CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 745 



the products of protein decomposition and carbo-hydrates, and 

 between carbo-hydrates and fat. From what we know of the 

 production of lactic acid both outside the body and in the intestine 

 from carbo-hydrates, it might seem a most plausible suggestion 

 that in the active muscle it comes from glycogen. 



Glycogen is the one solid constituent of muscle which has been 

 definitely proved to diminish during activity. It accumulates in 

 a resting muscle, especially in a muscle whose motor nerve has been 

 cut; but rapidly disappears from the muscles of an animal made 

 to do work while food is withheld ; or from the muscles of an animal 

 poisoned by strychmne,which causes violent muscular contractions. 

 But the best evidence points the other way e.g., in rigor mortis 

 lactic acid is produced just as in muscular contraction. Nay, 

 more, the amount of lactic acid (as much as 0-5 per cent, expressed 

 as zinc lactate) produced in full heat rigor (at 40 to 45 C.) is con- 

 stant for similar excised muscles. This ' acid-maximum ' is the 

 same when fresh muscle is at once put into rigor; or when fatigue 

 is first induced, with formation of lactic acid, before rigor; or, 

 finally, when the lactic acid of the fatigued muscle is caused to 

 disappear under the influence of oxygen, and heat rigor is then 

 brought about in the muscle (Fletcher and Hopkins). Yet in rigor 

 mortis the quantity of glycogen is unaltered (Boehm). Further, 

 under certain conditions an excised muscle is capable of producing 

 a quantity of lactic acid much greater than could be derived from 

 the glycogen contained in it. 



An indirect argument against the view that the lactic acid pre- 

 cursor is glycogen has been based by Hill on the results of his studies 

 on the heat production of surviving muscle. From the amount 

 of heat evolved, he calculates that the precursor of lactic acid 

 must have a heat value 10 per cent, greater than that of lactic acid. 

 Now, the heat of combustion of dextrose is only about 3 per cent, 

 more than that of lactic acid. He concludes that the precursor 

 which yields lactic acid is a body of greater energy than dextrose. 

 This, of course, does not preclude the possibility that the complex, 

 whatever it is, from which lactic acid is liberated, contains a carbo- 

 hydrate group. But it would not be profitable to pursue these 

 speculations at present. The facts just mentioned suggest that it 

 is the same precursor which yields the lactic acid developed with 

 the onset of rigor. Further evidence of the close relations between 

 the chemical changes occurring in contraction and those occurring 

 in rigor will be developed in considering the latter phenomenon. 



The Substances metabolized in Muscular Contraction. If the 

 liberation of lactic acid were assumed to be the immediate cause of 

 the mechanical changes in muscular contraction, if the nature of 

 the body which yields lactic acid were known, and if it were proved, 

 which is far from being the case, that the whole of the energy con- 



