CHAPTER XXXII. 



THE FERMENTATION OF UREA, URIC ACID, AND 

 HIPPURIC ACID. 



185. Urea, the Final Product of Animal Metabolism. 



THE natural cycle pursued by the elements normally present in the vegetable or 

 animal body is, with a few exceptions, very simple and easy to follow. Those 

 resisting the action of fire, and therefore found in the ash viz., K 2 0, Na 2 0, 

 CaO, MgO, SiO,, S0 3 , P,O 5 , Fe,O 3 are taken up by the plant from the soil 

 (where they are generally present in sufficient quantity) and are returned thereto 

 in manures. Hydrogen and oxygen are, in the form of water, always plentifully 

 at hand. Carbon is absorbed from the air as carbon dioxide by the plant, and is 

 given up again by the animal in the same form. 



The natural circulation of nitrogen is much more complex. This element, 

 which is indispensable for the construction of albuminoid substances, is an object 

 of solicitude not only to the farmer, who balances the incomings and outgoings 

 of his soil, but also to the bacteriologist, who carefully watches the changes of 

 form nitrogen undergoes during its passage from the plant through the body of 

 the animal and back to the earth, where it is again gradually enabled to renew 

 the cycle. 



Only a portion of the nitrogen consumed by man and animals in the food 

 chiefly in the form of albuminoids, but also as amido-compounds, &c. and 

 transformed and again excreted, leaves the body vid the intestinal canal, i.e. in 

 the faeces. This portion consists, on the one hand, of indigestible or undigested 

 food constituents, and, on the other, of nitrogenous metabolic products ; such, 

 for example, as glycocholic acid and taurocholic acid from the bile ; leucine and 

 tyrosine from the gastric juices, &c. &c. The subsequent fate of these amido- 

 compounds passing into the excrement has already been dealt with in 168. 



The residual nitrogen takes another route in order to make its exit from the 

 body, namely, vid the kidneys, and is excreted in the urine. The most important 

 constituent of this substance is urea, but uric acid, hippuric acid, allantoin, &e., 

 are also present, though in much smaller quantities. The qualitative content of 

 these bodies differs in the various kinds of animals, whilst quantitatively they 

 depend on the amount and composition of the food. 



In human urine and that of the carnivorous mammalia, birds and reptiles 

 which has an acid reaction owing to the presence of acid sodium sulphate uric 

 acid is present to a greater extent than hippuric acid. The total amount voided 

 by a man of normal size is : 



Urea . 35-50 grams. 



Uric acid Q-5--75 gram. 



Hippuric acid . . . . . .0.3 gram. 



On the other hand, the urine of the herbivorous mammals and birds which 

 has an alkaline reaction owing to its content of KHCO 3 exhibits a different 

 substantive ratio, uric acid being in very minute proportion ( T ^-gth per cent.), 

 whilst hippuric acid is comparatively plentiful, e.g. in cows' urine up to | per 



