PECULIARITIES OF THE CARDIAC MUSCLE. 189 



1. The strength of the contraction does not vary with the strength 

 of the stimulus. The heart's motto, as Eanvier and Kronecker and 

 Meltzer put it, is, " All or none " ; either it will not contract at all, or it 

 will contract to the fullest extent possible at the time, whether the 

 stimulus be weak or strong. 



2. The cardiac muscle cannot be tetanised. 



3. The cardiac muscle possesses a long refractory period. 



These three facts of stimulation of cardiac muscle are due almost 

 certainly to one common cause, and to that same cause is due in all 

 probability the rhythmical power of the cardiac muscle and the peculi- 

 arities of the conduction which are manifested in the due sequence of 

 the contraction of the various chambers. 



At the present time it is impossible to give any explanation of 

 the difference between striated skeletal and cardiac muscle, which will 

 account for the fact that the extent of contraction in the former varies 

 with the strength of the stimulus, while in the latter the extent of the 

 contraction is independent of the strength of the stimulus. It is 

 possible to imagine that, in the former case, only a portion of the 

 muscle is thrown into contraction with a weak stimulus, while in the 

 latter the whole of the muscle is affected ; or it is possible to imagine 

 that the greater slowness of the metabolic processes, both anabolic and 

 katabolic, which take place in cardiac muscle, as evidenced by the greater 

 length of the refractory period, combined with a difference in the stability 

 of the contractile material when formed, may account for the fact that 

 minimal stimuli are at the same time maximal. Certainly, as far as the 

 other two peculiarities of cardiac muscle are concerned, the absence 

 of tetanus and the length of the refractory period, they are to be 

 explained as a necessary consequence of a slow rate of repair after a 

 contraction. The impossibility of tetanising the cardiac muscle is 

 a necessary corollary of the fact that the tissue is inexcitable to the 

 strongest stimulus for some time after the commencement of a con- 

 traction. 



In all striated muscles this refractory period is said to exist ; in all 

 striated muscles, therefore, a time must elapse after a contraction before 

 the tissue is ready for a new contraction ; the only difference then is, 

 that this time is longer in cardiac muscle than in skeletal. During this 

 inexcitable period, it is natural to suppose that a process of repair is 

 going on in the tissue, so as to bring it back to the condition it was in 

 before the contraction took place ; if this is so, we may express the 

 difference between cardiac and skeletal muscle, by saying that the process 

 of repair after a contraction takes place more slowly in the former than 

 in the latter. 



It is then possible, it seems to me, to find for all three phenomena — 

 the length of the refractory period, the absence of tetanus, and the 

 absence of any relation between strength of contraction and strength of 

 stimulus — one common hypothetical cause, namely, the metabolic pro- 

 cesses, which accompany and are caused by a contraction, take place in 

 the cardiac muscle much more slowly than in ordinary striated muscle. 

 The length of the period of inexcitability after an effective stimulus 

 explains why a continuously acting stimulus, such as the constant 

 current, or the pressure of suitable fluids within the cavity of the 

 ventricle, etc., or the tension of the suspended muscular strip, causes 

 a series of contractions instead of one continuous contraction : and if we 



