704 SECRETION OF HIPPURIC ACID. [BOOK n. 



nation 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, 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 meta- 

 bolic 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 

 constituent 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 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 pig- 

 ment 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 possibly 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 that in frogs acid sodium 

 phosphate is furnished by the cells of the tubules. 



In conclusion then we may say that the activity of the epi- 

 thelium 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 the kidney is not to provide 

 a fluid, urine, which can be made use of for the needs of the body, 

 but to cast out waste matters from the body. Hence its secretory 

 activity is limited largely to the mere discharge of matters which 



