126 THE LOWER PRIMATES 



The central gray matter is quadrilateral in outline surrounded by the 

 collicular eminences and contains a central canal considerably elongated 

 in the dorsdventral direction. It is of interest in this connection to note a 

 numl^er of small diverticula connected with the central canal. Such appear- 

 ances are common in the adults of many subprimate mammals, but become 

 less conspicuous in the higher members of the primate group. Ontogenetically, 

 these evaginations are boubtless related to those extensions of the Sylvian 

 aqueduct observed in manjf Io\\er mammals and connected w ith the de\'elop- 

 ment of both the inferior and superior colliculi. Traced further back they are 

 doubtless remnants of the phyletically ancient ventricles of the optic and 

 auditory lobes of the midbrain. 



In the ventral aspect of the gray matter is the trochlear nucleus (Ntr), 

 which is notable because of its indefinite boundaries and the profusion of 

 fibers interspersed throughout its entire mass. It is a nuclear collection of 

 unusual size, suggesting that the fourth nerve in tarsius must play an impor- 

 tant role. In conjunction with the closely set, protruding eyes, it may be that 

 convergence of the visual axes has assumed great physiological prominence 

 in these animals. Their acquisition of binocular vision has doubtless already 

 set on foot those specializations which culminate in the development of 

 stereoscopic function. On the other hand, the absence of a retinal macula 

 might call the validity of this view in question. However this may be, it is 

 clear that the tarsiers need and employ the superior oblique muscle of the 

 orbit in such a manner as to require a large nucleus for its innervation. 



The course of the trochlear nerve is of interest because in its emergence 

 it departs somewhat from the course pursued in other primates. In emerging 

 from its nuclear origin, it passes immediately dorsad following a path 

 restricted to the central gray matter. It has no descending portion character- 

 istic of members of this group. It leaves the dorsal rather than the dorso- 

 mesial aspect of its nucleus. Its entire course gives the impression of 



