SPINAL ACCESSORY NERVE. 561 



the pneumogastric upon the same side ; but inhibition followed stimulation 

 of the pneumogastric upon the opposite side, on which 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 in- 

 fluence of the spinal accessory upon the heart. After extirpation of the spi- 

 nal 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 accessory, 

 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 muscu- 

 lar exertion. The use of the corresponding extremities in the human subject 

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

 ments ; still, certain inferences may be drawn from them with regard to the 

 action of the external branch in man. 



In prolonged vocal efforts, the vocal chords are put upon the stretch, and 

 the act of expiration is different from that in tranquil breathing. In sing- 

 ing, 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 regu- 

 lated by the expiratory muscles, and the other acting upon the vocal chords. 



In what is known as 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 is probably due to the want of syn- 

 chronous 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 pecul- 

 iarities. Bernard has observed these irregularities in the dog and the horse, 

 but they are not so well marked in the cat. There have been no opportuni- 

 ties for illustrating these points in the human subject. 



SUBLINGUAL (TWELFTH NERVE). 



The last of the motor cranial nerves is the sublingual ; and its action is 

 intimately connected with the physiology of the tongue in deglutition and 



