THE VAGUS NERVE 97 



walls of the pharynx fail to grip the bolus of food and to help 

 it on its way to the oesophagus. In addition, the soft palate 

 is paralysed, and on this account the naso-pharynx is not shut 

 off from the oral pharynx during deglutition. As a result, the 

 food, taking the path of least resistance, regurgitates through 

 the nose. A further result of the palatal paralysis is an altera- 

 tion in the character of the voice, which acquires a distinctly 

 nasal tone. 



The superior laryngeal nerve leaves the vagus at the lower 

 end of the ganglion nodosum and passes downwards and 

 medially towards the larynx. It breaks up into the internal 

 and external laryngeal nerves. 



The internal laryngeal nerve contains sensory fibres only. 

 It enters the larynx through the lateral part of the thyreo- 

 hyoid membrane and supplies the mucous membrane of the 

 interior above the level of the vocal folds (true vocal cords). 

 A few of its fibres are distributed also to the mucous 

 membrane of the recessus piriformis (p. 332). Stimulation 

 of the terminal branches of the internal laryngeal nerve sets 

 up the cough reflex, and, on this account, the lodgment of a 

 particle of food in the recessus piriformis causes a severe 

 spasm of coughing, the patient experiencing sensations similar 

 to those induced by irritation of the interior of the larynx. 



The external laryngeal nerve is a purely motor nerve, which 

 descends in company with the superior thyreoid artery and 

 passes under cover of the upper pole of the lateral lobe of the 

 thyreoid gland. It gives off one or two small branches to the 

 inferior constrictor muscle of the pharynx, but it is mainly 

 reserved for the supply of the crico-thyreoid. The contraction 

 of this muscle puts the vocal folds (true vocal cords) on the 

 stretch, and, when it is paralysed, the pitch of the voice is 

 lowered, as the other laryngeal muscles cannot maintain tbe 

 requisite tension (p. 339). 



In the neck the vagus descends vertically in the posterior 

 part of the cartoid sheath, in which it lies between the internal 

 jugular vein and the common carotid artery. 

 7 



