MUSCLE 669 



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 strychnine, which causes 

 violent muscular contractions. 



What material is the lactic acid formed from ? There are 

 reasons for thinking that it is an intermediate substance which 

 in metabolism may serve as a link between 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. 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 constant 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. It is very possible 

 that the lactic acid arises from protein in the transformation 

 of the store of material whose decomposition is associated 

 with the act of contraction. The facts just mentioned suggest 

 that it is the same store which yields the lactic acid developed 

 with the onset of rigor. But if this be so the transformation 

 must be more complete in rigor than in the fatigue of excised 

 muscles, since the amount of acid produced by severe direct 

 stimulation of a muscle is not more than about one-half of that 

 reached in full heat rigor. 



Source of the Energy of Muscular Contraction. The facts 

 just mentioned show that glycogen may be one of the sources 

 of muscular energy, but it cannot be the only source, for its 

 amount is too small. 



For example, the heart of an average man, which weighs 280 

 grammes, contains about 60 grammes of solids, and among these 

 certainly not more than i gramme of glycogen. In twenty-four 

 hours it produces 120 calories of heat (pp. 127, 584), equivalent to 

 the complete combustion of a little less than 30 grammes of glycogen. 

 To supply this amount, the whole store of glycogen in the heart 

 would have to be used and replaced every fifty minutes. But the 

 accumulation of glycogen is immensely slower in the muscles of a 



