METABOLISM, NUTRITION AND DIETETICS 445 



Hippuric acid can undoubtedly be produced in the kidney. 

 If an excised kidney is perfused with blood containing 

 benzoic acid, or, better, benzoic acid and glycin, hippuric 

 acid is formed. The kidney cells must be intact, for if a 

 mixture of blood, glycin, and benzoic acid be added to a 

 minced kidney immediately after its removal from the body, 

 hippnric acid is produced, but not if the kidney has been 

 (-rushed in a mortar. In herbivora hippuric acid cannot 

 nonnally be detected in the blood; it is present in large 

 quantities in the urine ; it must therefore be manufactured 

 in the kidney, not merely separated by it. In certain 

 animals, as the dog, the kidney is the sole seat of the produc- 

 tion of hippuric acid. But in the rabbit and the frog some 

 of it may also be formed in other tissues, for after extirpation 

 of the kidneys the administration of benzoic acid causes hip- 

 pi nic acid to appear in the blood. The benzoic acid comes 

 mainly from substances of the aromatic group contained in 

 v< :;< -table food, but a small amount is produced in the body, 

 sinre hippuric acid does not entirely disappear from the urine 

 in starvation. It is not known in what form the nitrogenous 

 ^Ivein appears on the spot where it is wanted to form hip- 

 pmic acid, since glycin has not been found anywhere in the 

 tissues. Hut there is no doubt that it is a product of the 

 metabolism of proteids (and gelatine). It is also ?. con- 

 stituent of glycocholic acid, and may be derived in part from 

 the bile which is reabsorbed. 



Kreatinin can be so readily obtained from kreatin outside 

 the body, that it is very tempting to suppose that the 

 kivatinin of the urine is manufactured by the kidney from 

 the kreatin of muscle carried to it by the blood. It has been 

 shown, however, that most, at any rate, of the kreatinin of 

 the mine is derived from ready-formed kreatin in the food. 

 The kreatin of the muscular tissue may be ultimately changed 

 into nrca, but if this is the case, the intermediate steps are 

 unknown. A small amount of kreatinin is excreted in the 

 urine, even in starvation, and this may come from the 

 muscular kreatin. 



Formation of Carbon Dioxide from Proteids, We cannot say 

 whether any carbon dioxide is normally produced at thc- 

 moment when the nitrogenous portion of the proteid molecule 



