i 



THE CRANIAL NERVES. 131 



as that of the dorsal root of a spinal nerve, is from the first of 



considerable length horizontally. About the time of opening of 



the mouth the root divides into two ; an anterior one, which runs 



nearly straight outwards from the brain ; and a posterior one, 



which runs very obliquely forwards, to join the anterior root just 



before it reaches the ganglion. The ganglion lies immediately 



behind the ear; beyond it the nerve divides into a set of 



ranches which supply the three hinder branchial clefts, and 



set of visceral branches, which run to the heart and alimen- 



ry canal. All these branches are well established by the time 



he mouth of the tadpole is formed. 



IX. The glosso-pharyngeal, or ninth cranial nerve, is formed 



om the part of the neural ridge immediately in front of that 



om which the pneumogastric nerve is developed, the roots of 



he two nerves being at first continuous with each other. The 



erve is very similar to the pneumogastric, but of smaller size : 



the upper part of its course it lies immediately in front of the 



neumogastric ; it then runs forwards and outwards, round the 



inder border of the auditory vesicle, to the upper edge of the 



rst branchial cleft, where it expands to form the ganglion. 



The ganglion, like that of the pneumogastric, is formed in part 



from an independently arising thickening of the external epi- 



blast, which fuses with the nerve rudiment about the time of 



hatching of the tadpole. The ganglion of the glosso-pharyngeal 



nerve is separated from that of the pneumogastric by the anterior 



cardinal vein. Beyond the ganglion the glosso-pharyngeal 



runs downwards, as a slender nerve, along the anterior edge of 



the first branchial arch, giving a small prasbranchial branch to 



the hyoid arch. All the main branches are present at the time 



of opening of the mouth of the tadpole. 



VIII. The auditory, or eighth nerve, arises from the neural 

 crest, in common with the seventh nerve, opposite the middle of 

 the auditory vesicle ; the two nerves being absolutely continuous 

 with each other up to the time of formation of the mouth. 

 The auditory portion of the combined nerve forms a large 

 ganglionic swelling, which is continuous with the inner wall of 

 he auditory vesicle from its very earliest appearance. In the 

 later stages, as the various parts of the ear become differentiated, 

 the auditory nerve divides into separate branches supplying its 

 several parts. (Of. Fig. 75, vnL) 



K 2 



