Chapter XW'I 



RECONSTRUCTION OF THE GRA\ MATTER IN THE 

 HUMAN BRAIN STEM 



"^HE reconstruction ol the ij;ra\' matter in tlu- luinian brain stem 

 re\eals thosi' modllications, expansions and elaborations which 

 repri'st'nt the cuhnliiation ol the cNolutional process traced throngh 

 the i:)rimate order. Certam kvitnres made more evident in this nu'thod ol 

 visualization deser\e particular consideration. In this regard, attenti(ui lirst 

 directs itsell to the [iontile nuclei, the grc-atest mass formed b\ any one 

 s|)ecialized structure contributing to the composition of tlu' lU'uraxial gray 

 matter. This imposing nuclear collection in man contributes a greater pro- 

 portional amount ol gray matter to the make-up of the brain stem than it 

 does m any ol the lower members of the primate group. Not only is this 

 dewiopnieiit easily recognized in the actual amount of nuclear material but 

 also in the complicated disposition ol {he lil)t'rs which arise within it, and 

 coursing through the nuclear matrix l)reak it up into a more intricate 

 nuclear mass than is seen in any of tht' otiur reprcsentatix es of this series. 

 Another im])ressive mass ol gray matter is the arcilorm nucleus, w hich 

 re|)resents a marked acKancc' o\er that seen in tiu' lowt'r forms, consisting 

 ol a layer ol gray matter almost completel\ iiucloping the \entral surface of 

 the inlrapontiU- portion ol the brain stem. The mlerior olivary complex also 

 reaches a degree of complication considerabl\ in acKance of that hitherto 

 oijst'rM'd m the ph\ lum, w hile the colliculi show tlu- hnal steps in the process 

 which has alread\ been obst-r\'ed to be in progress, the inferior colliculus 

 continuing to show a steady reduction in size and comple\it\, while the 

 sujicnor colliculus barely holds its own m comparative and absolute 



dillerentiation. 



837 



