854 PHYSIOLOGY OF DIGESTION AND SECRETION. 



have a common physiological significance in regard to the metabo- 

 lism. The creatin is regarded as an end-product of the break-down 

 of organized or living protein tissue. It is produced constantly in 

 the tissues and is normally converted to creatinin before it is ex- 

 creted in the urine. Under exceptional conditions, such as starva- 

 tion or fever, the disintegration of the tissues is increased and the 

 amount of creatin produced is too large to be wholly changed to 

 creatinin, with the result that both creatin and creatinin appear in 

 the urine. Other observers * believe that the metabolic history and 

 significance of creatin and creatinin are different. The creatinin 

 formed in the tissues represents an end-product of the breaking 

 down of the organized tissue and, indeed, forms an index of the 

 amount of this tissue wear and tear, but it is given off to the blood 

 and excreted in the urine as creatinin. The creatin, on the con- 

 .trary, while also constantly formed in the tissues as a result of their 

 metabolism, is not converted to creatinin, but undergoes some fur- 

 ther and as yet unknown metabolic change. When creatin is fed 

 to a man, for example, it is not excreted in the urine as creatin or 

 as creatinin, but is used in some way in the body. On this view 

 the significance of the creatin remains undetermined, and its 

 genesis and fate are also left unsettled, except so far as to deny its 

 conversion to creatinin. 



Hippuric Acid. This substance has the formula C 9 H 9 NO 3 . Its 

 molecular structure is known, since upon decomposition it yields 

 benzoic acid and glycocoll, and, moreover, it may be produced 

 synthetically by the union of these two substances. Hippuric acid 

 may be described, therefore, as a benzoyl-amino-acetic acid (CH2- 

 NH[C 6 H 5 CO]COOH). 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 carnivora. In 

 human urine, on an average diet, about 0.7 gm. are excreted in 

 twenty-four hours. If the diet is largely vegetable, this amount may 

 be much increased. This last fact is readily explained, for it has been 

 found that if benzoic acid or substances containing this grouping 

 are fed to animals they appear in the urine as hippuric acid. Evi- 

 dently a synthesis occurs in the body, and Bunge and Schmie- 

 deberg proved conclusively that in dogs the union of benzoic acid 

 and glycocoll to form hippuric acid takes place in the kidney 

 itself. Later it was disco veredf that the same synthesis may be 

 effected by ground-up kidney tissue, mixed with blood and kept 

 under oxygen pressure. It seems possible, therefore, that the 

 synthesis is due to some specific constituent of the kidney cells, 



* Folin and Krause, loc. cit.; also Benedict and Osterberg, "Journal of Bio- 

 logical Chemistry," 18, 195, 1914. 



t Bashford and Cramer, "Zeitschrift f. physiol. Chemie," 35, 324, 1902. 



