CHAP, ii.] THE CONTRACTILE TISSUES. 101 



Thus while dead muscle contains myosin, albumin, and other 

 proteids, extractives, and certain insoluble matters, together with 

 gelatinous and other substances not referable to the muscle 

 substance itself, living muscle contains no myosin, but some 

 substance or substances which bear somewhat the same relation to 

 myosin that the antecedents of fibrin do to fibrin, and which give 

 rise to myosin upon the death of the muscle. There are indeed 

 reasons for thinking that the myosin arises from the conversion of 

 a previously existing body, which may be called myosinogen, and 

 that the conversion takes place, or may take place, by the action 

 of a special* ferment, the conversion of myosinogen into myosin 

 being very analogous to the conversion of fibrinogen into fibrin. 



We may in fact speak of rigor mortis as characterised by a 

 clotting of the muscle plasma, comparable to the clotting of 

 blood plasma, but differing from it inasmuch as the product is not 

 fibrin but myosin. The rigidity, the loss of suppleness, and the 

 diminished translucency appear to be at all events largely, though 

 probably not wholly, due to the change from the fluid plasma to the 

 solid myosin. We might compare a living muscle to a number of 

 fine transparent membranous tubes containing blood plasma. When 

 this blood plasma entered into the 'jelly' stage of clotting, the 

 system of tubes would present many of the phenomena of rigor 

 mortis. They would lose much of their suppleness and translucency, 

 and acquire a certain amount of rigidity. 



61. There is however one very marked and important 

 difference between the rigor mortis of muscle and the clotting 

 of blood. Blood during its clotting undergoes a slight change 

 only in its reaction ; but muscle during the onset of rigor mortis 

 becomes distinctly acid. 



A living muscle at rest is in reaction neutral, or, possibly from 

 some remains of lymph adhering to it, faintly alkaline. If on the 

 other hand the reaction of a thoroughly rigid muscle be tested, it 

 will be found to be most distinctly acid. This development of an 

 acid reaction is witnessed not only in the solid untouched fibre but 

 also in expressed muscle plasma ; it seems to be associated in some 

 way with the appearance of the myosin. 



The exact causation of this acid reaction has not at present 

 been clearly worked out. Since the coloration of the litmus pro- 

 duced is permanent, carbonic acid, which as we shall immediately 

 state, is set free at the same time, cannot be regarded as the active 

 acid, for the reddening of litmus produced by carbonic acid speedily 

 disappears on exposure. On the other hand, it is possible to ex- 

 tract from rigid muscle a certain quantity of lactic acid, or rather 

 of a variety of lactic acid known as sarcolactic acid 1 ; and we may 

 probably regard the acid reaction of rigid muscle as due to a new 



1 There are many varieties of lactic acid, which are isomeric, having the same 

 composition C 3 H 6 3 , but differ in their reactions and especially in the solubility of 

 their zinc salts. The variety present in muscle is distinguished as sarcolactic acid. 



