63 o NERVOUS SYSTEM. 



gastric. It is here that the anastomotic branch leaves the spinal accessory to go to the 

 pneumogastric. At this point, the external branch, with the anastomosing branch, is 

 seized with a pair of rather broad-billed forceps, and gentle but firm traction is applied 

 to the entire nerve. Soon there is a cracking sensation conveyed to the hand as the 

 roots give way, and the nerve may then be drawn out entire. With care, either the fila- 

 ments of origin from the medulla or those from the cord may be extirpated alone. 



When one spinal accessory is extirpated, the vocal sounds are hoarse and unnatural. 

 When both nerves are torn out, in addition to the disturbance of deglutition and the par- 

 tial paralysis of the sterno-mastoid and trapezius muscles, the voice becomes extinct. 

 Animals operated upon in this way move the jaws and make evident efforts to cry, but 

 no vocal sound is emitted. This condition is very striking ; and, inasmuch as Bernard 

 has kept animals, with both nerves extirpated, for months, the question of the function 

 of these nerves in phonation may now be regarded as definitively settled. 



It remains now to consider the experimental facts with regard to the influence of the 

 different filaments of origin of the spinal accessory upon the voice. These are simple and 

 entirely conclusive ; and they are due exclusively to the researches of Bernard. This 

 experimenter found that division of the roots of origin from the spinal cord not only did 

 not affect the voice, but sometimes it seemed to render it clearer ; but that division of the 

 roots of origin from the medulla oblongata abolished the voice, although the inferior roots 

 were intact. 



It is not necessary to discuss the action of the muscles of the larynx in phonation, as 

 this subject has already been considered in connection with the voice. The experiments 

 that have demonstrated the influence of the spinal accessory nerve over these muscles 

 have pointed out the destination of the fibres that join the pneumogastric, which could 

 never have been done so satisfactorily by dissection. They have shown farther that 

 the movements involved in phonation are more or less independent of the respiratory 

 movements of the larynx. 



If the larynx be exposed in a living animal, with all its nervous connections intact, 

 it will be seen to open widely during inspiration, being passive in expiration. The wide 

 opening of the glottis at this time is due to the fact that, after the operation, respiration 

 is .usually more or less labored ; but, if we carefully observe the parts when the respira- 

 tory acts are perfectly tranquil, the movements of the glottis seem to be very slight. 

 The larynx is then permanently opened to a moderate degree, but the chink of the glottis 

 is slightly dilated with each expiration. If the recurrent laryngeal nerves, which are 

 distributed to all of the muscles of the larynx except the crico-thyroid, be now divided 

 upon both sides, the larynx is entirely paralyzed, and in cats and young animals, in which 

 the cartilages are soft and flexible, the parts are occluded by the effort of inspiration, and 

 death takes place from suffocation. Of course the division of the recurrent laryngeal 

 nerves abolishes the voice, but it arrests the other movements of the larynx as well. 

 The distinction thus established between the action of the spinal accessory and of the 

 recurrent laryngeal nerves was fully illustrated by Bernard, in the following experiments : 



In a cat, in which the voice had been completely destroyed by extirpation of both 

 spinal accessory nerves, the larynx was exposed. The glottis was seen dilated so as to 

 permit the free passage of air in respiration. The mucous membrane retained its sensi- 

 bility, and, when the interior of the larynx was irritated, a very slight but ineffectual 

 effort was made to close the glottis. It was impossible for the animal to approximate 

 the posterior points of attachment of the vocal cords or to put the cords upon the stretch. 

 If such irritation be applied to the larynx of an animal with the spinal accessory nerves 

 intact, the glottis is instantly and firmly closed. 



^ In a cat about five weeks old, both spinal accessory nerves were extirpated, and the 

 voice was thus destroyed. Two days after, both recurrent laryngeal nerves were divided, 

 and the animal died almost immediately of suffocation. 



These experiments show conclusively that the internal, or communicating branch of 



