THE WORK OF ORGANS AND CELLS 49 



greatly from the food as swallowed. Especially to be noted 

 is the fact that it does not undergo sudden and profound 

 chemical change. When, on the other hand, a muscle is 

 stimulated to contraction, there occurs in it a chemical 

 change requiring less than the hundredth of a second for its 

 completion. This of course suggests the chemical change in 

 gunpowder or dynamite. Obviously the food delivered by 

 the blood to the muscle fiber has been transformed in the 

 fiber into something more unstable, something capable of 

 a very sudden chemical change. The meat, bread, butter, 

 potatoes, and the like have been changed into something 

 comparable to the phosphorus in a match or the gunpowder 

 in a percussion shell. This unstable material has not been 

 demonstrated as granules or other visible material within the 

 cell, as have the zymogen granules of a gland; nor has it 

 been extracted from the cell, as have mucin and enzymes ; 

 but the facts force the conclusion that, like the gland cell, 

 the muscle fiber has used its period of rest to make and store 

 within itself an unstable compound which undergoes upon 

 the application of a stimulus a very sudden chemical change. 

 This unstable compound we may call the fuel substance or 

 the fuel of the fiber. 



9. Available and reserve fuel. The analogy of a match is 

 useful to make clear these fundamental conceptions of mus- 

 cular activity. The phosphorus on the head of the match is 

 the unstable fuel substance ; the friction of the match when 

 it is rubbed against a rough surface is the stimulus, which 

 is followed by a sudden chemical change in the fuel when 

 the match " goes off." At this point, however, the analogy 

 ends; for when a second stimulus is applied to a muscle 

 'within one tenth of a second, there is a second contraction, 

 and in this second contraction there is the same sudden 

 chemical change in the fuel ; moreover, this stimulation may 

 be repeated over and over again with like results. Even 

 more striking is the fact that the same thing is true of a 



