312 THE INTERMEDIATE PRIMATES 



(Ref ). This is penetrated l^y numerous internal arcuate fibers extending 

 in long sweeping curves from the region of the nucleus of Burdach, forward, 

 inward and backward, to enter the decussation of the mesial hllet. Passing 

 diagonally through the reticular formation, and intersecting the arcuate 

 fibers, are axons of the twelfth nerve ( N 12 ) as they are making their way 

 from the hypoglossal nucleus at the ventrolateral border of the central 

 gray matter. 



LEVEL THROLIGH THE MIDDLE OF THE INFERIOR OLI\'E (fIG. 1 47) 



At this level the most conspicuous feature is the appearance of the 

 somewhat irregular mass of gray matter lying in the ventral field of the 

 section, but separated from the midline by the interposition of the pyramid 

 (Py) and the mesial fillet (Mf). This mass of gray matter is the inferior 

 olivary nucleus (10). It appears as a somewhat irregular, convoluted, 

 saccular structure ^\ hose fundus is directed toward the periphery and 

 causes a slight elevation upon the surface already referred to as the 

 olivary eminence of the oblongata. Its hilus is directed dorsomesiallv ; 

 its cavity is filled with heavily myelinized axons. Into it and through it pass 

 arcuate fibers, many of them in the direction of the restiform body on their 

 way to the cerebellum. Ventromesial to the olive is a detached portion, 

 the mesial accessory olive ( VO), and dorsal to it a small detached portion, 

 the dorsal accessory olive (DO). The significance of this large nuclear 

 mass in the oblongata has already been discussed. Its probable relation to 

 the function of controlling the simultaneous movements of head, eyes and 

 hand and of facilitating the coordination of all skilled learned performances 

 has been commented upon and the reasons for ascribing such regulation to 

 it have previously been given (see page 253). Its increase in prominence, 

 both as to size and configuration, when compared with the loA\er primates, 

 speaks for an expansion in the function over which it presides. 



