490 THE NERVOUS SYSTEM. 



II. If the pneumogastric nerve be divided, and the faradic current 

 applied to its central extremity, the heart's pulsations are not inter- 

 rupted ; but when the current is applied to the peripheral extremity of 

 the nerve, they cease as before. The influence therefore which arrests 

 the heart's action, under stimulation of the pneumogastric, is not a 

 centripetal influence, operating through the nervous centres ; it is a 

 centrifugal influence, passing from above downward through the pneu- 

 mogastric to the heart. 



III. After stimulating the pneumogastric nerve with a current suffi- 

 cient to stop the cardiac pulsations, if the current be continued the heart 

 does not remain motionless. At the end of ten or fifteen seconds it 

 performs a beat. A little later this is repeated, and the pulsations then 

 recur, with increasing frequency, until their normal rate is reestab- 

 lished, notwithstanding the continued faradization of the. nerve. This 

 shows that the nervous action which arrests the heart is exhausted 

 after a certain time. If the stimulation be now applied to the pneu- 

 mogastric nerve of the opposite side, the heart stops, as before. The 

 heart, accordingly, is still sensitive to the action of arrest ; it is the 

 nerve only which, by continued excitement, loses the power of exerting 

 this action. But after a pneumogastric nerve has been thus exhausted, 

 so that it no longer retards the cardiac pulsations, if allowed to repose 

 for a time, and again stimulated, it again stops the heart ; showing 

 that it has recovered the power which it had temporarily lost. In 

 these respects, the influence of the pneumogastric nerve on the heart 

 resembles that of a motor nerve on the muscles of the limbs. The 

 difference between the two is in their effect. An ordinary motor nerve, 

 when stimulated, causes contraction of the corresponding muscle ; stimu- 

 lation of the pneumogastric nerve, as connected with the heart, causes 

 relaxation. 



Eleventh Pair. The Spinal Accessory. 



This nerve, so named from its spinal origin and subsequent associa- 

 tion with the cranial nerves, consists of filaments emerging from the 

 cervical portion or the spinal cord, from the level of the fourth or 

 fifth cervical nerve upward (Fig. 127, *). They unite into a slender 

 cord, which ascends between the anterior and posterior roots of the 

 cervical spinal nerves, to the foramen magnum, where it enters the 

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

 medulla oblongata, arranged in a continuous line with those of the 

 pneumogastric. The nerve trunk, thus constituted by the union of its 

 spinal and medullary roots, accompanies the pneumogastric and glos- 

 sopharyngeal nerves in their passage through the jugular foramen. 



The central origin of this nerve is a collection of nerve cells situated 

 in the upper portion of the spinal cord and the medulla oblongata, 

 on the outer and posterior aspect of the anterior horn of gray sub- 

 stance. From this source its fibres curve downward and outward to 

 their point of emergence on the lateral surface of the medulla. 



