TARSIUS SPECTRUM 127 



shortening due to compression in tlie stem, particularly about the region 

 of the posterior isthmus. 



Suf^jacent to the trochlear nucleus are the dense bundle of the posterior 

 longitudinal fasciculus (PL) and the predorsal bundle (PD). On the 

 periphery of the central gray matter is the mcsenccphahc root of the fifth 

 nerve (Tmt). The reticular formation at this level is extensive, but its 

 details are considerably obscured by the decussation of the superior cere- 

 bellar peduncle (XScp). 



LEVEL OF THE SUPERIOR COLLICULUS (FIG. 6o) 



At this level certain features peculiar to Tarsius become apparent. 

 Most conspicuous among these is the unusually large size of the entire 

 tectal region. Yet, in spite of exceptional dimensions, the superior colliculus 

 (SC) presents less stratigraphical specialization than might be expected 

 from its size. The colliculus as a whole appears to be a region extremely rich 

 in cellular elements. 



In contrast to the large proportions of the colliculus, the extremely 

 small size of the cerebral peduncle (CP) is another striking feature of 

 the midbrain. Considered together, they impart to this segment of the 

 tarsial stem a characteristic appearance different from all other primates. 

 The small size of the cerebral peduncle is dependent primarily upon the 

 meagre contributions made by the neopallium to the pallio-pontile sys- 

 tem. It is also dependent upon the relatively small volume of the pyramidal 

 system. 



Thus, the peduncle is indicative of an animal whose behavioral patterns 

 are extremely rudimentary and limited. It is another fact confirming the 

 opinion that tarsius has little capacity in the acquisition of learned skilled 

 acts and that its biological formula in adaptive reaction is determined 

 largely by its automatic associative combinations. 



