INHIBITION-BACKWAED PROPAGATION OF STIMULI. 621 



refractory period. In Fig. 172, p. 585, we notice that the 

 auricular contraction succeeded by ventricular inhibition occurs 

 after the refractory period of the ventricle has passed ; we must, 

 therefore, look upon the inhibition as due to the propagation of 

 a nervous impulse from one cavity to another. In the auricle 

 we find that stimulation may produce inhibition of the auricu- 

 lar and ventricular beats, or of the ventricular beats alone. 

 We may, therefore, suppose that the stimulus applied to the 

 auricle acts upon two different nervous mechanisms; seeing 

 that it is enabled to inhibit the ventricular beats without affect- 

 ing the auricular ones, we are unable to say precisely what the 

 effect of a single stimulus applied to the venous sinus is upon 

 the sinus itself, but here we note that the same result will 

 follow stimulation of the sinus, as of the auricle, viz., inhibition 

 of the ventricular without inhibition of the auricular beat, or 

 inhibition of both together. 



As has been already pointed out by Professor Marey, the 

 refractory period is increased when the heart is artificially 

 cooled. We have also found that there is a prolongation of the 

 time during which stimulation causes an inhibition or omission 

 of the following systole. 



It is very seldom that stimulation of the auricles or of the 

 venous sinus causes a ventricular contraction without auricular 

 systole preceding it in tlie ordinary rhythm. In this respect 

 the action of the heart offers a contrast to the normal. Though 

 the muscular wave started in the auricle is usually succeeded 

 by a ventricular contraction, it may occasionally be succeeded 

 by a ventricular inhibition, or auricular stimulation may be 

 followed by inhibition of both auricle and ventricle. 



The propagation of the wave in an upward direction, viz., 

 from ventricle to auricle, is not so regular as in the normal 

 heart, the time elapsing, when it does occur between the ven- 

 tricular and auricular systole, bearing a relationship to the 

 degree of cold produced. Wliilst the ventricle is reduplicating 

 in response to direct stimulation, the auricle may maintain its 

 regular rhythm. Stimulation of the venous sinus almost in- 

 variably gives an auricular contraction at all times preceding 

 the ventricular. It has been already shown that in the case of 



