THE SPINAL ACCESSORY. 571 



heart increase in rapidity, when it is stimulated, they experience a 

 retardation. 



This influence, exerted upon the heart by the pneumogastric nerve, is 

 of the peculiar kind known as the "action of arrest." Such a power 

 certainly exists in the nervous system, though its nature is not easy 

 of explanation. An instance of it has already been given in the fact, 

 observed b}" Waller and Prevost, of suspension of the movements of 

 the diaphragm by galvanizing the trunk of the superior laryngeal nerve. 

 The natural stoppage of respiration in the act of swallowing, and the 

 relaxation of the sphincters preliminary to the evacuation of the rectum 

 and the bladder, are effected by nervous influences of a similar kind. 

 There are evidently nervous fibres which transmit their stimulus di- 

 rectly to the muscles, and which, in this respect, belong to the category 

 of motor nerves ; but which, when called into activity, instead of ex- 

 citing muscular contraction, serve to moderate or even suspend it. The 

 most palpable instance of this mode of action is that of the pneumogas- 

 tric nerves in their relation with the heart ; but there is evidence that 

 it occurs, in a more obscure manner, in various other parts of the 

 nervous system. 



Eleventh Pair. The Spinal Accessory. 



This nerve, which has received its name from the singularity of its 

 origin and subsequent course, consists of filaments which emerge from 

 the side of the cervical portion of the spinal cord, from the level of the 

 fourth or fifth cervical nerve upward (Fig. 179,4). These filaments 

 unite into a slender, rounded cord, which ascends in a vertical direction 

 between the anterior and posterior roots of the cervical spinal nerves, 

 gradually increasing in size from the addition of new root fibres from 

 the spinal cord, to the level of the foramen magnum, where it enters the 

 cranial cavity. Here it receives a new supply of accessory root fibres 

 from the side of the medulla oblongata, which emerge in a continuous 

 line with those of the pneumogastric nerve. The nerve trunk, thus con- 

 stituted by the union of its spinal and its medullary roots, joins the 

 pneumogastric and glossopharyngeal nerves in their passage through 

 the jugular foramen. 



The central origin of the root fibres of this nerve is a collection of 

 nerve cells situated in the upper portion of the spinal cord and the com- 

 mencement of the medulla oblongata, on the outer and posterior aspect of 

 the anterior horn of gray matter. In the remainder of the medulla, this 

 nucleus is situated farther backward, receding from front to rear with 

 the rest of the gray matter in this part of the nervous centres. At its 

 anterior extremity, it becomes continuous with the nucleus of the pneu- 

 mogastric. From the gray matter of its nucleus, the fibres of the spinal 

 accessory nerve curve downward and outward until they emerge, as 

 above mentioned, in a series of bundles, from the lateral surface of the 

 medulla. 



While passing through the foramen lacerum, the spinal accessory 



