THE PHYSIOLOGY OF THE HEARTBEAT 



177 



be made to behave like the other. They are dependent upon the pres- 

 ence or absence of anastomosis between the muscle fibers. 



1. When electric stimuli of varying strengths are applied to skeletal 

 muscle, the contraction produced by each stimulus is proportional to 

 the strength of the latter until this has become of such a strength that 

 the maximal response is elicited. In cardiac muscle, on the other hand, 

 an entirely different result is obtained, for the weakest stimulus, if it 

 produces any response at all, produces one that is maximal; that is, the 

 height of contraction is the same as it would have been had a much 

 stronger stimulus been applied. Expressing this result in general terms, 

 we may say that in cardiac muscle a minimal stimulus produces a maxi- 



A. Skeletal Muscle 



B. Cardiac Muscle 



Fig. 44. Effects of stimuli of increasing strength on skeletal and cardiac muscle to illustrate 

 the "all or nothing" principle in the latter. (From Practical Physiology.) 



mal effect, whereas in skeletal, the effect, as measured by the height of 

 contraction, is proportional to the intensity of stimulation. This is some- 

 times known as the "all or nothing phenomenon" (Fig. 44). 



2. If maximal stimuli are applied successively and at short intervals 

 of time to skeletal muscle, a slightly higher response results from each 

 succeeding stimulus, until about ten stimuli have been applied, after 

 which for some considerable time the same height of contraction follows 

 each stimulus. If each contraction is recorded, it will be seen that the 

 first few contractions give a staircase effect ; that is, if a horizontal line is 

 drawn from the top of each contraction to the next one, the effect of an 



