1884.] Bejiex Excitation of the Cardiac Nerves in Fishes, 31 



TIL "On Reflex Excitation of the Cardiac Nerves in Fislies." 

 By J. A. Mc William, M.D., Demonstrator of Physiology, 

 University College, London. From the Physiological Labo- 

 ratory, University College. Communicated by E. A. 

 S CHAFER, F.R.S. Received December 4, 1884. 



Cardiac inhibition of a very profound character can be reflexly 

 induced in the eel with a facility that is quite astounding. 



Slight stimulation of either gill brings about an immediate arrest 

 of the heart's action, an arrest that may last for several minutes 

 provided the stimulation be continued. Gentle friction or pressure 

 very readily leads to the same result ; indeed, a mere touch is some- 

 times sufficient. A weak interrupted current is likewise effective; 

 also a thermal or chemical stimulus. If water be injected through 

 the gill apertures at considerable pressure so as to distend the branchial 

 chamber, inhibition of the heart's action is similarly induced. 



Stimulation of the fifth branchial arch (which carries teeth but no 

 gill) has the same effect. 



The branchial nerves are the paths that conduct the afferent impulse 

 to the medulla, whence it passes as an inhibitory impulse down the 

 vagus nerve and along the cardiac branch to the heart. . Stimulation 

 of the central end of a cut branchial nerve leads to the same result 

 as stimulation of the gill, and if one vagus nerve be cut, stimulation 

 of a branchial nerve on the same side can still arrest the heart, pro- 

 vided the vagus nerve on the other side be intact. Section of all the 

 branchial nerves obviates the occurrence of cardiac standstill as a 

 result of gill stimulation, and, of course, destruction of the medulla or 

 section of both vagi has a similar effect. 



The administration of ether or chloroform does not suspend the 

 operation of this mode of reflex inhibition, provided the amount of 

 the anaesthetic be not so great as to arrest all manifestations of 

 activity in the medulla, e.g., the respiratory acts. Neither removal of 

 the various parts of the brain above the medulla, nor section of the 

 spinal cord immediately below the medulla, prevents the occurrence of 

 cardiac arrest. 



This phenomenon of reflex cardiac inhibition as a result of gill 

 stimulation is one that occurs with almost perfect constancy, and 

 usually with a readiness and profoundness that are very striking. It 

 occurs in the uninjured animal, as may be seen by watching the area 

 ■ of cardiac pulsation on the ventral aspect of the body, while the gill 

 as stimulated by pressure or in any other way. The recommeiici-nunit 

 of the heart's action is often delayed for a considerable time (half a 

 , minute or longer) after strong stimulation of the gill has been dis- 



