THE CEANIAL KERVES. 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 gfir; beyond it the nerve divides into a set of 

 branches which supply the three hinder branchial clefts, and 

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

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

 the mouth of the tadpole is formed. 



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

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

 from which the pneumogastric nerve is developed, the roots of 

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

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

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

 pneumogastric ; it then runs forwards and outwards, round the 

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

 first 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 praebranchial 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 swellin g, which is continuous with the inner wall o f 

 t he auditory vesicle fr om its very earliest appearance. In the 

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

 the auditory nerve divides into separate branches supplying its 

 several parts. (Gf. Pig. 75, viii.) 



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