1884.] Reflex Action of the Cardiac Nerves in Fishes. 35 



lation of those regions of the skin which most readily lead to reflex 

 excitement of the spinal and medullary motor centres also leads most 

 readily to excitement of the vagus centre. 



Stimulation of the parietal peritoneum very readily gives rise to 

 reflex cardiac arrest. 



Tapping the ventral aspect of the abdomen of an intact eel is 

 commonly followed by more or less marked slowing of the heart's 

 action. And when the abdominal cavity is being laid open, the heart 

 generally stands still for a considerable length of time. When this 

 standstill has passed away, the introduction of a finger-tip into the 

 abdominal cavity so as to come into contact with the parietal peri- 

 toneum is frequently followed by a temporary cardiac inhibition. If 

 the abdominal parietes be now fixed apart so as to expose the cavity 

 fully, it can be readily observed that stimulation of the parietal portion 

 of the peritoneum is a very effective cause of reflex cardiac arrest. 

 Application of slight friction, a weak interrupted current, or a hot 

 wire to the internal surface of the abdominal parietes, almost invari- 

 ably brings about cardiac inhibition in a very marked degree. Con- 

 traction of the neighbouring trunk muscles commonly occurs at the 

 same time ; such contraction at times involves the muscles along the 

 whole length of the body. These muscular contractions (accompanying 

 the reflex cardiac arrest) occur on both sides of the body, and are 

 evidently a reflex effect of the peritoneal stimulation. A very weak 

 interrupted current is sufficient to bring about the results mentioned : 

 such a current as is obtained from the secondary coil (of a du Bois- 

 Reymond's induction machine, with the Helmholtz modification) at a 

 distance of 11 cm. from the position where it covers the primary coil 

 completely, whilst the battery used is a single Daniell's element. 

 The effect on the heart's action caused by stimulation of the parietal 

 peritoneum seems to occur more readily in response to a weak inter- 

 rupted current than to slight mechanical stimulation. 



If the parietal peritoneum be stripped off from the subjacent tissues 

 at any part where reflex inhibition is found to be readily excited, the 

 -application of the electric current to the same spot — now denuded of 

 its peritoneal covering — fails to exert any influence on the heart's 

 action, unless the current spread to one of the spinal nerves. But if 

 the electrodes be moved to another part of the abdominal parietes — 

 to a part where the peritoneal layer is still intact — the same cardio- 

 inhibitory phenomena as before can be readily observed. Mechanical 

 and thermal stimulation give similar results. 



The afferent impulses generated by stimulation of the parietal peri- 

 toneum seem to pass to the medullary vagus centre chiefly through 

 the spinal cord. Section of the upper part of the cord obviates the 

 cardio-inhibitory effect of peritoneal stimulation. Section of the 

 vagus nerves below the origin of the cardiac branches does not 



d 2 



