458 PROTOPLASM 



in doing work derives the necessary energy not from oxidation, as 

 is true in many living and nonliving machines, but from the 

 rapid conversion of the carbohydrate glycogen into lactic acid. 

 The action is an exothermic one; i.e., it proceeds without elimina- 

 tion of heat. 



When the fatigued muscle recovers, it recharges its store of 

 energy by reconverting the lactic acid into glycogen, to do which 

 it needs a source of energy; this it derives from further oxidation 

 of some of the carbohydrates. The reaction is a coupled one; 

 that is to say, the synthesis of the glycogen cannot take place 

 except through oxidation of part of the lactic acid. As the latter 

 supplies energy for the former, more glycogen must come from 

 other sources — perhaps out of the surrounding solution by 

 adsorption on to the protein constituents of the muscle. 



Such was the story in 1924, when Embden stated and after- 

 ward often reiterated that part (now all) of the lactate is formed 

 after the contraction is over. Although it had long been known 

 that phosphorus compounds play a part in muscular action 

 (creatine was known to be present in large amounts), the con- 

 tention of Embden was not fully accepted until six years later, 

 when Lundegaard proved that muscle may contract without the 

 formation of lactic acid. Thus is lactic acid removed from the 

 central position that it heretofore occupied in muscular con- 

 traction. The initial reaction may be the breakdown either of 

 glycogen or of phosphagen. In the former case, the reaction is 

 apparently immediately followed by the combination of a hexose 

 sugar with phosphoric acid to form an ester. Lactic acid is 

 produced later. 



Again it must be said that in giving to carbohydrates the chief 

 role in muscular action, it is likely that other substances, notably 

 the fats, may serve as a source of the energy. Hemoglobin, 

 carnosine, and potassium are also present in muscle with no 

 known role. The present opinion seems to be that any foodstuff 

 may be used for the restoration of energy in muscle. 



We have a satisfactory (though not the only) explanation of the 

 chemistry of muscular action, but it tells us nothing of the 

 physical mechanism involved. We may learn all the details of 

 coal combustion and yet know nothing about the machinery of a 

 locomotive. As the mechanics of muscular action involves, 

 possibly, such forces as surface tension, imbibition, and struc- 



