CHAP, ii.] THE CONTRACTILE TISSUES. 91 



acid reaction is not a perfectly simple process but a more or less 

 complex one, other substances besides sarcolactic acid intervening. 



Coincident with the appearance of this acid reaction, though 

 as we have said, not the direct cause of it, a large development of 

 carbonic acid takes place when muscle becomes rigid. Irritable 

 living muscular substance like all living substance is continually 

 respiring, that is to say, is continually consuming oxygen and 

 giving out carbonic acid. In the body, the arterial blood going to 

 the muscle gives up some of its oxygen, and gains a quantity of 

 carbonic acid, thus becoming venous as it passes through the 

 muscle capillaries. Even after removal from the body, the living 

 muscle continues to take up from the surrounding atmosphere a 

 certain quantity of oxygen and to give out a certain quantity of 

 carbonic acid. 



At the onset of rigor mortis there is a very large and sudden 

 increase in this production of carbonic acid, in fact an outburst as it 

 were of that gas. This is a phenomenon deserving special attention. 

 Knowing that the carbonic acid which is the outcome of the 

 respiration of the whole body is the result of the oxidation of car- 

 bon-holding substances, we might very naturally suppose that the 

 increased production of carbonic acid attendant on the development 

 of rigor mortis is due to the fact that during that event a certain 

 quantity of the carbon-holding constituents of the muscle are 

 suddenly Oxidized. But such a view is negatived by the following 

 facts. In the first place, the increased production of carbonic acid 

 during rigor mortis is not accompanied by a corresponding in- 

 crease in the consumption of oxygen. In the second place, a 

 muscle (of a frog for instance) contains in itself no free or loosely 

 Cached oxygen ; when subjected to the action of a mercurial air- 

 pump it gives off no oxygen to a vacuum, offering in this respect 

 a marked contrast to blood ; and yet, when placed in an atmosphere 

 free from oxygen, it will not only continue to give off carbonic 

 acid while it remains alive, but will also exhibit at the onset of 

 rigor mortis the same increased production of carbonic acid that 

 is shewn by a muscle placed in an atmosphere containing oxygen. 

 It is obvious that in such a case the carbonic acid does not arise 

 from the direct oxidation of the muscle substance, for there is no 

 >xygen present at the time to carry on that oxidation. We are 

 driven to suppose that during rigor mortis, some complex body, 

 mtaining in itself ready formed carbonic acid so to speak, is split 

 up, and thus carbonic acid is set free, the process of oxidaticn by 

 which that carbonic acid was formed out of the carbon-holding con- 

 stituents of the muscle having taken place at some anterior date. 



Living resting muscle, then, is alkaline or neutral in reaction, 

 ind the substance of its fibres contains a plasma capable of clotting. 

 Dead rigid muscle on ths other hand is acid in reaction, and no 

 longer contains a plasma capable of clotting, but is laden with the 



