KIDNEY AND SKIN AS SECRETORY ORGANS. 851 



changed to creatinin, with the result that both creatin and creat- 

 inin appear in the urine. Other observers beheve that the meta- 

 boUc 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 contrary, while also constantly formed in the tissues 

 as a result of their metabolism, is not converted to creatinin, but 

 undergoes some further 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, possibly in the synthesis of new protein. On this view the 

 creatin is not to be considered solely as a waste product prepared 

 for excretion, but as a product which can be utilized, under cer- 

 tain conditions at least, for the reconstruction of the protein 

 molecule.* If not so utihzed, then presumably it may be excreted 

 as creatin or as creatinin. 



Hippuric Acid. — This substance has the formula C9H9NO3. Its 

 molecular structure is known, since upon decomposition it yields 

 benzoic acid and glycocoil, 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[C6H5CO]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 glycocoil to form hippuric acid takes place in the kidney 

 itself. Later it was discovered f 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, 

 possibly an enzyme. Vegetable foods contain benzoic acid com- 

 pounds, and we can understand, therefore, why when fed they in- 

 crease the hippuric acid output of the urine. Since in starving 

 animals or animals fed upon meat hippuric acid is still present 

 in the urine, although reduced in amount, it is evident that it 

 arises in part as a result of the body metabolism. The physio- 



* Steinbock and Gross, "Journal of Biol. Chem.," 36, ?Pi, 1P18. 



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



