384 THE RABBIT. 



these three nerves in the rabbit. From the time when they can 

 be clearly recognised, about the eleventh day, their course and 

 relations are the same as in the adult. 



V. The trigeminal or fifth cranial nerve. This nerve can 

 be recognised on the ninth day, at a stage immediately after 

 closure of the brain has been effected. The nerve appears at 

 this stage as a pyriform ganglionic mass, the narrow end of 

 which is in close contact with the dorsal surface of the brain, 

 while the rest of the nerve lies close alongside the brain, 

 extending about half way down its side. By the tenth day the 

 roof of the hind-brain has widened very greatly, and the root of 

 the fifth nerve is now attached to the junction of the thin roof 

 and thickened side of the brain. Before the end of the tenth 

 day, the nerve branches distally ; and by the twelfth day the 

 ophthalmic, maxillary, and mandibular branches are of con- 

 siderable length, and have courses and relations very similar to 

 those of the same nerves in a five-day chick embryo (cf. 

 Fig. 115). 



VII and VIII. The facial or seventh, and auditory or eighth 

 cranial nerves are, as in the chick, very intimately connected, or 

 actually continuous with each other, from their first appearance. 

 They can be recognised before the end of the ninth day ; and by 

 the tenth day the facial nerve has acquired its definite relation 

 to the hyoicl arch, as in a five-day chick embryo (Fig. 115). 

 The auditory nerve appears to be continuous with the epi- 

 thelium of the auditory vesicle from its earliest appearance. 



IX. The glossopharyngeal, or ninth cranial nerve, as in the 

 chick, arises by multiple roots from the side of the medulla 

 oblongata, immediately behind the auditory vesicle. Its main 

 stem lies in the first branchial arch. 



X. The pneumogastric, or vagus, or tenth cranial nerve has, 

 in the rabbit embryo of the twelfth day, almost exactly the 

 same course and relations as in a chick embryo of the fifth day 

 (Fig. 115). It arises by a considerable number of roots from 

 the side of the medulla oblongata, behind the roots of the glosso- 

 pharyngeal nerve ; the most anterior of the roots of the pneumo- 

 gastric nerve is, as in the chick, directly continuous with the 

 posterior root of the glosso-pharyngeal nerve ; while the hind- 

 most root of the pneumogastric is of great length, and runs 

 backwards along the side of the medulla oblongata to the gan- 



