CHAP, ii.] THE CONTRACTILE TISSUES. 67 



living muscular substance like all living protoplasm is continually 

 respiring, 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 muscular 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 itl 

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

 Knowing that the carbonic acid which is the outcome of the res- 

 piration of the whole body is the result of the oxidation of carbon- 

 holding subtances, we might very naturally suppose that the in- 

 creased 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 any 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 

 attached 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 

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

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

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

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

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

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



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

 and the substance of its fibres contains a coagulable plasma. Dead 

 rigid muscle on the other hand is acid in reaction, and no longer 

 contains a coagulable plasma, but is laden with the solid myosin. 

 Further, the change from the living irritable condition to that of 

 rigor mortis is accompanied by a large and sudden development of 

 carbonic acid. 



It is found moreover that there is a certain amount of parallel- 

 ism between the intensity of the rigor mortis, the degree of acid 

 reaction and the quantity of carbonic acid given out. If we 

 suppose, as we fairly may do, that the intensity of the rigidity is 



52 



