50 THE HUMAN MECHANISM 



muscle removed from the body and consequently shut off 

 from access to new fuel supply in the blood flowing through 

 it. Such an excised muscle will give a long series of con- 

 tractions upon the repeated application of stimuli. With the 

 match or the percussion cap, on the other hand, such repeated 

 discharges would not occur, for the entire stock of fuel is 

 used up with each discharge. 



In order to explain these facts, it is commonly assumed 

 that the fuel substance of the muscle fiber exists in two 

 forms: the one unstable and ready to be discharged by the 

 stimulus ; the other and larger part incapable of being dis- 

 charged by the stimulus, but rapidly providing, after each 

 discharge, the material to make good the loss of unstable fuel. 

 We may speak of the one as the available or unstable fuel and 

 of the other as the reserve fuel. 



10. The chemical change of unstable fuel into waste prod- 

 ucts involves cleavage and oxidation. Although our present 

 knowledge is inadequate to the full understanding of the 

 chemical changes in the muscle during activity, it can at 

 least be stated that changes of two kinds are involved, 

 namely oxidation arid cleavage. 



Oxidation is the union of the material with oxygen, one 

 of the gases of the atmosphere. When carbon (charcoal) is 

 burned, for example, it disappears by uniting with oxygen 

 to form the colorless gas, carbon dioxide ; when hydrogen is 

 burned, it unites with oxygen to form water; or if a chem- 

 ical compound of carbon and hydrogen (for example, kero- 

 sene) is burned, its carbon unites with oxygen to form carbon 

 dioxide, while its hydrogen unites with oxygen to form water. 

 Conversely, when we find that the products of any chemical 

 change contain more oxygen than the original substance, we 

 infer that the change is a combustion or, as it is generally 

 called, an oxidation. 



The second kind of chemical change, cleavage, takes place 

 without the addition of oxygen or, indeed, of any other 



