106 ZOOLOGY SECT. 



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

 gustatory nerve to the tongue and supplies the pharnyx. 



In Fishes a nerve known as the lateral (X. /.) 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 back- 

 wards, supplying the cutaneous sense-organs of the lateral line 

 (see below). 



The tenth nerve (X.), called the vagm 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, hranchial nerves (X. hr. 1-5), coitc- 

 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-jpoiich has its 

 own nerve while each ^i7Z 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 

 froin 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. 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 eleveiith or accessory nerve (XI.), w'hich is recognisable in 

 some Fishes as a part of the vagus, is a distinct nerve in higher 

 forms, and consists of cerebral and spinal portions, so that it 

 occupies an intermediate position between the purely cerebral 

 and the purely spinal nerves. It acts in higher Craniates mainly 

 as the motor nerve for certain muscles of the shoulder. 



The twelfth or hypoglossal (XII.) arises from the ventral aspect 

 of the medulla oblongata, after the manner of the ventral root of 

 a spinal nerve. It is purely motor, and supplies the muscles of 

 the tongue and certain neck-muscles. In the Amphibia its place is 

 taken by the first spinal nerve, and there is no doubt that it is 

 to be looked upon as a spinal nerve which has become included in 

 the cranial region : even in some Fishes it passes out through 

 the skull. 



The sympathetic nerve (sym.) is continued into the head and 

 becomes connected with some gf the cerebral nerves. 



Sensory Organs. — The whole surface of the body forms an 

 organ of touch, but special tactile organs are more or less widely 

 distributed. End-huds consist of ovoidal groups of sensory cells 

 supplied by a special nerve : touch-cells (Fig. 780, A) are nerve-cells 



