96 THE NERVOUS SYSTEM 



The auricular branch of the vagus (Arnold's nerve) arises 

 from the jugular ganglion and, having passed through a small 

 canal in the petrous portion of the temporal bone, supplies 

 branches to the skin which lines the lateral surface of the 

 tympanic membrane and the deep part of the external acoustic 

 meatus. This little nerve merits description because the 

 stimulation of its terminal fibres may produce symptoms 

 which are referred to the areas of distribution of the terminal 

 branches of the vagus. Thus, a small piece of wax, impinging 

 on the tympanic membrane, may be sufficient to set up gastric 

 symptoms, which naturally do not respond to ordinary treat- 

 ment. The condition of the ear is often overlooked, as there 

 are no local symptoms to direct attention to the cause of the 

 disorder (cf. p. 200). 



Immediately below the skull a second peripheral ganglion, 

 termed the ganglion nodosum (ganglion of the trunk), is placed 

 on the vagus. At this point the vagus receives a large branch 

 of communication, which represents the whole of that portion 

 of the accessory nerve which takes origin in the medulla 

 oblongata. The fibres contained in this communicating 

 branch are entirely motor and they are destined, principally, 

 for the supply of the muscles of the larynx and pharynx. 



In addition, the ganglion nodosum establishes communica- 

 tions with the hypoglossal nerve, the first cervical nerve and 

 the superior cervical ganglion of the sympathetic trunk, but 

 the explanation of these connexions is not known. 



The pharyngeal branch of the vagus arises from the ganglion 

 nodosum and assists the glosso-pharyngeal nerve in the form- 

 ation of the pharyngeal plexus. Through the plexus the vagus 

 nerve supplies motor branches, not only to the constrictor 

 muscles of the pharynx, but also to the muscles of the soft 

 palate, with the possible exception of the tensor veli palatini 



(P- 7)- 



In bilateral lesions of the lower neurone (e.g. in post- 

 diphtheritic neuritis of the vagus or in bulbar paralysis, p. 108) 

 difficulty in swallowing is very pronounced, as the muscular 



