PITHECUS RHESUS, MACACUS RHESUS 383 



already been discussed. The fibers constituting the pyramidal system are still 

 further separated by the interposition of transverse fibers and portions of the 

 pontile nuclei. The tegmentum is separated from the basis here by the 

 mesial fillet (Mf) at whose lateral extremity, and extending dorsally, 

 is the lateral fillet (Lf). Mesial to the lateral fillet is a dense bundle, 

 now somewhat scattered, the superior cerebellar peduncle (Sep), about 

 to make its characteristic swing forward and inward preparatory to decussa- 

 tion m the midbram. The reticular formation (Ref) occupies the major 

 portion of the tegmentum. It contains at this level no specialized aggregations 

 of nuclear diflerentiation. 



LEVEL OF THE INFERIOR COLLICULUS (FIG. l8o) 



Here the contour of the axis is considerably altered by the appearance of 

 two elevations on its dorsal surface. These elevations, the inferior colliculi 

 (IC), still show a marked degree of stratification. Their prominence, 

 together with this specialization, may be taken to indicate that some of the 

 primitive function over which they preside is still retained. Fibers of the 

 lateral fillet (Lf) ha\e already entered into the inferior colliculus; 

 still others are approaching it. The prominence of these tectal structures, 

 both in surface relief and histological differentiation, doubtless indicates that 

 some portion, at least, of the function of hearing still has representation in 

 these primordial centers of that special sense. It is noteworthy, however, that 

 the coefficients of this structure, as compared with the lower primates, show 

 an actual decrease in size. This fact uncjuestionably implies that the telen- 

 cephalizing process which is gradually transferring the major functional activi- 

 ties of the auditory sense to the temporal lobe of the cerebral hemisphere, has 

 made considerable advance in such animals as the macaque, as a consequence 

 of which the tectum of the midbrain has lost much of its primitive auditory 

 dominance. The central gray matter (Cen) surrounds the Sylvian aque- 



