PHYSIOLOGICAL PROPERTIES OF CARDIAC MUSCLE 341 



ically recurring catabolic and anabolic phenomena. No doubt, the 

 systolic movements necessitate the utilization of the largest store of 

 its energy-yielding material which must first be replaced before the 

 next contraction can take place. The systole, therefore, must be con- 

 sidered as the period of decomposition of the contractile substance and 

 the diastole as the period of assimilation. Moreover, as the irritabiUty 

 of all tissues depends upon a proper store of energy-yielding substances, 

 the power of cardiac muscle to respond to stimuli must be at a mini- 

 mum when catabolic processes are going on. This is the case during 

 systole. The stimulus to contract is given immediately preceding this 

 period. This implies that certain chemicophysical changes result at 

 this moment which eventually give rise to the visible contraction. 

 During systole, however, while the heart is thus engaged in converting 

 practically all its potential energy into kinetic energy, no other exci- 

 tation can be brought to bear upon it effectively. This means that it 

 is then in a non-responsive state and is, so to speak, imperrneable or 

 refractory to outside influences. Immediately upon the completion of 

 its refractory period, it again becomes receptive and more so later on 

 in the course of diastole. Its greatest irritabiUty it attains just before 

 the next contraction. 



These changes in irritability may be detected very easily if single in- 

 duction shocks are passed through the heart of a frog or turtle at any 

 time while it registers its contractions upon the paper of a kymograph. ^ 

 It will be noticed that a stimulus which reaches it during its systolic 

 state, does not alter the sequence nor the general character of its con- 

 tractions, whereas a stimulus which enters at the very beginning or at 

 any time during the diastolic period is followed by an extrasystole. 

 This- extra contraction, however, does not appear until the succeeding 

 normal one has been completed. In accordance with what has just 

 been said, it must be clear that a greater strength of stimulus is re- 

 quired to produce this second reaction when applied at the beginning 

 of the period of relaxation than when applied near its end. This dif- 

 ference, as we have just seen, is accounted for by the fact that the 

 restoration of the contractile substances has been practically completed 

 at the end of diastole. The height of these extrasystoles corresponds 

 very closely to that of the normal contractions. 



After the completion of an extrasystole, the heart most generally 

 remains in a condition of relaxation during the interim of one beat. It 

 then exhibits a so-called compensatory pause. This designation, how- 

 ever, is not especially pertinent, because this temporary inhibition does 

 not serve the purpose of compensating for the preceding hyper-effort, 

 but only to correct the disturbance in the rhythm. The correctness 

 of this statement may be proved without much trouble by studying 

 these extra contractions when generated in an isolated ventricle. 

 If this portion of the heart, or a strip thereof, is activated by subjecting 

 it to the stimulating influence of a solution of the inorganic salts, these 



1 Marey, Trav. du laboratoire, 1876. 



