652 NERVOUS SYSTEM. 



experiments the results have been quite satisfactory. The experiments in which a pow- 

 erful current of galvanism has been applied to the nerves also show an arrest of respi- 

 ration ; but it is argued that there is nothing special in the action of the superior laryn- 

 geals under these conditions, inasmuch as other sensitive nerves have been found to act 

 in the same way. This is undoubtedly true ; but it is well known that, in living animals, 

 strong impressions made upon any of the acutely sensitive nerves arrest respiration, and 

 that this is one of the phenomena commonly observed in animals struggling under painful 

 operations. In view of these facts, it seems unnecessary to discuss more fully the numer- 

 ous experiments with regard to the effects upon respiration of stimulation of the superior 

 laryngeals ; and we can assume that it has been demonstrated that an impression made 

 upon the terminal filaments of these nerves, such as occurs in the ordinary process of 

 deglutition, excites, by reflex action, contraction of the constrictors of the pharynx, and, 

 at the same time, momentarily suspends the movements of the diaphragm. 



Important experiments have been made within the past few years, upon the action of 

 the pneumogastrics on the circulation, in which it is claimed that nervous filaments, aris- 

 ing, in the rabbit, in part from the trunk of the pneumogastric and in part from the 

 superior laryngeal branch, act as reflex depressors of the vascular tension. These experi- 

 ments will be fully discussed in connection with the cardiac branches. 



Properties and Functions of the Inferior, or Recurrent Laryngeal Nerves. The 

 anatomical distribution of these nerves shows that their most important function is con- 

 nected 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 mem- 

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

 sent to the inferior constrictor of the pharynx. The function of these filaments is suffi- 

 ciently evident. 



The inferior laryngeals contain chiefly motor filaments, judging from their distribu- 

 tion as well as from the effects of direct irritation. All who have experimented upon 

 these nerves have noted little or no evidence of pain when they are stimulated or divided. 



One of the most important functions of the recurrents is connected with the produc- 

 tion of vocal sounds. We have already fully treated of the mechanism of the voice and 

 the action of the intrinsic muscles of the larynx ; and, in our account of the physiology 

 of the internal, or communicating branch from the spinal accessory to the pneumogas- 

 tric, it has been shown that this is the true nerve of phonation. In the older works 

 upon physiology, before the functions 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 they showed loss of voice following 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 recurrents contain as well motor filaments which pre- 

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



The muscles of the larynx concerned in phonation are, the crico-thyroids, animated 

 by the superior laryngeals, and the arytenoid, the lateral crico-arytenoids, and the thyro- 

 arytenoids, animated by the inferior laryngeals. The posterior crico-arytenoids are re- 

 spiratory muscles ; and it is curious that these are not affected by extirpation of the spinal 

 accessories, but that the glottis is still capable of dilatation, so that inspiration is not 

 impeded. If, however, the spinal accessories be extirpated, and the larynx be then 

 exposed in a living animal, the glottis still remains dilated, but will not close when irri- 

 tated. If the inferior laryngeals be then divided, the glottis is mechanically closed with 

 the inspiratory act, and the animals often die of suffocation. When we call to mind the 

 varied sources from which the pneumogastrics receive their motor filaments, it is easy to 

 understand how certain of these may preside over the vocal movements, and others, 

 from a different source, may animate the respiratory movements. 



