PITHECUS RHESUS, MACACUS RHESUS 385 



the superior medullary velum. Lateral to the fibers ol the trochlear nerve, on 

 the outer border of the eentral gray matter, are the scattered fibers 

 from the tractus meseneephalieus trigemini. The territory occupied by 

 the reticular formation (Ref) has at this level become somewhat 

 obscured by the libers of the superior cerebellar peduncle (Sep) which 

 are sweeping inward and forward, about to undergo their decussation before 

 coming into relation w ith the red nucleus. This decussation is shown as fully 

 developed in the next section. The boundary between the basis and tegmentum, 

 as at other levels, is indicated b\- the presence of the mesial fillet (Mf) 

 from whose lateral angle extend dorsally many of the fibers of the lateral 

 fillet ( Lf ), now making their way to the inferior colliculus. 



The basis of the section still contains some of the typical characteristics 

 of the pons Varolii in its three layers and also the disseminated condition of 

 the pyramidal fibers (Py). There is, however, a tendency for the ven- 

 tral groove of the pons to deepen and thus to foreshadow the beginning 

 divergence of the basal portioii of the axis, which ultimately occurs in the 

 cerebral peduncles where an assemblage of the cortico-pontile fibers takes 

 place and the pyramidal axons are reassembled into a single compact bundle. 



LEVEL OF THE SUPERIOR COLLICULUS (FIG. l8l) 



Here the contour of the brain stem has again undergone modification 

 due to the appearance of two fairly prominent elevations on its dorsal aspect 

 and the development of a wide cleft in the middle portion of its base. These 

 elevations are the superior colliculi (SO which represent remnants ol 

 the optic lobes of the lower vertebrates. That they are reduced in size in all 

 mammals, particularly the primates, is significant of the fact that the process 

 of telencephalization of the visual function has been steadily progressing 

 since the advent of mammalian forms. Visual function has for the most 

 part been taken over by the occipital lobe in the cerebral hemisphere, a 



