INTERNAL STRUCTURE OF THE BRAIN STEM 999 



acids to its motor appanitus an cciuipnimt which permits ol suspension in 

 midair and th lis frees the limd-and torehmhs tor purposes of defense, ollense, 

 gathering of food and other aelixities m whieh t\\v hancLs and feet may be 

 simultaneously emplovt'cL A s[)eeit"s tluis sjjeelalized would acquire a more 

 extensive' funelional power oi bahmemg than animals whose base ol support 

 and mode ol loenmotion are less jjreearioiis. In other s])eeies, the dimensional 

 differenees of tlu' central balanciiiij; nu'chanism arc almost negligible. At first 

 glance it seems inconsistent that the cc|uilibrat'>ry lunctioiis ot the arboreal 

 animal, which moxcs about upon such uncertain support as the branches ot 

 trees, should not be greater than that ot a ground-li\ ing Inrm, such as man. 

 ^ et there are undoubtedl\ c(|uali/.ing lactors which ser\e to ollset any tend- 

 ency toward markctl disparity. In the larger anthropoids, lor c\am|)le, the 

 attempt to assume the erect posture and to depart from an arboreal mode 

 of living introduces new equilibratorv factors. This speciali/ation reaches its 

 highest degree of differentiation in man, who, while no longer m need ot 

 balancing functions adapted to the conditions of arboreal life, has added to 

 his equilibratorv responsibilities by dcxclopmg a type ol locomntion in \\ Inch 

 he must support his body on two feet instead of four. The expansion m the 

 human balancing mechanism, although not incident to tree-climbing or tree- 

 dwelling, is, none the less, a direct response to the lactors introduced by 

 bipedal cc|uilibration. Thus, whiK' the archaic structures, re])resentiiig the 

 balancing mechanism and know n collectively as tlu' \ cstibular complex, show 

 certain limitt^d \ariations as more definite terrestrial lile is assumed, they, in 

 general, maintain a fundamental ccpiality in their coellicient expression. 1 heir 

 variations, howcxcr limited, all manifest the effects of aclaptni' inlluences and 

 are signilicant in tlu- sense that tlu'y disclose the exolut lonaix |)redisposition 

 of neural structures to adjust themselves to \ar\ing demands ol lunctional 

 specialization. 



