340 PECULIAKITIES OF THE CARDIAC MUSCLE TISSUE 



In explanation of this phenomenon, it should be mentioned that 

 Gotch and K. Lucas^ have shown that the amplitude of the contrac- 

 tions of striated muscle is determined by the number of fibers actually- 

 involved in this process. In other words, while a slight stimulus 

 activates only a relatively small portion of the total mass of the muscle, 

 a strong stimulus causes a much more general reaction. The cellular 

 components of heart muscle, however, are not functionally independent 

 of one another, and hence, are not adapted to give graded reactions. 

 Thus, even the slightest stimulus must produce a wave of excitation 

 which spreads far and wide through its different rows of cells and 

 involves even its most distant constituents. This explanation of the 

 ''all or none" law permits of the conclusion that the mode of contrac- 

 tion of cardiac muscle is not at variance with that of other contractile 

 tissues. It must be evident, therefore, that the functional difference 

 to which attention has just been called, is dependent upon the number 

 of the cellular units involved and not upon any chemicophysical 

 differences in the muscle substance. Consequently, the all or none 

 law merely serves to show that the different components of cardiac 

 muscle are more closely allied with one another than those of skeletal 

 muscle. It is easily noted, however, that this continuity is not the 

 same in all hearts, as is shown by the fact that the effects in those of 

 the frog, turtle and different mammals always possess a disseminating 

 character, while those obtained in the crustacean heart do not. 

 Regarded from the standpoint of hemodynamics,^ a maximally contract- 

 ing heart is of course to be preferred, because it gives rise to more 

 uniform discharges and more constant pressures. 



The assumption that cardiac muscle is a functional curiosity, is 

 disproved further by the fact that it gives rise to the phenomena of 

 summation of stimuli and summation of contractions, both of which are 

 conspicuous characteristics of skeletal muscle. Thus, it has been 

 found that if several subminimal shocks are sent into a quiescent 

 strip of frog's ventricle in rapid succession, these individual stimuU 

 are added to one another until they finally give rise to a contraction. 

 Furthermore, if the ventricle of a Stannius-heart is stimulated with 

 single shocks at the rate of one in every ten seconds, the first reactions 

 frequently tend to be somewhat smaller than those obtained later on, 

 so that an ascending series is produced, resembhng the ''staircase 

 contractions" of striated muscle. This result is obtained only under 

 experimental conditions and, hence, does not run counter to the "all 

 or none" law. 



In accordance with the well-established fact, that a mf.ss of living 

 substance cannot continue to react unless a sufficient time be allowed 

 it during which to replenish the material destroyed during its pre- 

 ceding period of activity, it may justly be assumed that the 

 successive systolic and diastolic phases of the heart represent period- 



1 Jour, of Physiol., xxxviii, 1909, 113. 



2 Woodworth, Am. Jour, of Physiol., viii, 1902, 213. 



