RECONSTRUCTION OF HUMAN 857 



sulcus. This l;i\cT of tiray niattiT continues to mow dorsally and then rapidly 

 expands as the fourth \entriclc opens. It is chsposed in an arched fashion 

 lati'rall\ corresponding w ith thi' opening of the h)urth MMitricle and de\-el()ps 

 into a layer of gray matter which lorms the lloor oi theh)urth \t'nt rich' and 

 is interposed between the t'pi'nd\ nial linmg oi the \entrii-le itself and the 

 dorsal medullarx luiclci. This thin layer of gray matter eoiUiiUH's upward, 

 increasing considerahlx in lateral extent, fusing with the dorsal cochlear 

 nucleus and forming a thin layer o\ t'r the triangular nucleus ot the \ estii)ular 

 complex. In its lower portion it is in direct contact xx'iitrally with the 

 li\ poglossal nucleus w hich a[jpears as the eminentia hypoglossi on the lloor oi 

 the \entricle as that structure o])ens. Lateral to this emineiux- ap|)ears 

 a de])ression which separates theem inentia hypoglossi i'rom another eleva- 

 tion, the eminentia vagi, which is ])roduced by the underlying dorsal 

 luick'us oi' the \agus. 



I^rom this jjoint uj:)ward tlu' lloor ol the lourth xcntricle is rather smooth 

 and appears as a thin sheet of gray matter extending from the median sulcus 

 of the x'entricle to the lateral xentricular wall. At about the mid-portion of 

 the ventricle another emiiu'nce appears near the midline oi the lloor of the 

 fourth \ entricle. This ele\ ation is produced b\ thede\elopment in this localitx 

 of the abducens nucleus and is calK'd the I'lninentia abducent is. These emi- 

 nentia' are relativelx high, the groo\e between them being c|uite deep. The 

 area w Inch corresponds w ith the \ t'stibular and dorsal cochlear lUicU'i, know n 

 as the acoustic area, is relati\c'l\ smooth and is continued laterallx' into the 

 lateral recess oi the lourth \entricle at the point oi entrance of the cochlear 

 dix'ision ot the acoustic ner\t'. \hv gray matter oi the lloor ol the fourth 

 ventricle is continued upward, furnishing a thin covering for the triangular 

 nucleus of Schwalbe and the nucleus ol von Bechtercw in the lateral wall of 

 the fourth xciitricle. 



The central gray matter i'ormmg the lluor of the fourth ventricle above 



