THE CHEMISTRY OF MUSCLE. 63 



During muscular activity the store of glycogen is used up, and if 

 the activity is sufficiently prolonged it may be made to disappear 

 entirely. 



Lactic Acid (CgH^O.^). — Lactic acid is found in varying amounts 

 in the extracts of muscle. The acid that is obtained is the so-called 

 ethidene lactic acid or «-hydroxyi3ropionic acid (CH.,CHOHCOOH), 

 and differs from the lactic acid found in sour milk in that it ro- 

 tates the plane of polarized light to the right. The lactic acid in 

 sour milk is produced by bacterial fermentation, and is inactive to 

 polarized light, ]:)ecause it exists in racemic form ; that is, it con- 

 sists of equal amounts of the right-handed form which turns the 

 plane of p.Giarization to the right and of the left-handed form 

 which turns it to the left. In the muscle the dextro-rotary form 

 is found mainly or only, and this form, therefore, is frequently 

 designated as sarcolactic (or paralactic) acid. Recent work 

 indicates that in the perfectly resting muscle lactic acid is 

 present only in traces. The amount is greatly increased during 

 contraction or in the processes leading to rigor. This substance 

 may be considered, therefore, as an intermediary product formed 

 in the course of the chemical reactions constituting the normal 

 metabolism of the muscle. Its origin is considered in more detail 

 below. 



The Nitrogenous Extractives (Nitrogenous Wastes). — Muscle 

 extracts contain numerous crystallizable nitrogenous substances 

 which are regarded as the end-products of the disassimilation 

 or catabolism of the living protein material of the muscle. The 

 number of these substances that have been found in traces or 

 weighable quantities is rather large. They have aroused great 

 interest because their structure throws some light on the nature 

 of protein catabolism. The one that occurs in largest amount is 

 creatin, C4H9N3O2, or methyl-guanidin-acetic acid, NHCNH2- 

 NCH3CH2COOH. Creatin is present in amounts equal to 0.4 

 to 0.5 per cent, of the weight of the muscle. The relations of this 

 substance to the creatinin (C4H7N3O) found in the urine are dis- 

 cussed on p. 849. Another nitrogenous body with basic proper- 

 ties which occurs in amounts about equal to the creatin is carno- 

 sin, C9H14N4O3.* It is probably a derivative of histidin, since 

 on hydrolysis it yields histidin and alanin (Gulewitsch). Noth- 

 ing is known of its physiological significance. In addition there 

 is a group of bodies supposed to represent the end-products of 

 the breaking up of the nucleins of the muscle, all of which be- 

 long to the so-called purin bases. These are : Uric acid (C5H4N4O3) , 



* Von Fiirth and Schwarz, "Biochemische Zeitschrift," 1911, 30, 413. 



