HYLOBATES HOOLOCK, THE GIBBON 441 



position are fibers forming tlie posterior longitudinal fasciculus ( PL) and 

 the fasciculus predorsalis. 



Tiie tegmentum at this level is separated from the basis by the libers of 

 the mesial fillet ( Mf ). The superior cerebellar peduncle (Sep) is about to 

 make its decussation prior to entering the red nucleus. 



LE\EL OF THE SUPERIOR COLLICULUS (PIGS. 20g, 2 1 o) 



At this level the configuration of the brain stem again shows some 

 conspicuous alteration. This is chiefly due to the appearance of the two 

 elevations in the dorsal region of the midbrain which form the superior coll 

 liculi (SC). In a ventral position appears a divergence of the two cerebra- 

 peduncles which forms the caudal boundary of the optico-peduncular space. 



The superior colliculus in the gibbon, while somewhat less prominent 

 than in the lower primates, even than in baboon or macacus, still retains a 

 degree of its previous specialization. Traces of stratification may yet be 

 detected in it. The central gray matter (Cen) surrounds the small ventric- 

 ular space of the aqueduct of Sylvius and contains the nuclear specializa- 

 tion forming the nucleus oculomotorius (Noc). Near the median raphe 

 many fibers from this nucleus cross to the opposite side, forming the oculo- 

 motor decussation. The nucleus ruber (NRu), whose large size and clear 

 definition are in contrast with all other forms heretofore discussed, occupies its 

 characteristic position in the tegmentum. Because of its much greater defini- 

 tion and its more apparent emergence from the surrounding reticular forma- 

 tion, it seems to show some further advance in the development of that 

 system which has control over the coordinative regulation of movement. On 

 the other hand, the functional duality of this nucleus as a relay station should 

 be borne in mind. It is possible that its increments, both in definition and in 

 size, are due to augmentation from the striatal structures of the endbrain 

 rather than to an increase of fibers arising in the cerebellum. In all probabilit}-, 



