112 THE HUMAN BODY 



and if so devoted would have the additional advantage of caring 

 for the removal of the acid. That either the second or third of 

 these possibilities represents the method ordinarily operating in 

 the muscle seems certain. Either of them would satisfy the 

 known energy relationships in most respects. There is, however, 

 one fact that seems to favor the replacement theory as against the 

 oxidation theory. This is that if all the lactic acid produced in a 

 simple contraction were oxidized in connection with the relaxation 

 the total energy liberation would be several times that which ac- 

 tually occurs. This fact seems to identify the oxidation that does 

 occur with the production of enough energy for the manufacture 

 of the precursor, rather than with the removal of the lactic acid, 

 and strengthens the view that the acid is removed by being rebuilt 

 into precursor. 



Under conditions of extreme muscular activity the blood is not 

 able to deliver oxygen to the tissues fast enough to enable the 

 oxidations by which lactic acid is removed to keep up with the 

 production of the acid. There is, therefore, under these circum- 

 stances, an excess of lactic acid which must be removed if normal 

 activity is to continue. In this situation the method of removal 

 suggested first above comes into play, namely, the neutralization 

 of the acid by sodium carbonate. The sodium lactate thus 

 formed escapes from the muscles into the blood, is carried with 

 the blood stream to the kidneys, and there eliminated. The 

 proof of this lagging behind of the oxidations when the exercise 

 is very vigorous is furnished by the appearance of considerable 

 sodium lactate in the urine of persons who have recently undergone 

 violent exertion. As compared with the removal of acid by 

 oxidation this is evidently a wasteful process. The acid which 

 escapes in combination with sodium is no longer available for 

 reconstruction into precursor, and some other of the constituents 

 of muscle must be used in its stead. 



Significance of Lactic Acid in the Contraction Process. The 

 relationship of lactic acid to the liberation of energy in muscle has 

 been discussed in detail. There remains for consideration the 

 manner in which the presence of the acid in the muscle brings 

 about the mechanical act of contraction. 



The most satisfactory explanation of this mechanism thus far 

 suggested is based upon a property possessed by colloids of show- 



