THE PHYSIOLOGY OF MUSCULAR WORK 213 



bring about a loss of about 90 per cent, of the glycogen of the; 

 muscle. During starvation glycogen disappears from the liver 

 sooner than from the muscles. These facts raise the question 

 whether glycogen is supplied to the muscles by the blood-stream 

 which has taken up glycogen from the liver or whether it can be 

 formed by the activity of the muscles themselves. There is no 

 conclusive evidence to show that the latter is the case, but it will 

 be better to defer the consideration of this point until the sources 

 of muscular energy are discussed in detail. 



Dextrin and Sugars. Between these bodies and glycogen there 

 is a close relationship. Glycogen appears to be the form in which 

 carbohydrate is stored up, probably in loose combination with 

 the proteins of the muscle ; dextrin, maltose, and glucose represent 

 the stages through which the reserve material passes on its way 

 to yield energy during combustion. The glycogen of an excised 

 muscle rapidly decreases and the sugar at the same time increases. 

 This conversion can be effected by the ferments which are found 

 in muscle ; there is an amylolytic enzyme and a maltase, and by 

 their action dextrin, maltose, and finally dextrose are formed. 

 The relation of sugar to muscular activity will be considered 

 later. 



Inosite is found in small quantities in the muscles. It was 

 formerly known as muscle sugar, owing to the fact that it has the 

 same molecular formula (C H 12 ) as glucose. It is not, however, 

 a sugar, but a crystallisable substance belonging to the aromatic 

 series. Nothing appears to be known of its physiological im- 

 portance, although it is found in other parts of the body besides 

 muscle. 



Fat is constantly present between the muscle fibres, but it is 

 uncertain whether it is a normal constituent of muscle ; there is 

 no definite evidence of its presence within the substance of the 

 fibre. Leathes ( 7 ) found that the red muscles of the rabbit contain 

 considerably more fat than the white muscles. 



Lactic acid, paralactic or sarcolactic acid, CH 3 (CH.OH)COOH. 

 There has been a great conflict of opinion upon the question whether 

 the normal muscle contains lactic acid, which is always present 

 in dead muscle. Recently, however, the discrepancies have been 

 explained by Hopkins and Fletcher ( 8 ) in an important research 

 upon lactic acid in amphibian muscle ; by this work the knowledge 

 of the conditions under which lactic acid is formed in muscle has 



