

THE PROPERTIES OF MUSCULAR TISSUE 99 



in the tissue. This disturbance in chemical relationship brings 

 about corresponding disturbance in the electric equilibrium which 

 finds expression in an altered electric potential in the part where 

 the chemical activity is going on. 



Just as the chemical changes which follow injury to the tissue 

 give rise to the change of electrical potential which we call the 

 current of injury, so the chemical changes which accompany 

 normal activity in the tissue give rise to electrical changes which 

 are designated currents of action. Action currents cannot easily 

 be demonstrated in an ordinary contracting muscle because the 

 whole muscle goes into contraction at once and so the electric 

 potential of its entire surface rises and falls uniformly. In the 

 heart we have a muscle, however, which does not contract all at 

 once, the contraction sweeping over it from base to apex. The 

 action currents of the heart, therefore, can be demonstrated with- 

 out difficulty if the apparatus used for detecting them is able to 

 respond quickly enough to recurrent changes of potential in 

 opposite directions. An ordinary galvanometer cannot do this 

 because of its too great inertia, but the capillary electrometer 

 answers admirably for the purpose. Another interesting method 

 of demonstrating the action currents of the heart is by causing 

 them to act as stimuli for an irritable tissue. If in a recently 

 killed frog the sciatic nerve is dissected out as far as the knee 

 and cut away from its connection with the spinal cord, being left 

 in connection with the leg below, and if this nerve is laid on the 

 exposed beating heart of the same frog or some other recently 

 killed animal, often the muscles of the lower leg and foot which 

 are connected with the nerve will contract at each beat of the 

 heart. The nerve where it lies on the heart serves as a conductor 

 for the action currents as they are generated in the heart, and 

 the action currents in turn stimulate the nerve during their flow 

 through it. 



The Source of Muscular Energy. In the physical sense a 

 muscle is a machine. By this we mean that whatever energy it 

 gives out must have been supplied to it previously from the 

 outside. The work which a muscle does in contracting is at the 

 expense of its available store of energy. We know that the 

 energy exhibited by a steam-engine is derived from the combus- 

 tion or oxidation of the fuel under the boiler. We know also that 



