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, Si0 2 , 

 S0 3 , P 2 5 , Fe 2 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 via the intestinal canal, i.e. in the faeces. This portion 

 consists, on the one hand, of indigestible or undigested food con- 

 stituents, and, on the other, of nitrogenous metabolic products; 

 such, for example, as glycocholic acid and tyrocholic acid from the 

 bile ; leucine and tyrosine from the gastric juices, &c. &c. The 

 subsequent fate of these amido-compounds passing into the excre- 

 ment has already been dealt with in 1 68. 



The residual nitrogen takes another route in order to make its 

 exit from the body, namely, via the kidneys, and is excreted in the 

 urine. The most important constituent of this substance is urea, 

 but uric acid, hippuric acid, allantoin, &c., are also present, though 

 in much smaller quantities. The qualitative content of these 



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