THE CRANIAL NERVES. 489 



According to Bernard, the finger, if introduced into the stomach 

 through a gastric fistula in a healthy dog, is compressed with considera- 

 ble force by the walls of the organ ; but this pressure disappears com- 

 pletely on division of the pneumogastric nerves. The absence of mus- 

 cular action in a paralyzed stomach is sufficient to account for the failure 

 of digestion. This action is necessary to bring successive portions of the 

 food in contact with the mucous membrane, and for the thorough admix- 

 ture of gastric juice w T ith the alimentary mass. The pneumogastric 

 nerves therefore supply to the stomach a sensibility and motor power, 

 which are practically essential to the digestive process. 



Influence on the Heart. The pneumosrastric filaments, destined for 

 distribution in the heart, are partly derived from its superior laryngeal 

 branch, whence they join the upper cardiac nerve coming from the 

 superior cervical ganglion of the sympathetic. Others are furnished 

 by the trunk of the pneumogastric in the neck, which inosculates with 

 the continuation of the upper cardiac nerve. The inferior laryngeal 

 branch, during its reascending course, supplies so many filaments to the 

 same plexus that, according to Cruveilhier, it sometimes appears dis- 

 tributed in almost equal proportions to the larynx and to the heart. 

 Finally other small branches of the pneumogastric in the chest lose 

 themselves at once in the cardiac plexus, beneath the arch of the aorta. 

 All the filaments, accordingly, finally reaching the heart through the 

 cardiac plexus, originate either from the sympathetic or the pneumo. 

 gastric ; and the entire group is characterized by the frequent and inti- 

 mate admixture of fibres from these two sources. 



The effect produced on the heart by irritation of the pneumogastric 

 is precisely the opposite to that usually caused by irritating the nerves 

 of a muscular organ. If the heart be exposed in a warm-blooded quad- 

 ruped by opening the chest, and the circulation maintained by artificial 

 respiration, the action of the pneumogastric may be studied by apply- 

 ing to its trunk the poles of a galvano-faradic apparatus. On stimu- 

 lating the nerve in this way with an interrupted current of moderate 

 strength, the first visible effect is a diminution in frequency of the car- 

 diac pulsations. If the intensity of the current be increased, the heart 

 acts still more slowly; and with a further increase of intensity it stops 

 altogether. 



When the faradization of the nerve is suspended, the cardiac pulsa- 

 tions recommence ; and this may be repeated for many successive trials. 



There are three important facts to be noted in regard to these phe- 

 nomena : 



I. When the heart ceases to move, under the faradization of the 

 nerve, it stops in the condition of muscular relaxation. It lies flaccid 

 and motionless, while its cavities are slowly filled with blood returning 

 from the venous system. On stopping the faradization, on the other 

 hand, the first sign of activity in the heart is a normal pulsation. 

 Stimulation of the pneumogastric nerve, accordingly, tends to arrest 

 the muscular action of the heart. 



