TROGI.On^TES GORILLA 667 



functions over which they preside". \'ct even such archaic, fixed structures in 

 tlie brain stem show a degree of modification which is worth\ of noteascmpha- 

 sizing the fact that they, for all their primitivencss, have felt and responded 

 to the influences of progress. Thus, in passing from the lowest to the highest 

 members of the group, these archaic structures, the cranial ner\e nuclei, 

 show a steadily increasing definition in their outline as they assume more 

 distincti\e indi\ iduality and greater dimensions in each successive grade of 

 differentiation in the prnnates. 



LEVEL OF THE \ ESTIBULAH AND CEHEBELLAK NL CLEI (hIGS. 301-304) 



Here the striking feature is the replacement of the nuclei of Goll and 

 Binxlach by a large vestibular complex composed of the nucleus of Deiters 

 (ND) and the triangular nucleus of Schwalbe ( NSc). These two nuclear 

 masses have gained in definition but are considerably less prominent from 

 the dimensional standpoint than in some of the lower primates, espe- 

 cially those whose life is preeminently arboreal. The central gray matter 

 (Cen) occupies a position under the sube|)end\mal gray in the floor of 

 the fourth ventricle (Fig. 301). It contains in its most lateral portion the 

 nucleus of Schwalbe (NSc). Lateral to this is a large-celled nuclear aggre- 

 gation, the nucleus of Deiters (ND). Ventral to Deiters' nucleus is the 

 somewhat reduced substantia gelatinosa trigemini (NR) on whose outer 

 margin descends the trigeminal tract ( Trd). In a position lateral to 

 Deiters' nucleus lies the massive collection of fibers constituting the resti- 

 form body (ICP) about to form the inferior cerebellar peduncle and 

 thus consummate the connection between the cercbelluin, the s|)inal cord and 

 the oblongata. In their usual ventromesial position are the collected bundles 

 constituting the p\ramid (Py), and dorsal to it the fasciculus of the 

 mesial fillet (Mfj. The cephalic extremity of the inferior olive occupies 



