THE CHEMISTRY OF MUSCLE. 61 



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 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, 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 supposed to be given off 

 to the blood and eventually excreted in the urine as creatinin 

 (C 4 H-N 3 0), which is formed from creatin by the loss of a molecule 

 of water (seep. 780). 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 O 3 ), xanthin (C 5 H 4 N 4 O 2 ), hypoxanthin 

 (C 5 H 4 N 4 O), guanin (C 5 H 5 N 5 O), adenin (C 5 H 5 N 5 ), and carnin (C 7 H 8 - 

 N 4 O 3 ). They will be referred to more fully in the section on Nutri- 

 tion. Still other bodies of similar physiological significance have 

 been described from time to time. These nitrogenous products 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 

 indirectly 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 

 the development of special enzymes. A proteolytic enzyme capable 

 of digesting proteins has been described by Brucke and others; 

 an amylolytic enzyme capable of converting the glycogen to sugar 

 by Nasse ; a glycolytic enzyme capable of destroying the sugars 

 by Brunto'n, Cohnheim, and others ; a lipase capable of splitting 



