xiii PHYLUM CHORDATA 99 



accompanies the ophthalmic division of the fifth. In the higher 

 Vertebra ta the seventh becomes a purely motor nerve, supplying 

 the muscles of the face. 



The eighth or auditory nerve (VIII.) arises immediately behind 

 the seventh, with which it is intimately connected at its origin. 

 It is a purely sensory nerve, supplying the organ of hearing, i.e., the 

 epithelium of the membranous labyrinth presently to be described. 



The ninth or glossopharyngeal (IX.) is a mixed nerve : it arises 

 from the lateral region of the medulla, behind the organ of 

 hearing, and is connected at its origin with the vagus ganglion 

 (see below). Its trunk passes downwards and forks over the 

 second gill-cleft, sending an anterior branch to the hyoid arch 

 which bounds the cleft in front, and a posterior branch to the first 

 branchial arch which bounds it posteriorly. Thus the entire 

 nerve supplies the second gill-pouch, including both branchial 

 filaments arid muscles : its anterior branch goes to the posterior 

 hemibranch of the hyoid arch, its posterior branch to the anterior 

 hemibranch of the first branchial arch. In the air-breathing 

 Vertebrata, in which gills are absent, the glossopharyngeal sends a 

 gustatory nerve to the tongue and supplies the pharynx. 



The tenth nerve (X.), called the vagus or pneumogastric, is dis- 

 tinguished by its wide distribution. It arises by numerous roots 

 from the side of the medulla, the roots uniting into a stout 

 trunk with a vagus ganglion at its origin. From the trunk are 

 given off, in the first place, branchial nerves (X. l>r. 1-5), corre- 

 sponding in number and position to the gill -slits from the third to 

 the last inclusive. Each branchial nerve behaves in exactly 

 the same way as the glossopharyngeal : it forks over the gill-pouch 

 to which it belongs, sending one branch to the anterior, another 

 to the posterior wall of the pouch. Thus each gill-pouch has its 

 own nerve while each gill receives its supply from two sources ; for 

 instance, the gill of the second branchial arch has its anterior 

 hemibranch innervated from the first, its posterior hemibranch 

 from the second branchial branch of the vagus. The vagus also 

 gives off a cardiac nerve (X. c) to the heart, a gastric nerve (X. g) to 

 the stomach, and a lateral nerve (X. /) which passes backwards 

 along the side of the body and supplies the cutaneous sense- 

 organs (see below). In the air-breathing Craniata there are, of 

 course, no branchial nerves ; but the vagus still retains control of 

 the respiratory organs by giving origin to pulmonary nerves to the 

 lungs and laryngeal nerves to the larynx. 



The above mentioned ten nerves are all that exist in most of the 

 lower Craniata : the eleventh or accessory nerve (XI.) appears first in 

 Reptiles. It arises by numerous roots from the anterior part of 

 the spinal cord, passes forward, between the dorsal and ventral 

 roots of the spinal nerves, and finally leaves the medulla just 

 behind the vagus. It is thus a spinal nerve as regards its origin, 



H 2 



