RECONSTRUCTION OF PAPIO CYNOCEPHALUS ^ 345 



the lateral geniculate body. Mesially both of these nuclear structures are 

 continued across the midline by the respective superior and inferior collic- 

 ular commissures. 



The Substantia Nigra 



In the reconstruction, the substantia nigra appears as a relatively 

 massive nuclear collection in the pes peduncuh of the mesencephalon. 

 Situated ventral to it and leaving impressions upon its ventral surface are 

 located the pallio-spinal and pallio-pontile fiber systems. It is supported 

 ventrally by the heavy mesial and lateral buttresses of the pontile nuclei 

 and throughout its transverse extent by the deep layer of the pontile nucleus, 

 from a specialization of which it seems to take origin. It is in contact mesially 

 with the indifferent interpeduncular gray matter and is pierced at this junc- 

 tion by the emerging fibers of the oculomotor nerve. Its dorsal surface is 

 concave dorsally and is separated from the reticular formation oi the mesen- 

 cephalic tegmentum by the circumferential fiber accumulations which 

 appear in this region, coursing longitudinally and transversely through the 

 stem. At its lateral extremity it presents a nuclear condensation, from which 

 fibers pass into the mesencephalic tegmentum. Laterally it sends a prolonga- 

 tion toward the mesial and lateral geniculate bodies. As the diencephalon 

 is approached, the substantia nigra diminishes in size and seems to merge with 

 the zona incerta of the diencephalon. 



The Nucleus Ruber 



The nucleus ruber appears in reconstruction in the more cephalic por- 

 tion of the mesencephalic tegmentum and is encapsulated by the fibers 

 received from the decussation of the superior cerebellar peduncle. It is 

 fairly well developed and of moderate size. Its cephalic extremity approaches, 

 if it does not fuse with, the reticular formation of the diencephalon. 



