RECONSTRUCTION OF Hl'M AN 8,-i 



thr nu't(,'iUT|)lial()ii. In the region w Iutc the superior cerebellar jjeduncle 

 enters the brain stt'm, the pontile nucleus arri\'es at its greatest expansion, 

 lor hvw tlu' (li'cp layer ot the pontik' nut'leus swings dorsally to einbract- the 

 reticular ionnation almost as lar dorsal as the junction of the dorsal and mid- 

 dle thirds ol the brain stem. The \arious laminae of the pontile nucleus are 

 arranged somewhat conci'ntricall\ . all corres])onding to the curve of the 

 surface c)f the brain stem, and end in irregular, jagged extremities from between 

 which stream the middle c\'r(.'l)ellar i)eduncular libers as they coiuerge to form 

 the middle peduncle of the cerebellum on the lateral as])ect of the meten- 

 cephalic portion ol the brain stem. The delmite condensation of the pontile 

 nuclei to lorm the buttresses upon which rests the substantia nigra is not so 

 cU'arl\ exident m this reconstruction ol the human brain stem as it was in 

 some ol the lower lorms, on account ot the lact that the greater mass of the 

 peduncular libers ser\ es to break up the lateral buttress into a number of 

 small laminae and groups of nuclear material. 



The transition Iroin tin- pontile nucleus to the substantia nigra is so 

 gradual and indistinct that it is dillicult to sa\- at which point the deep la\er 

 of |)ontilc' nucleus and tht- \entral la\ cr ol the reticular formation cease and 

 the substantia nigra begins. In the ccjihalic portion of the metencephaloii the 

 pontile nucleus is delicient on its lateral and latt'ro-dorsal aspects, sup])lving 

 a point ot entrance lor the cerebral |)edunclc as it passes ventrolateral to the 

 substantia nigra, the pcduncU' being envelo|jed on its dorsal, mesial and 

 \ciitral surfaces but free on its lateral surface as it enters the meteiicephalon. 



The Vestibul.-\r Nuclei 



This mass ol gray matter tirst ajjpears at about the level of the opening 

 ol the lourth \entricle as the nuclc-us ol the descending root, interposed 

 between the gray matter ot the I loor ot the lourth \entricle and the diminish- 

 ing nucleus of Burdach and appearing almost as a cephalic continuation of 



