260 HUMAN PHYSIOLOGY 



it resides in intermingled nerve-cells continues to have 

 able defenders. 



The All or None Principle. The bloodless ventricle, 

 or a strip cut from it, may be used to study the response 

 of the local muscle to stimuli just as is done so often with 

 the gastrocnemius. (It will be recalled that the ventricle 

 without blood is not likely to make spontaneous con- 

 tractions.) When we observe the effect of electric 

 shocks upon the ventricle we note an apparent dif- 

 ference between its response and that of skeletal muscle. 

 The contractions of a skeletal muscle are proportional 

 in height to the strength of the shocks employed to pro- 

 duce them, within a certain range of intensity. The 

 heart muscle, on the contrary, is said to obey the All 

 or None Principle. If any response follows a shock it 

 is a typic, full-sized contraction. We cannot cause the 

 ventricle to execute small beats by reducing the strength 

 of our stimulation to a minimum. 



A satisfactory explanation of the difference between 

 skeletal muscle with its graded responses and cardiac 

 muscle with its uniform contractions can probably be 

 given. The cells of the cardiac tissue are so linked that 

 the activity of one excites its neighbors. A disturbance 

 once initiated will sweep through the whole fabric. The 

 fibers of skeletal muscle are, so to speak, insulated and 

 the contraction of certain ones does not tend to bring 

 others into action. It has therefore been urged that the 

 small contractions often made by skeletal muscles are 

 due to the contraction of a small proportion of the 

 fibers, the great majority remaining idle. 



Cardiac muscle does not, as a rule, show summation 

 or tetanus when stimulated in a manner which would 

 develop these phenomena in skeletal muscle. Rapidly 

 repeated shocks do not throw the heart into intense and 

 continuous contractions but cause it to flutter. The 

 fact seems to be that whenever the cardiac muscle goes 

 into contraction it expends all its resources for the 

 moment and is bound to relax somewhat while it is re- 



