690 THE HIGHER ANTHROPOIDS 



is in contact witli tiic central <i;ra\ matter hut cicnuircatcd I'rom It by the 

 circumferential liher Inindles ot the retieuhir h)rniati()n. 



The reticuhir formation is a massive structure in the tegmentum ol tlie 

 metencephalon. It pro\idcs a matrix m whicli dcNclop the \arious nuclear 

 specializations characteristic of the metencephalic tegmentum. It also allords 

 a sup])orti\e medium through w liich pass many longitudinal fasciculi. 



In the metencephalon the retiCLilar lormation is in contact with the cK'e]) 

 layer of the pontile nuclei. Here its ventral surface fuses with the dorsal por- 

 tion of the mesial and lateral buttresses of the pontile nuclei. 



Between these two points of contact with the pontile nucleus, the reticu- 

 lar formation is separated from the basis pontis by longitudinal liber bundles. 



In the Lipper ]jontilc levels the reticular formation becomes superlicial- 

 At the point where the superior cerebellar peduncle begins to sweep forward 

 in the isthmus mesenccphali a reticular prolongation extends lateral to the 

 peduncle, between the latter and the surface of the isthmus. This prolongation 

 gradually extends dorsally, covering the superior cercl)ellar peduncle with a 

 heavy investment of reticular substance. At length the superior cerebellar 

 peduncle is completely surrounded by reticular formation. The cerebellar 

 brachium then passes into a tunnel ol the reticular lormation which extends 

 toward its jjt'duncular decussation. A similar prolongation ot the nu'sence])h- 

 alic ri'ticular lormation presents itself at the transition of the trapezoid body 

 into the lateral lilU't. At (irst the lateral fillet lies superlieially upon the outer 

 surface of tlu' reticular formation. It then penetrates this structure, to pass 

 upward toward tlu' lateral extremit\ ol tlu' interior colliculus. 



These dorsal extensions of the ri'ticular lormation form a smooth, con- 

 tinuous layer situated close to the |:)t'riphery ol tlu' brain stem. The \entro- 

 lateral reticular angk's are continued forward and come into contact w ith the 

 deep la\i'r of the pontile nucleus. Dorsally these lateral extensions recen e the 

 lateral extremities of the coUiculi. 



