THE CIRCULATION. 273 



as at first. The height of the contraction is independent of the 

 intensity of the electric current. In other words, with very weak 

 induction currents the heart either does not contract, or, if it con- 

 tracts, does its best. With skeletal muscle, increasing the strength of 

 the electrical stimulus increases the height of contraction. 



As the cardiac contractions are always maximal, they make the 

 heart independent of the strength of the stimulus and prevent strong 

 stimuli from producing, in man, excessive contractions. 



Staircase Contraction of Bowditch. In a Stanniused heart 

 where the ventricle is quiescent, single induction shocks of a con- 

 stant strength, applied at intervals of five seconds, will produce con- 

 tractions. It is found that for the first five beats the contractions 

 become longer with each irritation. After that they remain about 

 the same. This is called the staircase phenomena of cardiac muscle. 



Fig. 86. "Staircase" of Beats of a Stanniused Frog's Heart 

 Excited by Maximal Induction Shocks at Intervals of Three Seconds. 

 ( WALLER. ) 



This is also seen in skeletal muscle Dr. F. S. Lee holds that this is 

 due to fatigue-products. 



Refractory Period of the Heart. Marey observed that the 

 irritability of the heart to weak electrical currents decreased during 

 systole and increased during diastole. Thus, a weak induction cur- 

 rent applied to a heart during systole does not call out a contraction, 

 but only during diastole. 



When a stimulus is thrown in at any point between the maxi- 

 mum of the systole and the beginning of the next contraction, it 

 causes what is denominated an extra-contraction, which is followed 

 by a longer pause than usual, the compensating pause. 



The irritability being much lowered from the decomposition of 

 energy-holding compounds is the cause of no contraction when the 

 stimulus is thrown in during systole. 



This compensatory pause is due to the dependence of the ven- 

 tricular beat upon the beat of the auricle. When the auricle or 

 ventricle is isolated the refractory period will not ensue. In a heart 



18 



