SPINAL ACCESSORY NERVE 



gastric of the opposite side, where the connections with the spinal 

 accessory were intact. In these observations it seemed necessary that 

 a sufficient time should elapse after extirpation of the spinal accessory 

 for the excitability of the filaments that join the pneumogastric to 

 become extinct ; but the experiments are sufficient to show the direct 

 inhibitory influence of the spinal accessory on the heart. After ex- 

 tirpation of the spinal accessory, degenerated fibres are found in the 

 trunk of the pneumogastric. The mechanism of inhibition of the heart 

 has already been considered in connection with the physiology of the 

 circulation. 



Uses of the External, or Muscular Branch of the Spinal Accessory. 

 Observations have shown that the internal branch of the spinal acces- 

 sory, 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 irregularity of the 

 movements of the anterior extremities and suffer from shortness of breath 

 after violent muscular exertion. The use of the corresponding extremi- 

 ties in the human subject is so different that it is not easy to make a 

 direct application of these experiments ; still, certain inferences may be 

 drawn from them in regard to the action of the external branch in 

 man. 



In prolonged vocal efforts the vocal chords are put on the stretch, 

 and the act of expiration is different from that in tranquil breathing. 

 In singing, for example, the shoulders frequently are fixed; and this is 

 done to some extent by the action of the sterno-cleido-mastoid and the 

 trapezius. It is probable, then, that the action of the branch of the 

 spinal accessory which goes to these muscles has a certain synchronism 

 with the action of the branch going to the larynx and the pharynx ; the 

 one fixing the upper part of the chest so that the expulsion of the air 

 through the glottis may be more nicely regulated by the expiratory 

 muscles, and the other acting on the vocal chords. 



In violent muscular effort, the glottis is closed, the thorax is 

 fixed after a full inspiration and respiration is arrested so long as the 

 effort, if it be not too prolonged, is continued. The same synchronism, 

 therefore, obtains in this as in prolonged vocal efforts. In experiments 

 in which the muscular branch only has been divided, shortness of breath, 

 after violent muscular effort, is observed ; and this probably is due to 

 lack of synchronous action of the sterno-cleido-mastoid and trapezius. 

 The irregularity in the movements of progression in animals in which 

 either both branches or the muscular branches alone have been divided 

 is due to anatomical peculiarities. 



