90 THE HUMAN BODY 



or dilute saline solution and which appears to form a framework 

 within the fiber. This latter is called the muscle stroma and con- 

 stitutes 9 per cent of the weight of striated muscle. Muscle 

 tissue contains three or four times as much inyogen as myosin. 

 Both of these proteins possess the property of passing over into 

 insoluble forms known respectively as myogen fibrin and myosin 

 fibrin. 



Heart muscle contains relatively much less myogen and myosin 

 and much more stroma than does ordinary striated muscle, its 

 stroma constituting 56 per cent of its weight. Smooth muscle 

 contains an even larger proportion of stroma, 72 per cent. In 

 striated muscle the proteins appear to be confined largely to the 

 colloidal sarcostyles, except in so far as the sarcolemma is protein 

 in constitution. 



Another constituent of muscle which is apparently part of the 

 contractile machinery is the relatively simple nitrogenous com- 

 pound creatine (p. 14). Recent studies of the part played by this 

 substance in tissue indicate that living protoplasm always con- 

 tains it, and suggest that it may be an essential part of the chem- 

 ical complex upon which life depends. 



The known fuel substances found in muscle are two, dextrose, 

 which is found in very small amount, and glycogen, which forms 

 about 1 per cent of the weight of the muscle. The muscle is 

 probably able to use other substances as fuel, fats, for example, 

 and perhaps proteins themselves on occasion, but chemical analy- 

 sis does not enable us to distinguish these from similar substances 

 which belong to the mechanism. 



Urea and other nitrogenous extractives, and sodium carbonate 

 are found in muscle also. These are to be classed as waste prod- 

 ucts, formed during the operation of the machine. An interest- 

 ing and very significant fact is that a muscle analyzed imme- 

 diately after a period of vigorous activity is found to contain 

 lactic acid, or the compound of this acid with sodium, sodium lac- 

 rate, whereas muscles that have been resting do not contain lac- 

 tates. This fact, as we shall see, has important bearing on the 

 question of the nature of muscle contraction. We shall recur to 

 it in connection with the analysis of the process of contraction. 



Beef Tea. From the facts about proteins, stated above, it is 

 clear that when a muscle is boiled in water its myogen and myosin 



