336 ZOOLOGY FOR MEDICAL STUDENTS chap. 



The Superficial Ophthalmic branch (Fig. 138, A, VIIo and Fig. 140, 

 VUs.o) runs along the inner wall of the orbit, dorsal to the similarly 

 named branch of the trigeminal which it accompanies in its distribution. 

 In Acanthias, as already indicated, the two superficial ophthalmic nerves 

 (V and VII) are enclosed in a common sheath. It innervates the lateral- 

 line sense-organs on the dorsal surface of the snout. 



Auditory (Fig. 138, VIII).^ — ^The Eighth nerve is the nerve of 

 hearing. It is formed of branches from the various parts of the otocyst 

 which are collected into a short thick trunk and enters the side-wall of 

 the medulla oblongata in close proximity to the Fifth and Seventh nerves. 



Glosso-pharyngeal (Figs. 138 and 140, IX). — The Ninth nerve 

 arises from the side of the medulla oblongata, well down towards the 

 ventral surface and some distance behind the auditory. It passes out- 

 wards and backwards through the cartilage of the auditory capsule and 

 forks over the first ordinary gill-cleft, just as the hyomandibular does 

 over the spiracle. Its anterior branch is sensory while the posterior 

 branch is also motor, supplying muscles connected with the first 

 branchial arch. 



Vagus (or Pneumogastric — Figs. 138 and 140, X). — The' Tenth nerve 

 gets its name Vagus ' (wandering) from the wide extent of its dis- 

 tribution ; although a cranial nerve it is not confined to the head but 

 extends far into the trunk region. - It arises from the side of the medulla 

 oblongata by a number of roots which converge in a fan-like manner 

 to form a common trunk. This (visceral trunk — Xw) passes back 

 supplying the stomach and neighbouring parts of the alimentary canal 

 and also the heart. Four branchial branches (Fig. 138, bi, &2, etc.) 

 pass off from the visceral trunk to supply the walls of the last four gill- 

 clefts : each forking over the cleft in precisely the same manner as the 

 glosso-pharyngeal over the first cleft. This fact taken in conjunction 

 with the numerous roots leads to the conclusion that the vagus really 

 represents not a single cranial nerve but a series of nerves, each equivalent 

 to the glosso-pharyngeal, which have become fused together. 



In addition to these branches there is the lateral branch (Fig. 140, lat) 

 which arises by its own root dorsal to the root of the glosso-pharyngeal. 

 It accompanies the visceral branch for some distance and then passes 

 backwards along the side of the body to the tip of the tail, lying deeply 

 embedded in the muscle and giving off at intervals twigs to the sense- 

 organs of the lateral line. 



In the fact that it is the sensory nerve of the lateral-line organs the 

 lateral branch of the vagus recalls the buccal and superficial ophthalmic 

 divisions of the facial, and the interesting discovery has been made that 



