298 STRUCTURE OF THE VERTEBRATES 



Nerve IX, Glossopharyngeal. The name is descriptive of 

 the mammalian condition, the nerve being distributed to the 

 muscles of the tongue and pharynx. In gilled vertebrates the 

 nerve supplies the first gill slit, dividing into anterior and pos- 

 terior branches. The anterior branch goes to the oral cavity and 

 the hyoid muscles; the posterior branch goes to the muscles of 

 the first gill arch, with a small branch to the taste buds of the 

 primitive tongue. With the modification of the gill arches in the 

 higher vertebrates, the nerve supplies the homologous muscles 

 in the hyoid (which supports the tongue), some laryngeal 

 muscles, the pharyngeal region, and some of the taste buds. 



Nerve X, Vagus. The vagus is the most widely distributed 

 of the cranial nerves, supplying hypomeric (visceral) muscles. 

 Vertebrates with gills have two major branches, (1) the lateral 

 and (2) the hranchio -intestinal. The former is a sensory branch 

 in connection with the lateral line system. It persists throughout 

 life in fish and urodele amphibia, but disappears at metamor- 

 phosis in the anura. The lateral branch is missing in the am- 

 niotes. 



The branchio-intestinal nerve of gill bearing animals sends 

 a branch to each of the gill slits posterior to the first. The main 

 trunk passes posteriorly and innervates the heart, stomach, part 

 of the intestine, and the swim bladder of the three groups which 

 have one. There is a degeneration of the branchial nerves in 

 the amniotes, the laryngeal and pharyngeal branches of the 

 vagus apparently being homologous structures. The intestinal 

 branch goes to the heart, stomach, parts of the intestine, and 

 the lungs. The last is contributory evidence in the undoubted 

 homology of swim bladder and lungs. 



The vagus has numerous connections with the autonomic 

 system, and the relationships are very close due to the inner- 

 vation of organs by both types of fibers — cranial and autonomic. 

 The relationships between the two parts of the nervous system 

 were discussed under the sympathetic nervous system. 



Nerve XI, Spinal Accessory. The accessory nerve is evi- 

 dently a modified spinal nerve. Its origin is on both the posterior 

 medulla and the spinal cord, the spinal roots fusing to form a 

 nerve which passes into the cranium, joins the roots from the 

 medulla, and then passes out through the foramen to the ster- 



