CHANGES IN A MUSCLE DURING CONTRACTION, 89 



sarkin), xanthiu, taurin, etc., occur in small quantity, and need not be 

 dwelt on here. 



Among non-nitrogenous extractives the most important is the sarcolactic 

 acid, of which we have already spoken ; to this may be added sugar in some 

 form or other, either coming from glycogen or from some other source. 



The ash of muscle, like the ash of the blood corpuscles, and, indeed the 

 ash of the tissues in general, as distinguished from the blood, or plasma, or 

 lymph on which the tissues live, is characterized by the preponderance of 

 potassium salts and of phosphates ; these form, in fact, nearly 80 per cent, of 

 the whole ash. 



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 con- 

 traction ? These changes are most evident after the muscle has been sub- 

 jected 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 contraction 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 shown 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 oxy- 

 gen. 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 split- 

 ting up of some highly complex substance. But here the resemblance 

 between rigor mortis and contraction ends. We have no satisfactory evi- 

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

 this difference in chemical results tallies with an important 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 translucent, 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 evi- 

 dence that kreatin, or any other nitrogenous extractive, is increased by the 

 contraction of muscle ; we have no evidence of any nitrogen waste at all as 

 the result of a contraction ; and, indeed, as we shall see later on, the study 

 of the waste products of the body as a whole lead us to believe that the 

 energy of the work done by the muscles of the body comes from the poten- 



