450 THE INTERMEDIATE PRIMATES 



trigcmini which is situated in the dorsolateral angle of the reticular formation. 

 Dorsal to the substantia gelatinosa this formation is covered by the fibers of 

 the columns of GoII and Burdach and gives origin to the basal portions of the 

 dorsal sensory nuclei. Mesially the reticular formation is continuous with 

 the central gray matter. Its mesial surface is separated from its fellow of the 

 opposite side by the longitudinal bundles situated adjacent to the raphe. 



The pontile portion of the reticular formation on its ventral surface 

 presents a deep excavation produced by the trapezoid body. Lateral to this 

 excavation are located two nuclear masses of moderate size, forming the supe- 

 rior olive and the lateral reticular nucleus. As the superior cerebellar peduncle 

 sinks deeper into the reticular formation, a lateral reticular prolongation 

 envelops the peduncular bundle over its lateral surface. 



In the mesencephalic portion of the neuraxis the reticular formation 

 comes to the surface laterally, dorsal to the pontile nucleus and ventral to the 

 colliculi. Somewhat ventral to the point at which the superior peduncle 

 extends into the tegmentum is the pathway of the lateral fdlet. 



The reticular formation, as it is followed further upward in the stem, 

 becomes more disseminated by the superior cerebellar peduncle. It is finally 

 separated into a mesial and a lateral portion by the appearance within it of 

 the red nucleus. At the level of this nucleus and above this level the reticular 

 formation becomes irregular and forms a matrix in which the mesial genicu- 

 late body develops. In the diencephalon the reticular formation seems to 

 merge with the zona inccrta and other less w ell-defined nuclear masses in the 

 caudal portions of the diencephalon. 



The Pontile Nuclei 



The pontile nuclei begin in the more cephalic levels of the medulla 

 oblongata by the appearance of the arciform nuclei which partially surround 

 the pyramidal tract on its ventral, mesial and dorsal aspects. This arciform 



