100 THE HUMAN BODY 



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 under 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 substance used by 

 contracting muscle is probably for the most part a certain sugar, 

 dextrose, or its anhydride, glycogen. When dextrose is completely 

 oxidized it yields carbon dioxid and water. This reaction is 

 represented by the equation C 6 H 12 O 6 + 12O = 6C0 2 + 6H 2 O. 



The manner in which this energy of oxidation is converted 

 within the muscle to energy of motion is not certainly known, 

 although many interesting theories have been proposed to 

 explain it. Most physiologists agree that the mechanism of 

 skeletal muscle is quite different from that present in smooth 

 muscle. 



The longitudinal fibrils which form such a characteristic feature 

 of skeletal muscle are believed by many physiologists to repre- 

 sent the actual contractile elements of this type of muscular 

 tissue. The precise way in which they perform their function is 

 at present a matter of conjecture. It has been suggested that 

 contraction is due to a rush of fluid into the fibrils from the sur- 

 rounding sarcoplasm. Those who hold this view believe that the 

 oxidation of dextrose to carbon dioxid and water bring about 

 conditions within the muscle which result in movement of fluid 

 of the sort indicated. 



Fatigue of Muscle. A muscle which is made to work continu- 

 ously for a long time responds less and less vigorously as the 

 stimuli are repeated until finally it will contract scarcely any, 

 if at all. It can be proven that this failure to respond to stimu- 

 lation is not due to exhaustion of the fuel supply, but to the 

 accumulation within the muscle of certain chemical products of 

 its activity known as " fatigue products," which by their presence 

 hinder the contractile process. Important among these is 

 sarcolactic acid (C 8 H 6 O 3 ), previously mentioned (see p. 15). 



Fatigue t comes . on much more quickly in a muscle which is 

 mace ttx work after removal xr.ctra 'the body than in one which is 



