426 THE INTERMEDIATE PRIMATES 



the median raphe. In their passage they ahiiost completely separate the 

 central gray matter (Cen) from connection with the substantia gela- 

 tinosa of Rolando (NR). The central gray matter has manifested decisive 

 alteration. It is now almost quadrilateral in outhne, with a narrow dorsal 

 extension reaching backward into the dorsomedian septum. This tongue- 

 lil:e process indicates the prehminary rearrangement incident to the open- 

 ing of the fourth ventricle. In the lateral field the circumferential zone is 

 characterized by the usual dorsal migration of the spinocerebellar tracts. 

 The dorsal spinocerebellar fasciculus (Fie) has changed its course to an 

 obhque passage along the outer side of the descending trigeminal tract where 

 its heavily myelinized fibers may be readily seen. The intermediate zone 

 contains the rubrospinal ( Rst), the spinothalamic (Spt) and the Deiterso- 

 spinal (DT) tracts. The ventral held contains the fibers which constitute 

 the pyramid (Py), dorsal to which are the bundles of the posterior 

 longitudinal and predorsal fasciculi. The internal arcuate libers, as they 

 cross the raphe to form the mesial fillet (Mf), somewhat obscure the 

 outlines of the predorsal and posterior longitudinal bundles. Dorsolateral to 

 the pyramid is a slender body of gray matter, the caudal extremity of the 

 inferior olive (10). An extensive, diffuse reticular formation (Ref), lies 

 between the olivary body and the nucleus of Rolando. It is traversed by 

 numerous internal arcuate fibers and contains several indefinite nuclear 

 aggregations, one constituting the lateral reticular nucleus and a second, the 

 dorsal reticular nucleus. 



A comparative survey of the six species of primates thus far considered 

 must make impressive the striking similarities which characterize the cor- 

 responding cross sections of the brain stem. Level for level in lemuroid and 

 anthropoid, the component structures are remarkably identical. So alike are 

 they that the repeated descriptions of the several levels in the different 

 species creates a degree of tedium felt by the reader no doubt as much as by 



