NERVOUS SYSTEM 885 



is no trace of these nerves, even in the embryo, although in the frog and 

 other amphibia which pass through a gilled larval stage, these nerves are 

 outlined in the development, but are lost during metamorphosis. 



VIII. Acusticus (Auditory) Nerve. This nerve is quite closely 

 related to the facial (Fig. 494), the two ganglia being fused, although 

 the roots are quite distinct. The auditory nerve is entirely sensory, and 

 has two branches, the cochlear and the vestibular. Both of these are 

 distributed to the sensory structures of the inner ear. Because of the 

 peculiar relations of this nerve to the ear, it is often assumed that, in 

 higher vertebrates, this is the remains of the original lateral line system. 



IX. Glossopharyng-eal Nerve. This is the first of the cranial nerves 

 lying posterior to the ear. It arises from the medulla close to the tenth 

 nerve, and in amphibia, both its roots and its ganglion, called the petro- 

 sal ganglion, fuse with those of the vagus. In gilled vertebrates the 

 nerve goes to the second gill cleft, there dividing into a pre-trematic 

 and a post-trematic branch. The pre-trematic passes to the region of the 

 hyoid arch as well as to the oral cavity. In teleosts it also passes to 

 the pseudobranch. The larger of the two branches, the post-trematic 

 supplies the muscles of the first cleft and sends a single branch to the 

 taste organs, called the lingual branch. In amniotes, the post-trematic 

 branch is called the pharyngeal nerve, and while there are no gill clefts, 

 the distribution is practically the same as though there were. 



There may be a dorsal nerve given off from the glossopharyngeal 

 near the petrosal ganglion, which is somatic sensory in function, and 

 supplies the skin on the upper side of the head. This is quite similar to 

 the auricular nerve in mammals. In fishes and amphibia the glosso- 

 pharyngeal has a connection with the fifth nerve ; the connection is 

 called Jacobson's commissure. 



X. Vagus or Pneumogastric Nerve. This is made up of several 

 metameric nerves. In comparative anatomy it is important to remem- 

 ber that in fish and amphibia this nerve differs considerably from that 

 of the higher forms. 



In all gilled vertebrates the vagus arises by a number of tiny roots, 

 while it has two closely associated ganglia, the anterior being called the 

 lateralis, and the posterior being called the jugular. The jugular 

 ganglion contains both somatic and visceral sensory cells. It is from 

 the jugular ganglion that a branchio-intestinal nerve arises, which sends 

 branches to each gill cleft behind the first. In the dogfish there is an 

 epibranchial ganglion on each of these branchial nerves. In the higher 

 fishes these ganglia are fused in the main trunk. Beyond the ganglion, 

 each branchial nerve divides into a pre-trematic and a post-trematic 

 branch, just as does the ninth. Beyond the last cleft, the nerve-trunk 

 continues as the intestinal nerve, going to the heart, stomach, and air 

 bladder when that is present. It is for this reason that the nerve is called 

 pneumogastric in human anatomy. The lateralis part of the nerve fol- 



