m'LOBATES HOOLOCK, THE GIBBON 435 



icance is equally important. It indicates the pronounced accessions in coordi- 

 native control ^vhich tliis more advanced primate has acquired in response 

 to its specialized arboreal locomotion. This detail of its neural organization 

 approaches much nearer to the higher anthropoids than any of the inter- 

 mediate primates. 



Dorsomesial to the dentate nucleus is the nucleus fastigii (Nfg), while 

 the inferior vermis projects into and fills most of the fourth ventricle. 



LEVEL OF THE EMERGENCE OF THE SLXTH CRANLA.L NERVE (fIG. 20j) 



At this level the most important features in the section are the abducens 

 nucleus (Nab) situated in the iloor of the fourth ventricle, the superior 

 olive (SO) connected with the secondary cochlear pathway, and the 

 appearance of the transverse fibers constituting the trapezoid body. 

 The pyramid (Py) occupies its usual position in the ventromedian portion 

 of the section, while dorsal to it, stretching across the section in a trans- 

 verse manner, are many fibers connected with the secondary cochlear path- 

 way of the trapezoid body. At the lateral extremity of this is a nuclear 

 collection which forms the superior olive (SO), from which many fibers 

 make their way to the nucleus abducentis. These latter constitute the 

 peduncle of the superior olive, the fibers of which participate in reilex acts 

 necessary to adjusting lateral gaze to the direction from which sudden sounds 

 may arise. The importance of such a direct reflex mechanism is apparent 

 when the needs of instantaneous visual detection are considered in relation 

 to acts of self-protection. Mesial to the nucleus abducentis (Nab) are the 

 fibers which form the second part in the emergent course of the facial nerve. 

 Other fibers constituting the fourth part of this nerve sweep forward and 

 outward, mesial to the substantia gelatinosa ( N R). The central gray matter 

 occupies the floor of the fourth ventricle whose roof is formed by the vermis 

 of the cerebellum. 



