SPINAL ACCESSORY NERVE. 631 



the spinal accessory is the nerve which presides over the movements of the larynx in 

 phonation. The filaments undoubtedly pass to the larynx in greatest part through the 

 recurrent laryngeal branches of the pneumogastric ; but the recurrent laryngeals also 

 contain motor filaments from other sources, which latter are chiefly concerned in the 

 respiratory movements of the glottis. 



Influence of the Internal Branch of the Spinal Accessory upon Deglutition. There 

 are two ways in which deglutition is affected through this nerve : 1. When the larynx is 

 paralyzed as a consequence of extirpation of both nerves, the glottis cannot be completely 

 closed to prevent the entrance of foreign bodies into the air-passages. In rabbits par- 

 ticularly, it has been noted that particles of food penetrate the trachea and find their 

 way into the lungs. 2. The spinal accessory furnishes numerous filaments to the pharyn- 

 geal branch of the pneumogastric, and, through this nerve, it directly affects the muscles 

 of deglutition ; but the muscles animated in this way by the spinal accessory have a ten- 

 dency to draw the lips of the glottis together, while they assist in passing the alimentary 

 bolus into the oesophagus. When these important acts are wanting, there is some diffi- 

 culty in the process of deglutition itself as well as danger of the passage of foreign 

 particles into the larynx. 



Influence of the Spinal Accessory upon the Heart. When we come to study the varied 

 functions of the pneumogastrics, we shall discuss fully the mechanism by which the con- 

 tractions of the heart are arrested by galvanization of both of these nerves in the neck. 

 A very curious and interesting observation by Waller has demonstrated that this influ- 

 ence, whatever be its mechanism, is derived from the spinal accessory and necessarily 

 comes through its communicating branch. It has been found that a powerful current of 

 galvanism passed through the pneumogastric upon one side will arrest the action of the 

 heart. Waller found that, if he extirpated the spinal accessory upon one side, the action 

 of the heart could not be arrested by galvanizing the pneumogastric upon the same side ; 

 but this result followed galvanization of the pneumogastric upon the opposite side, on 

 which the connections with the spinal accessory were intact. These phenomena, how- 

 ever, could not be observed until from ten to twelve days had elapsed after the extirpa- 

 tion of the spinal accessory. We have already seen, in treating of the general properties 

 of the nerves, that the irritability of the motor nerves disappears in about four days after 

 their separation from the nerve-centres. In the observation just referred to, it seemed 

 necessary that a sufficient time should elapse after extirpation of the spinal accessory for 

 the irritability of the filaments that join the pneumogastric to become extinct ; but the 

 experiment is sufficient to show the direct inhibitory influence of the spinal accessory 

 upon the heart. This subject will be more fully considered, however, in connection with 

 the functions of the pneumogastrics. 



Functions of the External, or Muscular Branch of the Spinal Accessory. The most 

 interesting feature in the recent researches into the functions of the spinal accessory is. 

 that experimentalists have been able to separate physiologically the internal from the 

 external branch. Observations have conclusively demonstrated that the internal branch, 

 and the internal branch only, is directly concerned in the vocal movements of the larynx, 

 and, to a great extent, in the closure of the glottis during deglutition. It has been noted, 

 in addition, that animals in which both branches have been extirpated present irregu- j 

 larity of the movements of the anterior extremities and suffer from shortness of breath I 

 after violent muscular exertion. The use of the corresponding extremities in the human I 

 subject is so different, that it is not easy to make a direct application of these experi- 

 ments ; still, we can draw from them certain inferences with regard to the functions of 

 the external branch in man. 



In prolonged vocal efforts, the vocal cords are put upon the stretch, and the act of 

 expiration is very different from that in tranquil breathing. In singing, for example, the 

 shoulders are frequently fixed; and this is done to some extent by the action of the 

 sterno-cleido-mastoid and the trapezius. We may suppose, then, that the action of the 



