570 THE CRANIAL NERVES. 



the cardiac filaments are, therefore, made up of fibres originally belong- 

 ing to the pneumogastric nerve. 



The effect produced upon the heart's action by irritating the pneumo- 

 gastric in the region of the neck is precisely the opposite to that usually 

 caused by irritating the nerves going to a muscular organ. This effect 

 may be seen by opening the chest, and exposing the heart to view, at the 

 same time that the pneumogastric nerves are separated from their con- 

 nections in the neck for a sufficient distance to apply to them the poles 

 of a galvano-electric apparatus. In the cold-blooded animals, as the 

 frog or the turtle, no other precaution is required; in the dog and 

 other warm-blooded species, artificial respiration must be maintained 

 by the nozzle of a bellows inserted in the trachea. 



When a galvano-electric current of moderate strength is applied to 

 the pneumogastric nerves prepared in this way, the cardiac pulsations 

 are reduced in frequency ; and if the current be increased in strength, 

 the heart's action stops altogether, and remains suspended so long as 

 the stimulus continues to be applied to the nerve. When the galvani- 

 zation ceases, the cardiac pulsations recommence ; and the same thing 

 may be repeated for an indefinite number of successive trials. 



There are two important facts to be noted in regard to these effects 

 of irritating the pneumogastric : 



1. When the heart ceases its movements under the galvanization of 

 the nerves, its walls are not in a contracted condition, but in a state of 

 relaxation. Neither are its cavities distended with blood ; but the organ 

 simply remains quiescent, lying at rest without any indication of mus- 

 cular activity. 



2. If the pneumogastric nerves be divided at their point of exposure 

 in the middle of the neck, and if the central extremities be galvanized, 

 no effect is produced upon the heart. But if the stimulus be applied to 

 their peripheral extremities, the above phenomena are reproduced, the 

 heart remaining flaccid so long as the galvanization is continued. The 

 effect in question, therefore, is not due to reflex action, but to a direct 

 influence convej^ed through the pneumogastric filaments to the muscu- 

 lar substance of the heart. This conclusion is fully confirmed by the 

 fact that a similar retardation or stoppage of the cardiac pulsations is 

 caused in frogs and turtles by galvanization of the medulla oblongata 

 itself, the pneumogastric nerves remaining entire ; but if the nerves be 

 previously divided, no such effect is produced. On the other hand, 

 division of the pneumogastric nerves, or sudden destruction of the 

 medulla oblongata, causes increased rapidity of the cardiac pulsations. 

 Section of these nerves, accordingly, in the warm-blooded animals, pro- 

 duces opposite effects upon the respiration and the pulse, one being 

 accelerated and the other retarded. According to Bernard, these 

 effects, though opposite in direction, are produced in similar propor- 

 tions ; so that, if the respirations are diminished one-half, the cardiac 

 pulsations are increased to double their former frequency. Thus when 

 the influence of the pneumogastric nerve is cut off, the motions of the 



