86 THE CONTRACTILE TISSUES. 



serum-albumin, a globulin differing somewhat from and coagulating at a 

 lower temperature than paraglobulin, and whic,h to distinguish it from the 

 globulin of blood has been called myoglobulm, some other proteids which 

 need not be described here, and various " extractives " of which we shall 

 speak directly. Such muscles as are red also contain a small quantity of 

 haemoglobin, and of another allied pigment called kistohcematin, to which 

 pigments, indeed, their redness is due. 



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 characterized by a coagulation 

 of the muscle-plasma, comparable to the coagulation 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 coagulation, 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 be- 

 tween the rigor mortis of muscle and the coagulation of blood. Blood 

 during its coagulation 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 produced 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 extract from rigid muscle a certain quantity of lactic 

 acid, or rather of a variety of lactic acid known as sarcolactic acid; 1 and 

 it has been thought that the appearance of the acid reaction of rigid muscle 

 is due to a new formation or to an increased formation of this sarcolactic 

 acid. There is much to be said in favor of this view, but it cannot at 

 present be regarded as established beyond dispute. 



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 



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

 sition, CsHeOs, but differ in their reactions and especially in the solubility of their zine 

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



