432 THE INTERMEDIATE PRIMATES 



system is here represented by the nucleus triangularis of Schwalbe (NSc) 

 and the nucleus magnocellularis of Deiters (ND). Both of these nuclear 

 structures are of relatively larger size than in most other primates and indi- 

 cate a balancing mechanism commensurate with the locomotor specializa- 

 tion of the gibbon's arboreal life. 



The fibers of the vestibular division oi the acoustic nerve penetrate 

 the substantia gelatinosa trigemini (NR) to enter Deiters' nucleus. Lateral 

 to Deiters' nucleus is the corpus restiforme (ICP) superposed upon 

 which is the tuberculum acusticum (Tub ). The circumferential area is now 

 occupied largely by the ventral spinocerebellar tract (Gow ), mesial to which 

 is the heavy bundle of the descending trigeminal tract (Trd). The inferior 

 olivary nucleus with its accessory bodies (10) occupies its characteristic 

 position and has its usual form. There is evident in it a slightly greater tend- 

 ency toward convolution than observed in lower primates. Its definition is, 

 however, somewhat more hazy than is true of higher members of this order. 

 Dorsolateral to the olive is the central tegmental tract (Ctt), important 

 because of its probable connections with the oculomotor nuclei and the 

 mesencephalic root of the trigeminal nerve. This mesencephalic root, accord- 

 ing to good authority, represents the proprioceptive conduction from the 

 eye muscles. In this light the central tegmental tract may be regarded as 

 the intersegmental link between the primary midbrain nuclei which receive 

 afferent stimuli from ocular muscles, and the inferior olivary nucleus. 

 Immediately dorsolateral to the olive and mesial to the central tegmental 

 tract is the spinothalamic tract ( Spt ), while ventral to the substantia gela- 

 tinosa trigemini (,NR) is the rubrospinal tract ( Rst). The reticular forma- 

 tion (Ref) is relatively large. Ventromesial to the olive is the pyramid 

 ( Py) and dorsal to it in succession toward the lloor of the ventricle are the 

 mesial fillet (Mf), the predorsal fasciculus ( PD) and the posterior longi- 

 tudinal fasciculus (PL). 



