540 THE SKIN AND THE KIDNEYS. [BOOK n. 



kidney, and is thus momentarily available for the conversion of 

 benzoic into hippuric acid. 



It seems probable therefore that, with regard to this par- 

 ticular 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 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 pig- 

 ments 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 

 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 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 reach it preexistent in the blood, though in 

 several cases it gives the final shape to the excreted substance 

 before this passes into the ureter. 



340. We may illustrate the preceding discussions by briefly 

 passing in review some of the more usual ways in which the 

 secretion of urine is in ordinary life modified. 



In the preceding section the composition of urine was illus- 

 trated by the daily output of the several constituents rather 

 than by a percentage account of any specimen of urine, for the 

 reason that the composition of urine varies within extremely 

 wide limits. This is especially the case as regards the propor- 

 tion of water to solids. One urine may be of high specific 

 gravity with a small amount of water relatively to the solids, 



