856 MAN 



lU'uial tissue of the substantia nigra appears. It is supported by the pontile 

 nucleus, the formation of the two buttresses ah-eady mentioned not being 

 nearly so delinite in man as it is in some of the lower forms. The substantia 

 nigra rapidly increases in bulk and spreads laterally, its deep surface being 

 separated from the tegmental reticular formation by the beginning conden- 

 sation of libers which in the diencephalon will form the fields of Forel. 

 Laterally the substantia nigra is continued outward into two definite pro- 

 longations which are producc^d by the appearance of a mass of white fibers 

 in the lateral jjortion of the substantia nigra, arising from a definite nuclear 

 condensation. In the interpeduncular region the mesial extremity of the sub- 

 stantia nigra is separated from its fellow of the opposite side by the indiflerent 

 gray matter forming the interpeduncular space, w hile the reticular formation 

 as it gradually becomes reconstituted alter the decussation of the superior 

 cerebellar |)eduncles moves forward and then swings laterally to parallel the 

 dorsal surface of the substantia nigra thus completely separating the sub- 

 stantia nigra from the nucleus ruber. 



The Nucleus Ruber 



This large mass of gray matter appears in the ventromesial portion of 

 the mesencephalic tegmentum aboxc the ca\ Ity excavated from the reticular 

 formation by the decussation of the superior cerebellar peduncles. It is a dis- 

 tinct globular nuclear mass and ra|3idi\ increases in siz.e, presenting the 

 spherical a|>ix'arance so typical of it and becoming the most striking structure 

 in the mesencephalic tegmentum. It is rouiuk'd m form and completely sur- 

 rounded by a capsule or mantle of w hilt' hbtrs w Inch separates it from the 

 reticular formation. 



The Central Gray Matter 



A ihm lannna of gra\ matter is set'ii t'xtt'iuling l)ackward from the 

 central gray matter as a narrow tongue lining each side of the dorsomedian 



