THE SPINAL ACCESSORY NERVE. 443 



centre in the medulla indications of such conditions of the 

 heart as require a lowering of the blood pressure in the vessels; 

 as, for example, when the heart cannot, with sufficient ease, 

 propel blood into the already too full or too tense arteries. 



Physiology of the Spinal Accessory Nerve. 



In the preceding pages it is implied that all the motor in- 

 fluence which the pneumogastric nerves exercise, is conveyed 

 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, is composed 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 enumerated 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, convul- 

 sive movements ensue in some of the muscles of the larynx ; 

 all of which, as already stated, are supplied, apparently, by 

 branches of the pneumogastric ; and (which is a very signifi- 

 cant fact) Vrolik states that in the chimpanzee the internal 

 branch of the accessory does not join the pneumogastric at all, 

 but goes direct to the larynx. On the whole, therefore, al- 

 though in some of the experiments no movements in the larynx 

 followed irritation of the accessory nerve, yet it may be con- 

 cluded that this nerve gives to the pneumogastric some of the 

 motor filaments which pass, with the laryngeal branches, to 

 the muscles of the larynx, especially to the crico-thyroid (Ber- 

 nard) ; although it is certain that the accessory nerve does not 



