102 ZOOLOGY SECT. 



nerve, arising from the ventral region of the medulla, and sup- 

 plying the external rectus muscle of the eye. We thus have the 

 remarkable fact that out of ten, or at the most twelve, cerebral 

 nerves, three are devoted to the supply of the six small muscles 

 by which the eye-ball is moved, and of those by which the accom- 

 modation of the eye for varying distances is effected. 



The seventh or facial (Fig. 788, VII.) is, like the fifth, a mixed 

 nerve in the lower Craniata, i.e., contains both sensory and motor 

 fibres. It arises from the side of the medulla, a short distance 

 behind the fifth, and is dilated near its origin into a facial ganglion. 

 It has two chief branches, a palatine (VII. p.), which passes in 

 front of the mandibulo-hyoid gill-cleft, and supplies the mucous 

 membrane of the palate, and a hyomandibular (VII. A.), which 

 passes behind the same cleft and sends branches to the lower 

 jaw and to the hyoid arch. In most aquatic Vertebrata an 

 ophthalmic branch is given off from the trunk of the nerve, and 

 usually accompanies the superficial ophthalmic division of the 

 fifth. In the higher Vertebrata 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 hyo-branchial 

 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 hyo-branchial gill-pouch, including both branchial filaments 

 and muscles : its anterior branch goes to the posterior hemi- 

 branch 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. 



In Fishes a nerve known as the lateral (X. I.) takes its origin 

 above the glossopharyngeal, sometimes in front of the latter, 

 sometimes behind it. It usually joins the trunk of the following 

 or tenth nerve, but becomes separate again and runs backwards, 

 supplying the cutaneous sense-organs of the lateral line (see below). 



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. br. 1-J), corresponding 

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



