SENSITIVE PHASE AND REFRACTORY TERIOD. 577 



one, to which we have given in the following paper the term of 

 the " sensitive phase/* lasting from the maximum of systole to 

 its end. The refractory period varies in duration according to 

 the intensity of the stimulus, and the conditions under which 

 the heart is operated upon. The feebler the stimulus, the 

 longer is the refractory period. When the stimulus is very 

 slight the refractory period may persist during the whole ven- 

 tricular systole; as the stimulus is increased, the refractory 

 period becomes shorter, and finally, when it is very strong, disap- 

 pears altogether. 



Heat applied to the heart shortens the refractory period or 

 abolishes it altogether. Cold has an opposite effectj and 

 lengthens the refractory period. The contractions caused by 

 artificial stimulation do not much alter the cardiac rhythm, for 

 the accelerated beat is followed by a longer pause than usual 

 which compensates for the diminished interval between the two 

 first beats. Sometimes no ventricular contraction is induced, 

 and then instead of acceleration there is apparent inhibition, the 

 application of the stimulus being followed simply by a longer 

 diastolic pause than usual. 



Marey's observations were confined entirely to the movements 

 of the ventricle, but we have extended ours to the movement of 

 the auricle as well. We employed two levers : one resting upon 

 the ventricle, and the other upon the auricle, which recorded 

 movements upon a revolving cylinder covered with smoked paper 



It is unnecessary to enter here into a fuller description of the 

 apparatus, which is given elsewhere.* 



By the method employed we are able to study the effects of 

 maximal and minimal stimulation applied to the ventricle, 

 auricle, and venous sinus upon the movements both of auricle 

 and ventricle. 



By minimal stimulation we understand the smallest shock 

 that produces any visible effect that in any way modifies 

 the course of contraction or the rhythm of the organ ; and by 

 maximal stimulation we mean the electrical irritation of such a 

 strength that its intensification produces no visible increase in 

 its effect. 



• Cash, Journal of Physiology^ vol. ir, N"©. 2. 



2 P 



