64 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



xanthin (C5H4N4O2), hypoxanthin (C5H4N4O), guanin (C5H5N5O), 

 adenin (C5H5N5), and carnin (C7H8N4O3). They will be re- 

 ferred to more fully in the section on Nutrition. Several other 

 nitrogenous extractives have been isolated and named, but there 

 is perhaps some doubt as to their chemical individuality. These 

 nitrogenous products are found in the various meat extracts and 

 meat juices used in dietetics. While they possess no direct nutri- 

 tive value, it seems probable (see chapter on Gastric Digestion) 

 that they may have a stimulating action upon the secretion of the 

 gastric glands. 



Pigments. — The red color of many muscles is believed to be 

 due to the presence of a special pigment which resembles in its 

 structure and its properties the hemoglobin of the red blood 

 corpuscles, and perhaps is identical with it. This pigment is known 

 as myohematin or myochrome. It belongs presumably to the 

 group of so-called respiratory pigments, which have the property 

 of holding oxygen in loose combination, and by virtue of this 

 property it takes part in the absorption of oxygen by the muscular 

 tissue. 



Enzymes. — Very active chemical changes take place in muscle 

 during contraction as well as during rest. These metabohc changes 

 involve processes of hydrolysis, of oxidation, of reduction, and of 

 synthesis, and, in most cases, they are supposed to be effected 

 through the agency of enzymes. In accordance with this general 

 statement very many different kinds of enzymes have been shown 

 to occur in muscular tissue: proteolytic, amylolytic, and lipolytic 

 enzymes, oxidases or peroxidases, reductases, an enzyme capable 

 of splitting off urea from arginin (arginase), probably deaminases 

 that split off ammonia from the amino-acids, etc. Most of these 

 enzymes will be referred to more or less specifically in the section 

 on Nutrition. 



The Inorganic Constituents. — Muscle tissue contains a number 

 of salts, chiefly in the form of the chlorids, sulphates, and phos- 

 phates of sodium, potassium, calcium, magnesium, and iron. As 

 in other tissues, the potassium salts predominate in the tissue 

 itself. In frog's muscle the entire ash constitutes about 0.88 

 per cent, of the dry material of the muscle, and of this ash the 

 potassium and the phosphoric acid together make up more 

 than 80 per cent. (Urano). These inorganic constituents are 

 most important to the normal activity of the muscle, and, 

 indeed, in two ways: first, in that they maintain a certain os- 

 motic pressure within the substance of the fibers and thus con- 

 trol the exchange of water with the surrounding lymph and 

 blood; second, in that they are necessary to the normal structure 

 and irritability of the hving muscular tissue. Serious variations 



