564 THE NERVOUS SYSTEM. 



Physiology of the Spinal Accessory Nerve. 



In the preceding pages it is implied that all the motor 

 influence which the pneumogastric nerves exercise, is con- 

 veyed through filaments which, from their origin, belong 

 to them : and this is, perhaps, true. Yet a question, which 

 has been often discussed, may still be entertained, whether 

 a part of the motor filaments that appear to belong to 

 the pneumogastric nerves are not given to them from the 

 accessory nerves. 



The principal branch of the accessory nerve, its external 

 branch, supplies the sterno-mastoid and trapezius muscles; 

 and, though pain is produced by irritating it, it is com- 

 posed almost exclusively of motor fibres. It might appear 

 very probable, therefore, that the internal branch, which 

 is added to the trunk of the pneumogastric just before the 

 giving off of the pharyngeal branch, is also motor ; and 

 that through it the pneumogastric nerve derives part of 

 the motor fibres which it supplies to the muscles enumer- 

 ated above. And further, since the pneumogastric nerve 

 has a ganglion just above the part at which the internal 

 branch of the accessory nerve joins its trunk, a close 

 analogy may seem to exist between these two nerves and - 

 the spinal nerves with their anterior and posterior roots. 

 In this view, Arnold and several later physiologists have 

 regarded the accessory nerve as constituting a motor root 

 of the vagus nerve ; and, although this view cannot now 

 be maintained, yet it is very probable that the accessory 

 nerve gives some motor filaments to the pneumogastric. 

 For, among the experiments made on this point, many 

 have shown that when the accessory nerve is irritated 

 within the skull, convulsive movements ensue in some of 

 the muscles of the larynx ; all of which, as already stated, 

 are supplied, apparently, by branches of the pneumo- 

 gastric ; and (which is a very significant fact) Vrolik states 

 that in the chimpanzee the internal branch of the accessory 



