68 CHEMICAL CHANGES. [BOOK i. 



dependent on the quantity of myosin deposited in the fibres, and 

 the acid reaction to the development if not of lactic acid, at least 

 of some other substance, the parallelism between the three products, 

 myosin, acid-producing substance, and carbonic acid, would suggest 

 the idea that all three are the results of the splitting-up of the 

 same highly complex substance. But we have not at present 

 succeeded in isolating or in otherwise definitely proving the exist- 

 ence of such a body, and though the idea seems tempting, it may 

 in the end prove totally erroneous. 



We may now return to the question, What are the chemical 

 changes which take place when a living resting muscle enters into 

 a contraction? These changes are most evident after the muscle 

 has been subjected to a prolonged tetanus ; but there can be no 

 doubt that the chemical events of a tetanus are, like the physical 

 events, simply the sum of the results of the constituent single 

 contractions. 



In the first place, the muscle becomes acid, not so acid as in 

 rigor mortis, but still sufficiently so, after a vigorous tetanus, to 

 turn blue litmus distinctly red. The cause of the acid reaction 

 like that of rigor mortis is doubtful ; but is in all probability the 

 same in both cases. 



In the second place, a considerable quantity of carbonic acid is 

 set free ; and the production of carbonic acid in muscular contrac- 

 tion is altogether similar to the production of carbonic acid during 

 rigor mortis. It is not accompanied by any corresponding increase 

 in the consumption of oxygen. This is evident even in a muscle 

 through which the circulation of blood is still going on, for though 

 the blood passing through a contracting muscle gives up more 

 oxygen than the blood passing through a resting muscle, the increase 

 in the amount of oxygen taken up falls below the increase in the 

 carbonic acid given out ; but it is still more markedly shewn in a 

 muscle removed from the body. For in such a muscle both the 

 contraction and the increase in the production of carbonic acid will 

 go on in the absence of oxygen. A frog's muscle suspended in an 

 atmosphere of nitrogen will remain irritable for some considerable 

 time, and at each vigorous tetanus an increase in the production of 

 carbonic acid may be readily ascertained. 



Moreover there seems to be a correspondence between the 

 energy of the contraction and the amount of carbonic acid and 

 the degree of acid reaction produced, so that, though we are now 

 treading on somewhat uncertain ground, we are naturally led to the 

 view that the essential chemical process lying at the bottom of a 

 muscular contraction as of rigor mortis is the splitting up of some 

 highly complex substance. But here the resemblance between rigor 

 mortis and contraction ends. We have no evidence of the formation 

 jiuring a contraction of any body like myosin. Now the contracted 

 and rigid muscle differ essentially in the fact that while the former, 

 as compared with living resting muscle, increases in extensibility and 



