CHAP, n.] THE CONTRACTILE TISSUES. 105 



63. We may now pass on 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 con- 

 stituent 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 not quite clear, 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 resembles the production of carbonic acid during rigor mortis 

 in that it is not accompanied by a 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 satisfactory evidence of 

 the formation during a contraction of any body like myosin. And 

 this difference in chemical results tallies with an important physical 

 difference between rigid muscle and contracting muscle. The 

 rigid muscle as we have seen becomes less extensible, less elastic, 

 less translucent; the contracting muscle remains no less trans- 

 lucent, elastic, and extensible than the resting muscle, indeed 

 there are reasons for thinking that the muscle in contracting 

 becomes actually more extensible for the time being. 



But if during a contraction myosin is not formed, what changes 

 of proteid or nitrogenous matter do take place? We do not know. 

 We have no evidence that kreatin, or any other nitrogenous 

 extractive is increased by the contraction of muscle, we have no 



