THE SECRETION OF URINE. 423 



acid introduced into the alimentary canal or injected into the blood reap- 

 pears 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 bloodvessels of the kid- 

 ney 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-hsemoglobin, carbonic-oxide haemoglobin not producing the 

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

 moved 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 ben- 

 zoic acid and glycin to the kidney by means of a blood-stream. If a " sur- 

 viving " 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, cannot 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 momentary 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 kidney, and 

 is thus momentarily available for the conversion of benzoic into hippuric 

 acid. 



It seems probable, therefore, that, with regard to this particular con- 

 stituent of urine, hippuric acid, the cells of the tubules have the power of 

 effecting a combination between the benzoic acid brought to them by the 

 blood and the glycin which they furnish by means of their own metabolism, 

 and in this way produce hippuric acid. 



Not only benzoic acid, but many other bodies taken into the system, 

 reappear in the urine combined with glycin, and in their cases also the 

 combination probably takes place through the activity of the cells of the 

 tubules of the kidney. Moreover, other changes than the assumption of 

 glycin, the various changes which many chemical substances taken into the 

 system undergo before reappearing in the urine, probably also take place to 

 a large extent in the kidney, and are also carried out by means of the 

 epithelium of the tubules. 



What other constituents of normal urine are produced in this or a similar 

 manner we do not as yet definitely know. The pigment urobilin, which, as 

 we have seen, is supposed to be a derivative from bilirubin, may be brought 

 ready-formed from the liver or may have the finishing touches given to it in 

 the kidney itself; and the other normal or abnormal urinary pigments pos- 

 sibly arise either directly from haemoglobin or indirectly from that body 

 through the biliary pigment by a transformation taking place in the cells of 

 the tubules. There is also evidence in frogs that acid sodium phosphate is 

 furnished by the cells of the tubules. 



In conclusion, then, we may say that the activity of the epithelium of the 

 kidney appears especially modified, as compared with other secreting glands, 

 to meet the special object which the kidney has to secure. The purpose of 



