lo- 

 1 



542 THE HIGHER ANTHROPOIDS 



iilar commissures which join the inrenor conifuli and tlu' superior culheiih' 

 respectively. The sujx-rior collieuhis is separated Irom the dieneei)halic 

 structures by another dorsal extension of {\\v nieseneephanc reticular 

 formation. 



The Substantia Nigra 



The substantia nigra appears at tiie transition between the meten- 

 ccphalon and the ineseneephalon. It seems to be a speeiahzation from tiie deep 

 layer of the jjontile nuclei and is supported by the lateral and mesial but- 

 tresses. Its ventral surface is in direct relationship with the descending pall 

 spinal and pallio-pontile systems ol hbers. Mcsiallx' it is eontiiiuous wit 

 the indilferent interpeduncular gray matter from which it is separated by the 

 emerging fibers of the oculomotor nerve. It expands laterally as far as the 

 reticular bed containing the mesial and lateral geniculates. Dorsally it lies 

 in relationship with the ventral surlace ol the mesencephalic Testicular 

 formation. As the dicncejjhalon is approached it diminishes m extent and 

 finally disappears, seeming to gi\ e place to the zona incerta. 



The Nucleus Ruber 



The nucleus ruber appears at about the le\el ol the oculomotor nucleus 

 as a roundi'd, well-demarcated speeiali/ntion in the mesial portion of the 

 mesencephalic reticular tormation. It rapidl\ incri-asi.'s in si/.e to its maximum 

 and then as rapidly diminishes. It is surrounded at all points and separated 

 from the rt'tieular formation by the encapsulating libers derived from the 

 superior cerebellar peduncle. 



The Central Grav Matter 



The central gray matter has already btin described in conncx'tion with 

 the upper cervical levels and there presents the ([uadrilateral form which 



