OH. xin.] CHEMISTRY OP MUSCLE. 159 



Chemical Composition of Muscle. 



The phenomena of rigor mortis will be more intelligible if we 

 consider the chemical composition of muscle. 



The connective tissue of muscle resembles connective tissue 

 elsewhere : the gelatin and fat obtained in analyses of muscle are 

 derived from this tissue. The sarcolemma is composed of a sub- 

 stance which resembles elastin in its solubilities. 



The contractile substance within the muscular fibres is, during 

 life, of semi-liquid consistency, and contains a large percentage of 

 proteids and smaller quantities of extractives and inorganic salts. 

 By the use of a press this substance can be squeezed out of per- 

 fectly fresh muscles, and it is then called the muscle-plasma. 



After death, muscle-plasma, like blood-plasma, coagulates (thus 

 causing the stiffening known as rigor mortis). The solid clot 

 corresponding to the fibrin from blood-plasma is called myosin, and 

 the liquid residue is called the muscle-serum. 



Pursuing the analogy further, it is found that the coagulation 

 of both muscle-plasma and blood-plasma can be prevented by cold, 

 by strong solutions of neutral salts, and by potassium oxalate, 

 which precipitates, as the insoluble oxalate of calcium, the lime 

 salts essential for the coagulation process. In both cases the 

 clotting is produced by the action of a ferment developed after 

 death. In both cases the precursor of the solid clot is a proteid 

 of the globulin class which previously existed in solution. 



Fibrin in the blood-clot is formed from the previously soluble 

 fibrinogen of the blood-plasma. Myosin in the muscle-clot is 

 formed from the previously soluble myosinogen* of the muscle- 

 plasma. When the blood-clot contracts it squeezes out blood- 

 serum ; when the muscle-clot contracts it squeezes out muscle- 

 serum. The muscle-serum contains small quantities of albuminous 

 material, together with the extractives and salts of the muscle. 

 The origin of the sarco-lactic acid is a controversial question ; some 



* The myosin precursors are really two in number, paramyosinogen, which 

 is coagulated by heat at 47 50, and myosinogen, which is coagulated by 

 heat at 56 C. 



When the muscle is gradually heated at a certain temperature it con- 

 tracts permanently, and loses its irritability. This phenomenon is called 

 lirat rigor. It is undoubtedly due to the coagulation of the proteids in 

 the muscle. If a tracing is taken of the shortening, it is found that the 

 first shortening occurs at the coagulation temperature of paramyosinogen 

 (47* 50), and if the heating is continued, a second shortening occurs at 

 56, the coagulation temperature of myosinogcn. If frog's muscles are used, 

 there are three shortenings, viz., at 40, 47 and 56 C. : frog's muscle- 

 plasma containing an additional proteid which coagulates at 40. (T. G. 

 Brodie.) 



