232 



THE ESSENTIALS OF HISTOLOGY. 



The floor of the fourth ventricle is covered by a layer of ciliated 

 epithelium-cells, continuous below with those lining the central canal, 

 and above, through the Sylvian aqueduct, with the epithelium of the 

 third and lateral ventricles. The epithelium rests upon a layer of 

 neuroglia known as the ependyma of the ventricle. The fourth ventricle 

 is roofed over by a thin layer of pia mater, with projecting choroid 

 plexuses, the under surface of which is covered by a thin epithelial 

 layer continuous at the side with the ciliated epithelium of the floor. 

 The roof becomes somewhat thickened as it is continued into the 

 ependymal layer of the floor of the ventricle; this thickened part 

 (tcenia or ligula, figs. 259, 260, t) is often left attached when the thin 

 epithelial roof is removed along with the pia mater which covers it. 



n.PZZZp. 



nVlfiai 



FIG. 262. TRANSVERSE SECTION OF THE UPPER PART OP THE MEDULLA 

 OBLONGATA. (Schwalbe. ) . 



py, pyramid ; o, olivary nucleus ; V.a., ascending root of the fifth nerve ; VIII., inferior (pos- 

 terior) root of the auditory nerve, formed of two parts, a, dorsal, and b, ventral, which 

 enclose the restiform body, c.r.; n. VHLp., one of the nuclei of the eighth nerve; n. VIH.ac., 

 accessory nucleus ; g, ganglion-cells in the dorsal root ; n.f.t., nucleus of the funiculus 

 teres ; n.XII., nucleus of the hypoglossal ; r, raphe. 



A section taken through the uppermost part of the olivary prominence will 

 still show very much the same form and structural arrangements as 

 that just described. The nucleus of the hypoglossal (fig. 262, n. XII.) 

 is still visible in the grey matter of the floor of the ventricle, but the 

 nerve which is now seen arising from the outer part of that grey 

 matter is the eighth or auditory (J^llL), the bundles of which, as they 

 leave the medulla, embrace the inferior crus of the cerebellum (corpus 

 restiforme, c.r.), which is now passing into that organ. The origin of 



