302 THE OUTGO OF ENERGY 



For many years the weight of physiological 

 opinion has been largely on the side of the alter- 

 ation hypothesis ; but it would be unsafe without 

 further evidence to decide finally against the 

 molecular theory. 



ACTION CURRENT OF MUSCLE 



The demarcation current (current of injury, 

 current of rest) just studied has been shown to 

 be due to the injury of the tissue. We have 

 now to examine the electromotive forces which 

 appear when a nerve or muscle becomes active. 



1. Rheoscopic Frog. Make two nerve-muscle 

 preparations, A and B. Let the nerve of B 

 rest on muscle A. Stimulate the nerve of A 

 with single induction shocks, and with the 

 tetanizing current. 



Muscle B will contract once for each contrac- 

 tion of A. The current of action of muscle A 

 stimulates the nerve of B. 



Secondary contraction can take place also from 

 muscle to muscle, but only under circumstances 

 that suggest increased irritability, as, for example, 

 through partial drying. No secondary contrac- 

 tion has been secured from voluntary muscular 

 contraction. 



2. That the stimulus to the nerve of the rheo- 

 scopic muscle is really an electrical current, is 



