PITHECUS RHESUS, MACACUS RHESUS 379 



extends backward and inward, apparently terminating in the large nuclear 

 mass situated on the lloor of the fourth ventricle. This hitter is the nucleus 

 abducentis (Nab) from which arise the libers of the sixth nerve to supply 

 the external rectus muscle of the eyeball. They pass successively through 

 the reticular formation ( Ref), the trapezoid body (Trp) and the stratum 

 superficiale pontis. The fiber connections between the superior olive and 

 nucleus of the sixth nerve constitute the superior olivary peduncle. Axons 

 from the superior olive to the sixth nerve nucleus serve the purpose of turn- 

 ing the eyes rellexly toward the side from which the animal may hear a 

 sudden or unexpected sound. The immediate dellection of the gaze toward 

 the source of possible danger would thus be of service in the quick detection 

 of an approaching enemy or other unfavorable factor. 



Dorsomesial to the nucleus of the sixth nerve, lying between it and the 

 subependymal gray matter on the lloor of the ventricle, is a dense bundle 

 of fibers constituting the second portion of the facial nerve (N7) in its 

 intramedullary course. The beginning of the third portion of this nerve, the 

 genu facialis, lies immediately dorsal to the abducens nucleus, while the 

 fourth division of the facial nerve forms a long bundle of heavily myelinized 

 fibers extending obliquely forward and outward between the nucleus of the 

 seventh nerve and the substantia gelatinosa trigemini (NR). 



Extending dorsolaterally from the substantia gelatinosa of Rolando is 

 a radiating fasciculus which makes its way backward and inward toward the 

 angle of the fourth ventricle where it apparently terminates in a heavily 

 myelinized bundle of libers, the fasciculus mesencephalici trigemini. The 

 fourth ventricle is much reduced preparatory to transition into the Sylvian 

 aqueduct. Above it lies the superior medullary velum upon which rests the 

 lingular lobule of the cerebellum (Lin). Dorsal to the mesencephalic root 

 of the fifth nerve is a dense bundle of fibers constituting the superior 

 cerebellar peduncle (Sep). 



