Chap, hi.] ELIMINATION OF WASTE PRODUCTS. 539 



urea the work of the epithelium of the tubules is largely if not 

 exclusively confined to simply picking the urea out of the blood 

 and pushing it so to speak into the lumina of the tubules. We 

 might perhaps say exclusively, for there is no evidence that any 

 urea at all is actually manufactured in the kidney. 



How the urea, which is in this peculiar manner taken out of 

 the blood, comes to make its appearance in the blood is a problem 

 in which the kidney is not concerned and with which we shall 

 deal in treating of the metabolic events of the body generally. 



§ 339. In the case of some other constituents of the urine 

 we have evidence that the cells do something more than simply 

 pick the constituent out of the blood. Hippuric acid, as we have 

 seen, occurs in small quantity in the urine of man, and in larger 

 amount in the urine of herbivora. Now hippuric acid may be 

 formed by the combination, with dehydration, of benzoic acid 

 and glycin (C 7 H 6 2 + C 2 H 5 N0 2 — H 2 = C 9 H 9 N0 3 ) ; and benzoic 

 acid introduced into the alimentary canal or injected into the 

 blood, reappears in large measure in the urine as hippuric acid. 

 Somewhere in the body the benzoic acid meets with and com- 

 bines with glycin. And we have experimental proof that the 

 combination may and probably does take place in the kidney. 



If a circulation of blood be kept up through the blood vessels 

 of the kidndy freshly removed from a living animal, and benzoic 

 acid and glycin be added to the blood as it is about to enter 

 into the kidney, hippuric acid will be found in the blood issuing 

 from the kidney, especially if the same blood be passed through 

 the kidney several times ; the blood used must be blood contain- 

 ing oxyhemoglobin, carbon ic-oxide-haemoglobin not producing 

 the effect. The mere mixing with the blood itself is insufficient; 

 and if the blood be sent not through a kidney just removed from 

 the living body but through one taken from a dead body or one 

 which has been left to itself for some time after removal from a 

 living body, the synthesis will not be effected. To carry out 

 the combination by means of the kidney which has been removed 

 from the body the kidney must retain for a while its own life, it 

 must be a " surviving " kidney. Nor is it absolutely necessary 

 to bring the benzoic acid and glycin to the kidney by means of 

 a blood-stream. If a "surviving" kidney be divided rapidly 

 into small pieces and the, benzoic acid rapidly mixed with the 

 pieces, hippuric acid is formed. Nor is it necessary to furnish 

 the glycin. If benzoic acid alone be used, hippuric acid is 

 formed all the same. Glycin, as we have previously said, can- 

 not be recognized as a normal constituent of any of the tissues ; 

 nevertheless, as we have seen in speaking of glycocholic acid in 

 the bile and as we shall see later on, glycin must make a momen- 

 tary appearance in various metabolic processes of the body, being 

 immediately on its appearance converted into something else, so 

 that it never remains as glycin. It apparently is formed in the 



