THE MUSCULAR SYSTEM 417 



with difficulty, at o° C. The fluid filtrate is opalescent, neutral, 

 or faintly alkaline in reaction, and is known as ' muscle-plasma.' 

 When its temperature is allowed to rise it coagulates in the same 

 way as does blood-plasma, yielding a clot which, unlike fibrin, is 

 granular and flocculent, and forming a liquid serum. During the 

 clotting the liquid becomes acid, as the result of a formation of 

 sarcolactic acid, and the clot consists of myosin. Assuming, as 

 we may reasonably do, that the muscle-plasma represents more 

 or less closely the muscle-substance in the living fibre, we may 

 take these phenomena of the clotting of the muscle-plasma as 

 indicating the most characteristic chemical differences between 

 living and dead muscle (though there are others), and thus we 

 gain considerable insight into the composition of living muscle 

 as based upon an analysis of the dead tissue. 



With this preliminary caution we may now state the com- 

 position of muscle to be approximately as follows : 



Water ----- 75 per cent. 



Proteins - - - - 20 



Fat 3 



Carbohydrates - - - 0-4 to 1 per cent. 

 Nitrogenous waste products - 0-2 ,, 



Salts 1 to 15 



Our knowledge of the nature of the proteins of muscle is a 

 matter of no slight uncertainty, which is not made less by the 

 existing confusion in the terminology employed by various 

 investigators. Into this we cannot here enter. It must suffice 

 to say that the chief and characteristic protein of dead muscle 

 is the myosin formed in the clotting of muscle-plasma ; it belongs 

 typically to that class of proteins known as globulins. Bearing 

 in mind the phenomena of the clotting of muscle-plasma, and 

 using the nomenclature employed for blood-plasma, we may say 

 that living muscle contains myosinogen, which on the death of 

 the muscle is converted into myosin, just as in blood-plasma 

 fibrinogen gives rise to fibrin. It has not as yet been shown 

 that calcium salts play a part in the coagulative formation 

 of myosin, as they necessarily do in the case of fibrin and of the 

 casein-clot in milk. The proteins of living muscle are not 

 entirely myosinogen, nor are those of dead muscle entirely 

 myosin. Other members of the globulin class are present in 

 both, as also an ordinary albumin closely resembling serum 

 albumin. 



The carbohydrate material is composed chiefly of glycogen, 

 which diminishes in amount by conversion into sugar on the 

 death or after the contracting activity of muscles ; these sub- 

 stances have already been fully dealt with in a previous chapter. 

 The nitrogenous waste products or ' extractives ' are kreatin, 



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