CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 703 



ligature of the ureters be so damaged as to be unable to carry on 

 their work, an accumulation takes place in blood, not as was once 

 thought of some antecedent of urea such as kreatin, but of urea 

 itself. In the case of birds and reptiles which excrete not urea but 

 chiefly uric acid the accumulation is one of uric acid. Obviously 

 in secreting 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. 



But even this mere picking up the urea is after all not a 

 simple process; the epithelial cell of the tubule is not a mere 

 passive sieve of peculiar structure especially adapted to strain off 

 the urea from the blood. As we have already seen, when urea or 

 uric acid is injected into the blood the result is not a mere 

 increase in the proportions of urea (or uric acid) present in the 

 urine which is being secreted. The injection leads to an increased 

 flow of urine, the whole activity of the cell is stirred up, and other 

 constituents, not at the moment like the urea existing in excess in 

 the blood, are discharged into the lumina of the tubules together 

 with the urea. 



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. 



419. 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 O 2 + C 2 H 5 N0 2 - H 2 = C 9 H 9 N0 3 ) ; and benzoic acid intro- 

 duced into the alimentary canal or injected into the blood, re- 

 appears in large measure in the urine as hippuric acid. Somewhere 

 in the body the benzoic acid meets with and combines 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 kidney 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 containing 

 oxy haemoglobin, carbonic-oxide-hsemoglobin 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 combi- 



