662 THE HIGHER ANTHROPOIDS 



olivary fissure. A small intermediate suleus in this region demarcates the 

 position of the bundle of libers constituting the ascending cerebellar tracts, 

 particularly tlu' dorsal spmocerebellar tract (Fie). These delnniting sulci 

 separate corresponding ele\ations noted on the external surface. The most 

 ventral of these elevations is the pyramid ( Py). Lateral to it is the olivary 

 eminence. The lirst elevation on the dorsal surlace is the tubercuhini 

 trigemini structurally corresponding to the substantia gelatinosa trigemini 

 (NR). In the dorsal lields are two other cininences, the cuneus (CB) and 

 the clava (CG ). Especial attention is called to these elevations and then- 

 demarcation upon the surface of the axis as this definition of outline appears 

 increasingly more pronounced in the brain stem of the higher |:)riniates. It 

 reaches its most marked prominence in man. 



The central gray matter (GEN) appt'ars considerably- larger and 

 more (|uadnlateral m outline than in lower le\els. The entire mass has 

 migrated into a more dorsal position preparatory to the opening oi the 

 fourth ventricle. Both tlu' nucleus ol Goll (NG) and the luicleus ol 

 Burdach (NB) have increased in size and atford an opportunity for 

 estimating the relative sizes ol the two ma|or nuclei m the dorsal sensory 

 Held. The inecjuality of the two columns of Cioll and Burdach is still appart-nt, 

 thus indicating a greater sensory inllux from the arm, lorearm and hand than 

 from the lower extremity. The substantia gelatinosa of Rolando (, N R ) 

 occupies the most lateral position in the dorsal held and is surrounded by 

 the descending trigeminal tract (Trd). The rtlative size and importance 

 of the i^yramidal system may be estimated by tlu' dimensions of the pyramid 

 (Py). Some of the crossing libers ha\e already assumed their typical 

 descending position in the dorsolateral area. The caudal extremitx ol the 

 hypoglossal nucleus is seen located intlu' xi'iilral portion ol the central 

 gray matter and arising from it sonu' ol its lowermost libers make tht'ir way 

 outward toward the inferior olivary nucleus. 1 he more caudal sections ol 



