PAPIO CVNOCEPHALUS 325 



peduncle (Mcp) which passes into the cerebeUum to terminate in the lateral 

 loiaes of this organ. The three layers of the pons, including the pontile nuclei 

 and the scattered bundles of the pyramidal system, constitute the basis poiitis. 

 The dorsal boundary of the base of the pons is established I)y the transversely 

 disposed fasciculus forming the mesial fillet ( Mf ), dorsal to which is the 

 Iciimeiitum pontis. In the dorsal region of the section, immediately lateral to 

 the much constricted fourth ventricle, is the superior cerebellar peduncle, and 

 dorsal to this the uncinate tract of Russel (Tur ). A more heavily myelinized 

 bundle lateral to this fasciculus is the ventral spinocerebellar tract on its way 

 toward the vermis of the cerebellum (Gow). Some fibers of the motor root 

 ol the trigeminal nerve emerge from the nucleus masticatorius (NM). 



LEVEL OF THE INFERIOR COLLICULUS (pIGS. I52 AND I 53) 



At this level certain features make their appearance which alter the 

 external configuration of the brain stem. Most prominent among these is 

 the extreme narrowing of the ventricular cavity to form the beginning of 

 the Sylvian aqueduct, while just above the roof of the aqueduct rise two 

 elevations, one upon cither side of a deep median sulcus. These structures 

 constitute the inferior colliculi ( IC), distinguishing characteristics of the 

 quadrigeminal plate of the midbrain. Ventrally the convex contour of the 

 section is undergoing some alteration due to the appearance of a deep 

 median groove which tends to separate the two halves of the pons Varolii 

 in the median line. This groove eventually becomes deeper in the next 

 succeeding section to form a depression, the beginning of the interpeduncular 

 space which separates one cerebral peduncle from the other. The separation 

 is foreshadowed in this section on the ventral surface of the pons, where it is 

 possible to discern the beginning of the right and left cerebral peduncles. 

 The stratum superficiale pontis and the stratum profundum pontis are well 



