538 NERVOUS SYSTEM 



it would seem that the impression which gives rise to the movements of 

 deglutition aids in protecting the air-passages from the entrance of for- 

 eign matters by temporarily arresting the inspiratory act. 



Properties and Uses of the Inferior, or Recurrent Laryngeal Nerves. 

 The anatomical distribution of these nerves shows that their most impor- 

 tant action is connected with the muscles of the larynx. The few fila- 

 ments given off in the neck to join the cardiac branches are probably not 

 very important. It is proper to note, however, that the inferior laryn- 

 geal nerves supply the muscular tissue and mucous membrane of the 

 upper part of the oesophagus and trachea, and one or two branches are 

 sent to the inferior constrictor of the pharynx. The action of these 

 filaments is sufficiently evident. 



The inferior laryngeals contain chiefly motor filaments, as is evident 

 from their distribution as well as from the effects of direct stimulation. 

 All who have experimented on these nerves have noted little or no evi- 

 dence of pain when they are irritated or divided. 



One of the most important uses of the recurrents relates to the pro- 

 duction of vocal sounds. In connection with the physiology of the 

 internal, or communicating branch from the spinal accessory to the 

 pneumogastric, it has been shown that this branch is the true nerve of 

 phonation. Before the uses of the spinal accessory were fully under- 

 stood, experiments on the inferior laryngeals led to the opinion that 

 these were the nerves of phonation, as loss of voice follows their divi- 

 sion in living animals. It is true that these nerves contain the filaments 

 which preside over the vocal movements of the larynx ; but it is also the 

 fact that these vocal filaments are derived from the spinal accessory, and 

 that the recurrents contain as well motor filaments which preside over 

 movements of the larynx not concerned in the production of vocal sounds. 



The muscles of the larynx concerned in phonation are the crico-thy- 

 roids, animated by the superior laryngeals, and the arytenoid, the lateral 

 crico-arytenoids and the thyro-arytenoids, animated by the inferior laryn- 

 geals. The posterior crico-arytenoids are respiratory muscles, and these 

 are not affected by extirpation of the spinal accessories, but the glottis is 

 still capable of dilatation, so that inspiration is not impeded. If, however, 

 the spinal accessories are extirpated and the larynx is then exposed in a liv- 

 ing animal, the glottis remains dilated, but will not close when irritated. If 

 the inferior laryngeals are then divided, the glottis is mechanically closed 

 with the inspiratory act, and the animals often die of suffocation. In 

 view of the varied sources from which the pneumogastrics receive their 

 motor filaments, it is easy to understand how certain of these may pre- 

 side over the vocal movements, and others, from a different source, may 

 animate the respiratory movements of the glottis. 



