CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 673 



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- 

 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 con- 

 stituent 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 con- 

 verted 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 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 deri- 

 vative 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 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 



r. 43 



