196 



THE ESSENTIALS OF HISTOLOGY 



third and lateral ventricles. 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. 



A section taken just above the level 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. 230, 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 (VIII.}, the bundles of which, as they 

 leave the medulla, enbrace the inferior crus of the cerebellum (c.r.), 

 which is now passing into that organ. The reticular formation still 



n.VHIp. 



FIG. 230. TRANSVERSE SECTION OF THE UPPER PART OF THE MEDULLA 



OBLONGATA. i 



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, (striae acusticae), and b, which 

 enclose the restiform body, c.r: ; n.VIII.p. principal nucleus of the auditory nerve; 

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

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



occupies the greater part of each lateral half of the medulla between 

 the grey matter at the floor of the fourth ventricle and the pyramids 

 (py), and a small portion of the olivary nucleus (o) may still be seen, 

 as may also the upward continuation of the grey matter of the tubercle 

 of Eolando ; this is intimately connected with some well-marked 

 bundles of nerve-fibres, which are passing up to the pons to join even- 

 tually the root of the fifth nerve (Fa). 



A section through the middle of the pons Varolii (fig. 231) shows 

 very much the same arrangement of grey and white matter as that 



