620 THE HIGHER ANTHROPOIDS 



ill the lloor of the fourth xcntriclc. It is travcrst'd Ijv a nuinlxr of the (ibcrs of 

 thesecoiichuN cochlear tracts w hich ha\ e ah-cacl\- received rehi\ in the ventral 

 cochlear nucleus and are passing across the lloor ol the toLirth \entricle. 



The Colliculi 



These masses of gray matter in the tectum ot the mesencephalon repre- 

 sent a relatively massive accumulation ol nuclear material. The inlerior 

 colliculus appears as a specialization in the dorsal extension ol the meseii- 

 ce])halie reticular lormation which supports the colliculi at their lateral 

 extremities. Dorsally the mierior and superior colliculi are continuous with 

 the indiilerent gray matter lying in the midline. Each colliculus is connected 

 across the midline by the decussating libers respectively of the interior and 

 superior collicular commissures. These decussating hirers support the dorsal 

 gray matter and j^roduce a defmite elelt between this element and the dorsal 

 surface of the central gray matter. 



Caudallx and laterall\ the t'litrance ol the lateral lillet into the area 

 immediati'ly sub|aeeiit to tlu' mienor colliculus is elearl\ show n in the recon- 

 struction. Tlu' inferior colliculus is separated from the superior colliculus by 

 the intercollicular sulcus which is occupied by a dorsal extension oi the 

 mesencephalic n-ticular formation and a similar extension serves to se]:)arate 

 the superior colliculus from the dieiicephalon. 



The SlBSTANTIA Nir.i^A 



The substantia nigra appears in its usual situation, dexeloping Irom the 

 dorsal layer of the pontile nucleus Irom which it seems to be (k'rnetl. It is 

 supported b\ the dorsal la\ cr and, to a U'ss I'xtent than is the case in tlu' lower 

 forms, by the lateral and mesial buttresses. It is marked \fiitrall\ by the 

 longitudinal libers of the pallio-s|)inal and pallio-|)ont ilc systems. It merges 

 mcsially with tlu' interpeduncular gra> matti'r, pierced at this point 1)\ the 



