THE NERVOUS SYSTEM. 



507 



the ear is the older part phylogenetically, the cochlea being a more recent special- 

 ized diverticulum of the older structure. (See p. 599 and Figs. 512 and 513.) 



The central processes of the acoustic ganglionic mass first develop from the 

 upper part, forming the vestibular nerve root which enters the marginal layer of 

 the medulla. A portion at least of its fibers bends caudally, forming a de- 

 scending tract. The central processes of the cells of the cochlear ganglion, 

 forming the cochlear nerve root, pass dorsally, cross the vestibular ganglion and 

 enter the medulla dorsal and lateral to the vestibular root fibers (Fig. 437). 



Roof plate 



Fig. 438. — Transverse section through the rhombic brain in the region of the trigeminus (V) nerve 

 of a 10.2 mm. human embryo. a.W., Spinal V; G.G., Gasserian ganglion; V.m., efferent 

 root of V nerve. His. 



The trigeminus is the most anterior of the ganglionic masses (Fig. 434). 

 Embryological evidence has been brought to show that it consists of two or 

 more nerves which subsequently fuse. Placodes have also been described. 

 It is possible that such placodes represent those belonging to the most anterior 

 division of the lateral line system in lower forms, and probably in this case 

 would not properly belong to the V (comp. Fig. 405). From the ganglionic 

 mass {Gasserian or semilunar ganglion) the three principal branches — oph- 

 thalmic, maxillary and mandibular — are formed, the two latter passing into the 



