PNEUMOGASTRIC NERVE. 579 



superior laryngeals do not receive their motor filaments from the spinal acces- 

 sory. 



The sensory filaments of the superior laryngeals have important uses con- 

 nected with the protection of the air-passages from the entrance of foreign 

 matters, particularly in deglutition, and they are also concerned in the reflex 

 action of the constrictors of the pharynx. When both superior laryngeals 

 have been divided in living animals, liquids often pass in small quantity into 

 the larynx, owing to the absence of the reflex closure of the glottis when 

 foreign matters are brought in contact with its superior surface and the occa- 

 sional occurrence of inspiration during deglutition. 



Aside from the protection of the air-passages, the superior lauyngeal is 

 one of the sensory nerves through which the reflex acts in deglutition operate. 

 There are certain parts which depend for their sensibility entirely upon this 

 nerve ; viz., the mucous membrane of the epiglottis, of the aryteno-epiglot- 

 tidean fold and of the larynx as far down as the true vocal chords. When 

 an impression is made upon these parts, as when they are touched with a 

 piece of meat, regular and natural movements of deglutition ensue. 



If the superior laryngeal nerves be divided and a stimulus be applied to 

 their central ends, movements of deglutition are observed, and there is also 

 arrest of the action of the diaphragm. From these experiments, it would 

 seem that the impression which gives rise to the movements of deglutition 

 aids in protecting the air-passages from the entrance of foreign 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 important 

 action is connected with the muscles of the larynx. The few filaments which 

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

 important. It is proper to note, however, that the inferior laryngeal 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 upon these nerves have noted little or no evidence 

 of pain when they are irritated or divided. 



One of the most important uses of the recurrents relates to the production 

 of vocal sounds. In connection with the physiology of the internal, or com- 

 municating branch from the spinal accessory to the pneumogastric, it has 

 been shown that this branch of the spinal accessory is the true nerve of 

 phonation. Before the uses of the spinal accessory were fully understood, 

 the experiments upon the inferior laryngeals led to the opinion that these 

 were the nerves of phonation, as loss of voice follows their division 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 exclusively from the spinal accessory, and that the 



