340 THE INTERMEDIATE PRIMATES 



very irregular surface marked by the entrances and exits of the various fiber 

 bundles connected with the colliculi and by the development of the lateral 

 fillet from the trapezoid body. The course of this fiber tract can be definitely 

 followed along the lateral surface of the reticular formation. 



Immediately prior to the formation of the lateral lillet the trapezoid 

 body occupies a considerable excavation in the central portion of the 

 ventral surface of the reticular formation. This bay is limited by mesial and 

 lateral extensions of the reticular formation which pass ventrally to afford 

 contact with the dorsal layer of the mesial and lateral buttresses of the pon- 

 tile nucleus. 



Cephalad to the inferior colliculus the inferior brachium makes 

 its appearance as the continuation of the acoustic pathway and produces a 

 groove on the surface of the mesencephalic reticular formation at which level 

 this fiber bundle acts as the next relay to the mesial geniculate body. 

 As the tract approaches this nuclear mass it again sinks into the reticular 

 matrix and is covered over by a lateral extension of the reticular formation. 



Dorsally the reticular formation is in close contact with the body 

 of the central gray matter, separated from it, however, by the circum- 

 ferential condensation of fibers mainly represented by the posterior longi- 

 tudinal fasciculus, the predorsal bundle and the mesial fillet. In the cephalic 

 portion of the pontile tegmentum the reticular formation is disposed in a 

 rather irregular fashion, presenting a lateral thin lamina overlying the supe- 

 rior cerebellar peduncle as it enters the upper part of the tegmentum from 

 the superior medullary velum. 



As the pontile reticular formation is succeeded by that of the mesenceph- 

 alon the circumferential layer of the reticular formation becomes relatively 

 thicker, whereas the central mass of the reticular formation diminishes in 

 bulk due to the fact that the superior cerebellar peduncle is approaching 

 the midline. 



