CARBOHYDRATES 457 



When flour is freed of starch, ghiten remains behind as a tenacious, 

 sticky mass. It is less abundant in foods than is starch but an 

 equally valuable foodstuff. 



Another amylose is dextrin; it is an intermediate product 

 between starch and glucose. Some of the so-called "soluble" 

 starches are probably dextrins. They are not abundant in 

 plants. Dextrin is used as a substitute for gum. 



Glycogen, or animal starch, occurs rarely in plants — in only a 

 few of the fungi. It has risen to great prominence of late as the 

 fuel for muscular action, though it has long been recognized as a 

 substance of great physiological importance, especially in the 

 liver where formerly it was thought to exist simply as stored 

 excess carbohydrate but now is viewed dynamically, that is to 

 say, as fuel for energy. (The distinction is probably not great, 

 and glycogen is still stored in part as food.) It is readily con- 

 verted into the soluble sugars maltose and glucose, by the starch 

 (amylum)-splitting enzyme amylase. 



Muscular Action. — The search for the source of energy in the 

 contraction of muscle dates back nearly a century, when Helm- 

 holtz concerned himself with the relation between heat production 

 and muscular energy. 



The names of three Englishmen and one German are intimately 

 associated with our present knowledge of muscular action — F. G. 

 Hopkins, W. M. Fletcher, A. V. Hill, andO. Meyerhof. The first 

 two investigators found that muscle when stimulated accumulates 

 lactic acid, CH3-CH(0H)-C00H (so named because it occurs 

 in sour milk). The lactic acid is formed from glycogen, and the 

 amount produced is proportional to the amount of work done. 

 Like a run-down electric accumulator, the exhausted muscle 

 requires recharging. This is done during the period of rest or 

 recovery. The accumulation of lactic acid, which possibly has 

 a toxic effect, causes the muscles to become "tired." (Exhaus- 

 tion of muscle is not fully comparable to that of an electric 

 accumulator, because in the latter, though there are "poisoning" 

 effects, the stored energy is actually used up; while in muscle, the 

 supply of glycogen is not yet gone even at complete fatigue.) 

 During recovery, which is the period of rest — of recharging or 

 reactivation — oxygen is absorbed, and part of the lactic acid is 

 oxidized and part converted back into glycogen. The reaction 

 becomes. Glycogen :^ glucose ^ lactic acid. Thus, muscle tissue 



