452 THE INTERMEDIATE PRIMATES 



upward, the superficial layer of the pontile nucleus remains in contact later- 

 ally with the ventral surface of the reticular formation of the mesencephalon. 

 As the pontile nucleus approaches the isthmus mesencephah it rapidly 

 decreases in size, leaving only the deep nuclear layer which continues upward 

 to merge directly with the substantia nigra. 



The Vestibular Complex 



The vestibuhu- complex hrst comes to view on the surface of the recon- 

 struction at the lower level of the fourth ventricle where it appears as a small, 

 triangular, wedge-shaped mass, the nucleus of Deiters. It is intercalated 

 between the lateral surface of the gray matter of the lloor of the fourth 

 ventricle and the cephalic portion of the nucleus of Burdach. The nucleus is 

 continued forward, presenting a prolongation which overhangs the sub- 

 stantia gelatinosa trigemini. It reaches its maximum at about the mid- 

 ventricular level. At this point its dorsal surface is covered by the beginning 

 development of the triangular nucleus complex which gradually increases in 

 size as the Deitersal nucleus diminishes. It passes upward iii this position 

 between the gray matter of the lloor of the fourth ventricle and the reticular 

 formation but diminishes in size as the upper pontile levels are approached. 

 The nucleus of von Bechterew is continued from the upper portion of the 

 triangular nucleus into the lateral wall of the fourth ventricle. 



The Cochlear Complex 



The cochlear complex is relatively large in contrast to that of Cyno- 

 cephalus babuin, which is relatively small. The ventral nucleus of hylobates 

 is well developed, having the same trough-like formation present in the other 

 members of the primate group. The roots of the nerves passing into the nucleus 

 are partially surrounded by the gray matter, leaving the mesial surface free. 



