TROGLODYTES GORILLA 663 



tlu' brain stem in gorilla aliorcl a basis for cstiinating the accessions to the 

 N'oluntary motor control h\ tlu' pidportioiis ol the |j\raniitls. They also dis- 

 close the relative sizes and hence the relati\e runctional significance of the 

 three elements t'litermg into the dorsal sensory held, re|)rt'senting collectivelv 

 the entire inllu.x of sensor\ impulses pertaining to discriminative sensibility. 



LEVEL OF THE MIDDLE OF IMF IXI ERIOK OLI\E (IIG. 300) 



At this level certain familiar changes have appeared incident to the 

 alterations in the dorsal held cansed b\ the opening of the fourth \entricle 

 with the gradual conversion ol the central gray mattt'r into the lloor of 

 this chamber ol tin- brain. Important also is the a|)pearance of the now 

 lull\' dcxelopcd inlerior olnarv body ( 10), togctlu'r with its two major 

 accessory portions, the \eiitral accessory olive (VO) and the dorsal accessory 

 olive (DO). 



The general contour ot the cross section is much altencl by the appear- 

 ance oi tlu' lourth Ncntricle. Upon either side ol the \ cntricle, the dorsal para- 

 median sulcus separates the cephalic extremity ol the much atttnuated 

 nucleus of Goll (NG) from the large adjacent nucleus of Burdach ( NB). 

 The latter iuicIclis is m turn separated by the dorsolateral sulcus Irom the 

 tuberculum trigemmi whose prominence ui)on the surlace now has been 

 bscured by the dorsal migration ol the libers entering into the restiform 

 )dy. The post- and preoli\7ir\- sulci are well delined, separating on the 

 one hand the olivary body Irom the corjjus restilorme, and on the other 

 the pyramid and olive. The central gray matter (Cen) spreads out in a 

 \-shape, lorming tlu' lowt-r portion ol the lloor ol the lourth \entricle. In 

 its most mesial portion is the nucleus of the twelfth nerve ( Nhy), from 

 which pass the liljcrs of the hypoglossal nerve. Immediatclx adjacent to the 

 nucleus of the tweltth nerve in the central gray matter is a collection of gray 

 matter which receives incoming libers of the pneumogastric nerve; this is the 



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