CHANGES IN A MUSCLE DURING CONTRACTION. 87 



place when muscle becomes rigid. Irritable living muscular substance, like 

 nil 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 phenomena deserving special attention. Knowing that the car- 

 bonic acid which is the outcome of the respiration of the whole body is the 

 result of the oxidation of carbon-holding substances, we might very natu- 

 rally 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 sud- 

 denly 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 increase 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 shown 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 sub- 

 stance, 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, con- 

 taining 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 car- 

 bonic 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 or parallelism 

 between the intensity of the rigor mortis, the degree of acid reaction and 

 the quantity of carbonic acid given ont. If we suppose, as we fairly 

 may do, that the intensity of the rigidity is 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. No one has at present, however, succeeded 

 in isolating or in otherwise definitely proving the existence of such a body, and 

 though the idea seems tempting, it' may in the end prove totally erroneous. 



62. As to the other proteids of muscle, such as the albumin and the 

 globulin, we know as yet nothing concerning the parts which they play and 

 the changes which they undergo in the living muscle or in rigor mortis. 



Besides the fat which is found, and that riot infrequently in abundance, 



