RECONSTRUCTION OF SIMIA SA IMUS 543 



is immcl in tlu- human hram stmi. It presents the usual lour an^K's: the 

 ventrolateral reeeivni^ the \cntral <ira\' eolunms, and the dorsolateral 

 rccei\in<i the dorsal gra\ eolunms. As the oljlon^^ata is approaehed the 

 eentral gray matter gixes rise to the dorsal nu'dullarx iiuelei, the nueU'us of 

 (m)II appearing shortlx alter the upper leAels ol the et'r\ leal legion are passed, 

 riu' nueleus ol Burdaeh appears at a eonsick'iahlx higlu-r le\el. As these 

 nuelei develop tlu' eentral grax' matter heeonies more llattened Irom side 

 to side and eontinuousl\ approaehes the dorsal surlaee of the stem. This is 

 very elearly show 11 h\ the diminishing anteroposterior extent ol tlu- dorsal 

 medullary nuelei. The eentral gra\ matter in the mid-ohlongatal region sends 

 out a narrow i^dlongation dorsallx which, ri'aehing the surlaee, bilureates 

 and extends laterally. The remainder ol the eentral gra\ a])parently follows 

 the li'ad of this extension, passing dorsall\- and llattening out laterally until 

 the lloor of the fourth \entricie is formed. The lloor of the fourth \entrielc 

 in the orang begins to show sonu'what more modelling than in the prec'eding 

 types. The suggestion of the hypoglossal and the \agal trigones ma\' be 

 diseerned in the lower half of the lourth \i,-ntriele. 1 lu' area aeustiea istairly 

 well dexeloped, erossed b\ the striae aeustieat'. In the u[jper portion ol the 

 xentriele the nu'dianix plaeed elexations eorresponding to the de\'elo|)ment 

 beneath them of the nueleus abdiieentis with the knee of the seventh nerve 

 are well marked. 



Above the mid-\ entrieiilar le\ el of the stem the \c-ntrieli' rather rapidly 

 narrows by tlu- approximation of its walls. As the juiietion with the mesen- 

 cephalon is ])assed, the wails ol the aqueduet ol S\ l\ius, tormed by the een- 

 tral gray matter, beeome progressixely more massi\-e. In the mesencephalon 

 itself thi' eentral gra\ is continued forward as a fairly well-defined tongue 

 l\ ing on either side i^\' \\\v nudian rapiie. in this region dexelop the nucleus 

 trochlearis and the nucleus oculoinotorius. Abo\ e the le\el ol the oculomotor 



