PITHECUS RHESUS, MACACUS RHESUS 381 



The central gray matter (Cen) surrounds the much reduced ven- 

 tricular space which is here approaching the caudal orifice of the Sylvian 

 aqueduct. The major portion of the tegmentum is occupied by the reticular 



FIG. 178. MACACUS. LEVEL THROUGH THE AUDDLE OF THE PONS VAROLIL 

 CEN, Central Gray Matter; ctt, Central Tegmental Tract; GOW, Ventral Spinocerebellar Tract; lf. Lateral 

 Fillet; mcp. Middle Cerebellar Peduncle; mf, Mesial Fillet; nbe, Nucleus of Bechterew; nr, Nucleus of 

 Rolando; N5, Trigeminal Nerve; N7, Facial Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fascicu- 

 lus; PN, Pontile Nuclei; pns. Pons; py. Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract; scp, 

 Superior Cerebellar Peduncle; spt. Spinocerebellar Tract; trd. Descending Trigeminal Tract; tlr, Tractus 

 Uncinatus of Russel (Hook Bundle). (Accession No. 149. Section 280. Actual Size 21X10 nim.| 



formation (Ret) ni which are many transverse decussating iibers, some 

 of which at least appear to take origin in the substantia gelatinosa trigemini 

 (NR), passing inward, it may be, as a secondary pathway for the con- 

 duction of impressions from the face and head. These fibers appear to decus- 

 sate, as is the case of all sensory axons, in the median raphe. 



LEVEL OF THE EMERGENCE OF THE FOURTH OR TROCHLEAR NERVE (fIG. I 79) 



At this level, the neuraxis passes through its isthmial portion at or near 

 the junction between the hindbrain and the midbrain. In its roofplate, which 



