110 THE HUMAN BODY 



acid. The amount of energy thus manifested has been found to 

 equal about 1.3 calories for each gram of muscle. By chemical 

 analysis the amount of lactic acid produced can be measured. It 

 does not exceed 0.004 gram for each gram of muscle. The pro- 

 duction of 0.004 gram lactic acid has given rise, then, to 1.3 calories 

 of energy. The conversion of this amount of sugar to lactic acid, 

 however, yields only 0.43 calories, or only one-third the amount of 

 energy actually liberated. Evidently the transformation which 

 actually occurs, the result of which is the production of 0.004 gm. 

 lactic acid and 1.3 calories of energy for each gram of muscle, is 

 some other than the direct conversion of sugar into acid. 



These facts, which hold for the energy manifested during the 

 contraction of rigor, are true also for the ordinary contractions of 

 muscle except in so far as the latter are attended by oxidation 

 processes, which liberate additional energy and complicate the 

 determinations. If, however, oxidation be prevented, as can be 

 done by surrounding the muscle with an atmosphere of pure nitro- 

 gen, substantially the same relationship between lactic acid pro- 

 duction and energy manifestation appears as in rigor. A curious 

 and important feature of the energy liberation of an ordinary con- 

 traction is that the oxidation that takes place in connection with 

 the contraction accompanies, not the contraction phase proper, 

 but the relaxation phase. For this reason it has been called the 

 " recovery oxidation." This oxidation, as stated previously, is 

 the ultimate source of the energy shown by the contracting muscle. 

 How are we to connect this chemical process, occurring after the 

 contraction is well under way-, or even after it is over, with the 

 energy shown during the contraction itself? The analogy that 

 has been suggested, and that corresponds with the facts so far as 

 we know them, is that of the pile driver. In this machine the 

 energy of the burning fuel under the boiler is used in raising the 

 weight to the top of the derrick. When the trip is operated the 

 weight falls and by the energy of its impact does the work for 

 which the machine is designed. In our analogy the production 

 of lactic acid corrresponds to the fall of the weight. The stimulus 

 which excites the muscle is represented by the operation of the 

 trip which releases the weight. The process, which, in the muscle, 

 is analogous to the raising of the weight, is pictured as the forma- 

 tion of a substance which is decomposed into lactic acid under the 



