NERVOUS SYSTEM. 63 



the time of metamorphosis. The hyomandibularis soon divides 

 into an anterior or mandibular branch and a posterior division, 

 which supplies the muscles of the gill cover, and some of those 

 of the jaw. When the first visceral cleft or spiracle is present, 

 this division takes place just above it, so that one branch (man- 

 dibularis) is pre-trematic, i.e., is in front of the opening, the 

 other being post-trematic (Fig. 61). The mandibularis goes to- 

 the lower jaw; and one of its branches, which unites with the 

 mandibularis branch of the fifth nerve, is known among the 

 higher vertebrates as the chorda tympani. The palatine branch 

 supplies the palate and the roof of the mouth. In the lower 

 forms it is a mixed nerve ; in the mammals it innervates only 

 the muscles of the soft palate. It may unite with either branch, 

 a or b, of the fifth. The buccal branch runs in the upper jaw, 

 uniting with the ophthalmicus profundus. 



The auditory nerve is closely connected with the seventh, 

 and is often regarded as its dorsal root. It goes directly to the 

 ear, dividing almost immediately into two branches, which may 

 leave the skull through separate foramina. 



The vagus complex is composed of the ninth, tenth, and 

 eleventh nerves, which are closely connected, and present many 

 similarities to each other. In many features they resemble 

 more closely the spinal nerves, especially in the presence of 

 distinct dorsal and ventral roots. The ear intervenes between 

 these and the nerves in front. The complex arises from the 

 side of the medulla by from four to eight or more roots, the 

 anterior pair being considered as those of the glossopharyngeal. 

 Usually in the aquatic vertebrates its ganglion is fused with 

 that of the vagus. 



The glossopharyngeal nerve splits into two branches, 1 the 

 anterior going to the pharyngeal region, the other (lingualis) 

 to the muscles and mucous membrane of the gill in fishes, and 

 to the sense organs of the tongue in the mammals, etc. The 

 pharyngeal branch also gives off a nerve (Jacobson's anasto- 

 mosis) which unites with the hyomandibularis of the facial. 



The vagus or pneumogastric has a wide distribution. In 



1 In the branchiate vertebrates the division occurs above the first true gill slit, so that 

 here, too, we have pre- and post-trematic branches. 



