CHEMISTRY OF DIGESTION AND NUTRITION. 339 



Xanthin (C 5 H 4 N 4 2 ), hypoxanthin (C 5 H 4 N 4 0), guanin (C 5 H 4 N 4 ONH), 

 and adenin (C 5 H 4 N 4 NH) arc substances closely related to uric acid, and are 

 found in traces in the urine. Since they also originate in the disintegration 

 of nucleins, it is probable that their physiological significance is the same as 

 that of uric acid, and that to the extent to which they occur they also repre- 

 sent an end-product of the katabolism of cell-nuclei. These bodies are 

 found in greatest quantity in muscle, and are present, therefore, in meat- 

 extracts. It is interesting in this connection to call attention to the fact that 

 theobromin (dimethyl-xanthin) and caffein (trimethyl-xanthin) are closely 

 related to the xanthin bodies. 



Creatinin. — Creatinin (C 4 H 7 X h O) is a crystalline nitrogenous substance 

 constantly found in urine. It is closely related to creatin (C 4 H 9 N 3 2 ), the two 

 substances differing by a molecule of water; the creatin changes to creatinin 

 upon heating with mineral acids. Creatinin occurs in urine to the extent 

 of about 1.12 grams per day in man. In dogs it has been found that 

 the amount may vary between 0.5 and 4.9 grams per day according to 

 the diet, an increase in the amount of meat in the diet causing an increase 

 in the creatinin. This is readily explained by the fact that creatin is a 

 constant constituent of muscle, and when taken into the stomach it is 

 eliminated in the urine as creatinin. It is evident, therefore, that part 

 of the creatinin of the urine is derived from the meat eaten, and does 

 not represent a metabolism within the body. A part, however, comes 

 undoubtedly from the destruction of proteid within the body. In this con- 

 nection the following facts are suggestive and worthy of consideration, although 

 they cannot be explained satisfactorily : The mass of proteid tissue in the body 

 is found in the muscles, and the end-product of the destructive metabolism of 

 proteid is supposed to be chiefly urea. Nevertheless, urea is not found in the 

 muscles, while creatin occurs in considerable quantities, as much as 90 grains 

 being contained in the body-musculature at any one time. Only a small 

 quantity (1.12 grams) of creatin is eliminated in the urine as creatinin during 

 a day. What becomes of the relatively large quantity of creatin in the mus- 

 cles? It has been suggested that it is one of the precursors of urea — that it 

 represents an end-product of the proteid destroyed in muscle which is subse- 

 quently converted to urea in the liver or elsewhere. This supposition is sup- 

 ported by the fact that creatin may be decomposed readily in the laboratory, 

 with the formation of urea among other products. But against this theory 

 we have the important fact that creatin introduced into the blood is not con- 

 verted to urea, but is eliminated as creatinin. 



Hippuric Acid. — This substance has the formula C 9 H 9 N0 3 . Its molecular 

 structure is known, since upon decomposition it yields benzoic acid and gly- 

 cocoll, and, moreover, it may be produced synthetically by the union of these 

 two substances. Hippuric acid may be described, therefore, as a benzoyl-amido- 

 acetic acid. It is found in considerable quantities in the urine of herbivorous 

 animals (1.5 to 2.5 per cent.), and in much smaller amounts in the urine of 

 man and of the earnivora. In human urine, on an average diet, about 0.7 



