118 APPLIED PHYSIOLOGY 



disregarded, and unequal volumes of blood transmitted ; 

 but the balance is always speedily rectified by an extra 

 effort on the part of the heart. 



Physiological Properties of Heart Muscle. 



The starting of the cardiac systole is due to the 

 excitability of the heart ; the propagation of the systolic 

 wave, as we have seen, to its conductivity ; the expul- 

 sion of the contained blood to its contractility ; and the 

 regularity of the contractions to its rhythmicity. 



The properties of excitability and rhythmicity will be 



p cub ed, 



FIG 3. SERIES OF VENTRICULAR SYSTOLES (AFTER WENCKEBACH.) 



p to a = Refractory period ; at a, b, c, and d lessening degrees of 

 stimulation will induce contraction. 



further discussed when we come to consider the rhythm 

 of the heart (p. 130). 



As regards the property of contractility, two pecu- 

 liarities must be emphasized. The first is that the 

 contractions of the heart are always maximal i.e., if a 

 stimulus is strong enough to excite the heart to contract 

 at all, the latter always responds with all the strength 

 of which it is capable at the moment. The advantages 

 of this, as pointed out by Wenckebach, are (1) that it 

 makes the heart independent of the strength of the 

 stimulus, and (2) that while weak stimuli produce the 



