THE CHEMISTRY OF MUSCLE. 61 



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

 polarized light, because it exists in racemic form; that is, it con- 

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

 plane of polarization to the right and of the left-handed form 

 which turns it to the left. In the muscle the right-handed form 

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

 designated as sarcolactic (or paralactic) acid. 



The Nitrogenous Extractives (Nitrogenous Wastes). 

 Muscle extracts contain numerous crystallizable nitrogenous sub- 

 stances which are regarded as the end-products of the disassimila- 

 tion or catabolism of the living proteid material of the muscle. 

 The number of these substances that has 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 proteid catabolism. The one that occurs in largest amount is 

 creatin, C 4 H 9 N 3 2 , or methyl-guanidin-acetic acid, NHCNH 2 NCH 3 - 

 CH 2 COOH. Creatin may be present in amounts equal to 0.3 per 

 cent, of the weight of the muscle. It is given off to the blood 

 and eventually excreted in the urine as creatinin (C 4 H 7 N 3 0), which 

 is formed from creatin by the loss of a molecule of water. The 

 creatinin itself may occur in the muscle in small quantities. 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 belong to the so-called purin bases. These are: Uric acid 

 (C 5 H 4 N 4 3 ), xanthin (C 5 H 4 N 4 2 ), hypoxanthin (C 5 H 4 N 4 0), guanin 

 <C 5 H 5 N 5 0), adenin (C 5 H 5 N 5 ), and carnin (C 7 H 8 N 4 3 ). They will 

 be referred to more fully in the section on nutrition. Still other 

 bodies of similar physiological significance have been described 

 from time to time. These nitrogenous extracts are found in the 

 various meat extracts and meat juices used in dietetics. While 

 they possess no direct nutritive value, it seems probable (see 

 chapter on gastric digestion) that they may be very effective in- 

 directly by stimulating 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. A number of unorganized ferments or enzymes 

 have been described by one observer or another. In this tissue 

 as in others the processes of nutrition seem to be connected with 



