THE PHYSIOLOGY OF THE HEARTBEAT 181 



These observations enable us to interpret the results obtained by ap- 

 plying electric shocks (extra stimuli) to the beating heart during different 

 phases of systole and diastole. During systole, the muscle being refrac- 

 tory, no effect is produced by the extra stimulus, but during diastole 

 extra systoles which are progressively more pronounced the later in 

 diastole they occur, follow the application of each stimulus. These re- 

 sults are so far exactly like those obtained with a quiescent heart. But 

 another phenomenon now becomes evident; namely, that following each 

 extra systole there is a compensatory pause in the action of the heart, 

 of such duration that, when the next natural beat occurs, it does so 

 practically at the same time as it would have occurred had no artificial 

 stimulus been applied. This will be apparent from the accompanying 

 diagram (Fig. 48). 



It should be noted that the refractory period is greatly diminished by 

 raising the temperature of the heart. Indeed, under these conditions 

 and with strong stimulation it may be possible to produce an almost 

 complete tetanus. , 



The importance of knowing the above facts is that we are thereby 

 enabled to explain the peculiar manner in which the ventricle responds 

 to stimuli transmitted to it from the sinus and the auricle. The muscu- 

 lature of the auricle and ventricle of the mammalian heart is not one 

 continuous sheet, but is separated by a space at the auriculoventricular 

 junction, across which, in specially organized structures, the beat of the 

 auricle is transmitted to the ventricle. Sometimes the stimuli are so 

 freqtient that the ventricular muscle is unable to respond to each stimu- 

 lus transmitted to it, with the result that marked irregularities in con- 

 traction occur (see page 280). In this way certain of the cardiac irregu- 

 larities observed in man can be explained. Thus, the so-called pulsus 

 bigeminus is due to every second beat being an extra systole. This second 

 beat is therefore generally weaker than the preceding and succeeding nor- 

 mal beats, and it is almost always followed by a compensatory pause. When 

 the intervals separating the beats are of uniform length, although every 

 second beat is diminished in size, the pulse is termed pulsus alternans. 



