45° 



THE PROPERTIES OF STRIPED MUSCLE. 



2. The "all or none" principle. — The second respect in which 

 cardiac muscle differs physiologically from skeletal muscle, namely, its 

 property of responding fully to any stimulus to which it responds at all, 

 whatever the strength of the stimulus, is much easier to explain, 

 inasmuch as it may be accounted for by the structural peculiarities 

 of cardiac muscle. In curarised skeletal muscle there is no communica- 

 tion of the excitatory state from element to element. In the ventricle, 

 such communication is essential to the production of a single contraction 

 or beat. In order that it may happen, the excitatory change produced 

 at the seat of excitation must have a certain intensity, for which a 

 certain strength of stimulus is necessary. 



The same " law " with regard to the propagation of the excitatory 

 process prevails, for a similar reason, in the excitable tissues of plants 

 as, e.g., in the leaf of Dioncva, where also a stimulus, whether mechanical 

 or electrical, is either fully adequate or else inadequate. If a stimulus 

 applied to one lobe is adequate to evoke a response in the opposite 

 lobe, the latter responds in the same way, whatever be the strength 

 of the stimulus. 



