MUSCULAR ACTIVITY 111 



influence of the stimulus and which in connection with this de- 

 composition yields the amount of energy manifested by the con- 

 traction. The substance so pictured has not been demonstrated 

 chemically. For lack of a definite name it has been called the 

 " lactic acid precursor" to indicate its position as the energy-yield- 

 ing antecedent substance to lactic acid. Since this substance 

 must contain more energy than sugar, or it would not by its de- 

 composition into lactic acid yield enough energy, it cannot be one 

 of the fuel substances, but must be built up within the muscle at 

 the expense of energy furnished by the fuel. If so built up it is 

 not necessary that the oxidation of fuel occur in immediate con- 

 nection with the contraction process, since all the oxidation has to 

 do is to provide energy by which a supply of precursor is kept on 

 hand. The muscle is then ready to respond whenever stimula- 

 tion occurs. 



To complete the picture we must account for the mechanical act 

 of relaxation. Since contraction depends on the production of 

 lactic acid in the muscle relaxation necessarily involves its disap- 

 pearance. That the removal of lactic acid is a definite process 

 is shown by the persistent contraction of rigor, which is due to the 

 loss in the dead muscle of the means for getting rid of the acid. 

 The immediate discharge of lactic acid in living muscles from the 

 contractile elements is in all probability a simple outward diffusion 

 from sarcostyles into sarcoplasm. Under ordinary, conditions 

 the relaxation is too rapid to suggest a more complicated action. 

 Unless the lactic acid is removed from the sarcoplasm, however, 

 equilibrium will soon be reached between it and the sarcostyles 

 and further diffusion will be impossible. There are at least three 

 ways in which lactic acid might be removed. The simplest one, 

 chemically, is by reacting with the alkaline salt of the muscle, 

 sodium carbonate, to form neutral sodium lactate. A second 

 possible method is by oxidation with the formation of carbon 

 dioxid and water and with the liberation of much energy that would 

 be available for the building up of the precursor. A third method, 

 suggested by the analogy of the pile driver above, is the rebuilding 

 of the acid itself into the precursor from which it was originally 

 derived. The energy of the burning fuel might as welt be devoted 

 to the reconversion of lactic acid into precursor as to the building 

 up of the precursor from some other substance than lactic acid, 



