THE NERVOUS SYSTEM. 



505 



masses. The changes taking place are similar to those exhibited in the 

 differentiation of the spinal nerves (p. 497). The central relations of the 

 nerves of this region of the medulla are shown in Fig. 436. (Comp. Fig. 407). 

 The glossopharyngeus at the same time develops its branches, most of the 

 peripheral fibers running in the third arch (lingual branch). Somewhat later 

 (12 to 14 mm. embryo) another bundle (tympanic branch) (Fig. 435) passes for- 

 ward to the second arch. This forms the typical branchiomeric arrangement 

 in which there is a forking of the nerve into prebranchial and postbranchial 

 branches, the latter being larger and containing the efferent element (see p. 471 

 and Fig. 405). 



- Roof plate 



Alar plate 

 Fourth ventricle 



Tractus solitarius - 

 (in marginal layer) 



Efferent nu. N. X. 

 Nucleus N. XII. 



Ganglion N. X. 



Sulcus limitans 



Inner layer 



Mantle layer 



of basal 

 plate 



- Ventro-lat. column 

 (in marginal layer) 



- Floor plate 



Fig. 436. — Transverse section through the rhombic brain of a 10.2 mm. human embryo (during the 

 fifth week). X, Vagus; XII, Hypoglossus. His. 



While the ganglia of the facialis and acusticus are derived from the same 

 mass of cells (p. 502, Fig. 434) and are later still in very close apposition, it must 

 be remembered that they are totally different in character. At four weeks they 

 are differentiated from each other (Fig. 437). The relations of the two ganglia 

 are shown in Figs. 435 and 437. It is probable that the ganglion of the facial 

 (geniculate ganglion) shows an early differentiation into dorsal and ventral 

 parts similar to the ganglia of the IX, and X, and also has associated placodes. 

 The peripheral branches of the cells of the geniculate ganglion develop into the 

 great superficial petrosal and chorda tympani. Both of these nerves enter into 

 secondary relations with the V. There is some doubt as to whether the chorda 

 is a prebranchial or postbranchial nerve (Fig. 43 5 J als0 compare p. 469 and 

 Figs. 405 and 406). 



