THE BRAIN OF MODERN MAN 81 1 



and \'(.'ntial to the nucleus ol I^Lirdach is the substantia o;cIatin<)sa trigcmini 

 ( N l-v I ni\ rstc'd on its outer surlace h\- the descenchn<i; tri^n'ininal tract ( T rd ). 

 Tins le\el represents a critK'al region m xhv bram steni. The nucleus of 

 Burdach is replact'd b\ another nuclear structure which also receixcs pro- 

 prioceptive hbcrs but in this case conung Irom the highly specialized 

 proprioceptors of the vestibular portion of the internal ear. The section is also 

 critical as showing the approach to the next higher segment of the axis repre- 

 sented i)v the pons \ arolii. The transition which is about to occur is indi- 

 cated on the ventral surlace by the appearance of the nucleus arciforniis. 

 In man this nucleus which appears to be the caudal expression of the exuber- 

 ant pontile nuclei extends much lurther downward along the axis than is 

 the case in the other anthropoids. (^See reconstruction of human stem.) 



LEVEL OF THE \ESTIBILAR NICLEI (fIGS. 346, 34-) 



At this le\el the contour of tiie section shows those changes incident to 

 the a|)pearance in the dorsal sensor\- field of the large vestibular complex 

 \\hich now- occupies the position lormerly held by the nuclei of Goll 

 and Burdach. The reliel ol this vestibular complex has pre\ious!y been 

 noted upon the lloor of the lourth xeiitricle. Its general proportions and 

 appearance in man give tiu' impression ol a structure somewhat less exten- 

 sively develo]X'd than m tlu' lower i^rimates, and particularl\ m those forms 

 which are highly specialized for arboreal life. It may be diiiicult to maintain 

 that the \estibular nuclei m man are less extensixe than m other primates. 

 On the other hand, it would seem likely that the balancing mechanism in 

 man had less ckmand made upon it than would be the case in those animals 

 \\ hich lead arboreal li\ fs. Man, w ith his more lirm supporting base upon the 

 ground, is provided with a surface which aflords him greater security. 

 There are reasons, however, why the balancing mechanism in man should 

 not undergo marked reduction. These arise from the fact that, ha\ing 



