592 THE HIGHER ANTHROPOIDS 



The reticular lurniatiori I R c I ) is c\U'nsi\c' l)iit presents no marked 

 specialization at this le\eL It is i^enctratecl l)\ nian\ internal aicuate libers. 

 Some of these arcuate libers occupying a more dorsal position in the reticular 

 formation take tht'ir origin in the Dcitersal nucleus and undergo decussation 

 to form tht' crossed Deiterso-spinal tract. Other decussating libers in a more 

 ventral position pass inward to iorm the caudal [jortion oi the corpus 

 trapezoideum. This level is significant as showing the general tendency 

 ()bser\ed in the alferent course of all sensory path\\a\s to undergo decussa- 

 tion. L'sualK the crossing of such pathways trom one side to the other takes 

 place in close relation to their nuclear origins. Such is the case with the 

 pathway serving for alferent im|)ulses from the upper extremity and trunk 

 and the lower extremity and tail. I^Or the impulses receixed trom the pro- 

 prioct'ptixe structure of the internal ear, namely, tlu' utricle, saccule and 

 semicircular canals, a similar decussation takers place immediately alter 

 relay in the primary receiving stations of Deiters' and Schwalbe's nuclei- 



LE\EL THROUGH THE PONS SHOWING THE EMERGENT FIBERS OF THE SI.XTH 

 NERVE, NER\US ABDUCENS (FIGS. 263, 264) 



At this level the contour of the section has undergone a marked change, 

 due to the presence of the massive structures constituting the pons \ aroiii. 

 The pons presents its characteristic three layers w hich consist ol tiie stratum 

 superliciale, the stratum complexum containing trans\-erse pontile libers, 

 the ponlik' nuclei and the disseminated bundles of the pxramidal tract, 

 and the stratum profundum consisting iargel\ of trans\t'rst' pontik' libers. 

 Passing throLigh the st'wral strata of the |)ons are scattered libers making 

 their way forward to a point of t'lnergence at the iunction ol the pons 

 Varolii and the oblongata. These are the emergent libers ol the abduceiis 

 nerve which suppl\ tlu' external rt'ctus muscli's ol the t\ve (N6). The 

 massive size of the pons, part lcularl\- tlu' large xolume ol the pontili' nuclei 



