THE CHEMISTRY OF MUSCLE 87 



40 per cent, of the former and 60 per cent, of the latter. By far the greatest 

 amount of organic phosphorus is present in the form of phosphatide. 



Lactic Acid. — Most generally muscle tissue also contains a certain amount of 

 ethidene lactic acid or sarcolactic acid, CH3CHOHCOOH. This acid is a product 

 of tissue metaboHsm. It is dextrorotary, while that contained in sour milk, is in- 

 active to polarized light and finds its origin in bacterial fermentations. In normal 

 resting muscle it is difficult to detect it, because it is oxidized as rapidly as it is 

 formed, but its removal may be greatly interfered with by restricting the entrance 

 of oxygen. The amount of this acid is greatly augmented during muscular activity. 



Extractives. — If muscle tissue is extracted with boiling water, a number of sub- 

 stances are obtained which are of especial interest, because they represent in all 

 probability the products of the metabolism of muscle. Chief among these are 

 those of nitrogenous origin, because they give rise to some of the substances ex- 

 creted in urine. As a rule, fresh muscle yields about 2 per cent, of its weight in 

 extractives of which 0.7 per cent, is of organic and 1.3 per cent, of inorganic origin. 

 The one present in largest amounts is creatin, C4H9N3O2, which equals 0.1 to 0.4 

 per cent, of the weight of the mammalian muscle. Creatinin, C4H7N3O, is present 

 in much smaller amounts, but constitutes 0.3 per cent, of the weight of the muscles 

 of fish. No definite conclusions have been reached as yet regarding the origin of 

 these bodies and even the statement of Liebig and Ranke, ^ that creatin is a fatigue 

 substance and increases with muscular activity, has not been substantiated by 

 the more recent and more exact quantitative determinations of these substances. ^ 

 Carnosin,^ CgHi4N405, is a basic extractive and is said to be derived from histidin, 

 because on hydrolysis it yields histidin and /3-alanin. It is present in about the 

 same proportion as creatin. Other bodies are carnitin, novain and taurin. 



The purins of muscle are relatively scanty in amount, because by far the great- 

 est part of the muscle cell is composed of cytoplasm. They are represented by 

 such bodies as uric acid (C5H4N4O3), xanthin (C5H4N4O2), hypoxanthin (C5H4N4O), 

 guanin (C5H5N5O), adenin (C5H5N5) and carnin (C7H8N4O3). Urea is present in 

 very small amounts in the muscles of mammals (0.04 to 0.08 per cent.), but in 

 much larger quantities in the muscles of certain fish (1 to 2 per cent.). 



Pigments and Enzymes. — The red color of muscle is said to be due to a pigment 

 which is known as myohematin or myochrome. Inasmuch as this body presents 

 several of the characteristics of hemoglobin, it is commonly said to be identical 

 with it. Its chief function is respiratory, because it furnishes the muscle with 

 oxygen which it holds in loose combination. 



The substances furnished to the muscles by the blood, are made available for 

 their metabolism b}^ hydrolysis, oxidation, reduction and synthesis. It is believed, 

 therefore, that muscle tissue is in possession of certain enzymes which are capable 

 of instigating these processes. Their function is proteolytic, lipolytic and amylo- 

 lytic. They also act as oxidases or peroxidases, reductases, deaminases, etc. The 

 products of muscular metabolism frequently exert a certain influence upon the 

 function of other structures. Thus, lactic acid and carbon dioxid serve as stimu- 

 lants to the respiratory center, while the accumulation of these and other bodies 

 in consequence of disturbances in their excretion, may give rise to toxic symptoms. 



The Chemical Changes in Contracting Muscle. — The metabolic 

 alterations in the contracting muscle are characterized by a constancy 

 of the catabolism of the proteins and an increase in the catabolism of 

 the carbohydrates, together with a production of lactic acid and carbon 

 dioxid. This is clearly shown by the fact that muscular work does not 

 augment the nitrogen output of the muscle nor of the body, but is 



* Tetanus, eine physiol. Studie, Leipzig, 1865. 



^jGrindley and Woods, Jour. Biol. Chem., .ii, 1906, 309; Urano, Hofmeister's 

 Beitrage, ix, 1906, 104, and Meyers and Fine, Jour. Biol. Chem., xv, 1913, 283. 

 ' Gulewitch and Amiradzibi, Zeitschr. phys. Chem., xxx, 1900, 565. 



