into small lasciculi, lorming the ascending mest'iicephalic tract of the 

 fifth nerve 1 Tint ). 



The most outstanding Icatures, howc'\er, art- tlu' two elexations abo\-e 

 the central gray constituting the interior colliculi 1 IC) to which some 

 fibers of the lateral fillet are making their way. This colhculus is a way- 

 station in the pathwax ol auditorx impulses destined lor the temporal lobe 

 ol the cerebral cortex. It still shows some ol its primiti\e histological details, 

 especially indicated by the rudimentary stratilication of cell and liber layers. 

 Comjxuatiw'ly speaking, however, the mierior colliculus is less prominent in 

 the chim])an/.ee than it is in the mtt'rmediatt' and lower primates, thus indi- 



