CH. XII.] 



CHEMICAL COMPOSITION OF MUSCLE 



137 



The connective tissue of muscle resembles connective tissue else- 

 where; the gelatin and fat obtained in analyses of muscle are 

 derived from this tissue. The sarcolemma is composed of a substance 

 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 

 proteins and smaller quantities of extractives and inorganic salts. 

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

 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 corre- 

 sponding 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 an enzyme developed after death. In 

 both cases the precursor of the solid clot is a protein 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 believe it originates from 

 the carbohydrate (glycogen and sugar) ; others think it comes from 

 the protein molecules in the muscle. 



The general composition of muscular tissue is the following : 



Water . . . 75 per cent. 



Solids 



Proteins . 

 Gelatin . 

 Fat 



Extractives 

 Inorganic salts 



18 

 J2 to 5 



0-5 

 1 to 2 



The proteins, as already stated, chiefly pass into the clot : very 

 little is found in the muscle-serum. 



The extractives comprise a large number of organic substances, 

 all present in small quantities, some of which are nitrogenous, like 

 creatine, creatinine, xanthine, and hypoxanthine : the rest are non- 

 nitrogenous namely, fats, glycogen, sugar, inosite, and the variety 



* For further details see small text at the end of this chapter. 



