MUSCULAR TISSUE 359 



Other extractives besides those enumerated above have been described 

 and there are undoubtedly still others whose presence remains undeter- 

 mined. A detailed consideration would, however, be unprofitable in 

 this place. 



Glycogen is an important constituent of muscle. The content 

 of this polysaccharide in muscle varies and is markedly decreased by 

 intense muscular activity. It is transformed into sugar and used as 

 fuel. The liver is the organ which stores the reserve supply of glycogen 

 and transforms it into glucose which is passed into the blood stream 

 and so carried to the working muscle where it is synthesized into gly- 

 cogen. The glycogen thus formed is then changed into glucose as the 

 working muscle may need it. 



Glycogen is a polysaccharide and has the same percentage com- 

 position as starch and dextrin. It resembles starch in forming an opal- 

 escent solution and resembles dextrin in being very soluble, in giving 

 reddish color with iodine and in being dextro-rotatory. Glycogen may 

 be prepared from muscle by extracting with boiling water and then 

 precipitating the glycogen from the aqueous solution by alcohol; dilute 

 or concentrated potassium hydroxide may also be used to extract the 

 glycogen. Glycogen may be prepared in the form of a white, tasteless, 

 amorphous powder. It is completely precipitated from its solution 

 by saturation with solid ammonium sulphate, but is not precipitated by 

 saturation with sodium chloride. It may also be precipitated by 

 alcohol, tannic acid, or ammoniacal basic lead acetate. It has the 

 power of holding cupric hydroxide in solution in alkaline fluids but 

 cannot reduce it. It may be hydrolyzed with the formation of glucose 

 by dilute mineral acids and is readily digested by amylolytic enzymes. 



Mendel and Leavenworth have drawn the conclusion, from the ex- 

 amination of embryo pigs, that embryonic structures do not contain 

 exceptionally large amounts of glycogen. The distribution of the 

 glycogen was not observed to differ from that in the adult animal ex- 

 cept that the liver of the embryo does not assume its glycogen-stor- 

 ing function early. They further draw the conclusion that the meta- 

 bolic transformations of glycogen in the embryo and the adult are 

 entirely analogous. 



The lactic acid occurring in the muscular tissue of vertebrates is 

 paralactic or sarcolactic acid, 1 



H OH 



I I 

 H C C COOH. 



I I 

 H H 



is dextro-rotatory, whereas fermentation lactic acid (<f-Mactic acid) is optically 



inactive. 



