104 THE HUMAN BODY 



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. Delicate galvanometers have been devised 

 which answer 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 re- 

 cently 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 

 the energy exhibited by a contracting muscle is derived from 

 the oxidation of fuel substances within it. The physical accom- 

 paniments of oxidation are not the same in the two cases; the 

 fuel wnder the boiler burns with flame and at a high temperature; 

 the fuel substance within the muscle burns without flame and at 

 a temperature only slightly higher than that of the body. The 

 energy yield, however, for corresponding amounts of fuel is as 

 great in one case as in the other. 



The fuel substances used in the Body are fhiefly dextrose (grape 

 sugar), or the closely related substance glycogen, and fats. That 

 the third group of energy yielding foods, the proteins, are not or- 

 dinarily used as fuel for the muscles was proven by a very inter- 



