348 EXCRETION. 



soda discharged by the healthy human subject is, according to 

 Lehmann, about 25 grains. This substance exists in the urine of 

 the carnivorous and omnivorous animals, but not in that of the her- 

 bivora. In the^latter, it is replaced by another substance, differing 

 somewhat from it in composition and properties, viz., hippurate 

 of soda. The urine of herbivora, however, while still very young, 

 and living upon the milk of the mother, has been found to contain 

 urates. But when the young animal is weaned, and becomes her- 

 bivorous, the urate of soda disappears, and is replaced by the hip- 

 purate. 



URATES OF POTASSA AND AMMONIA. The urates of potassa and 

 ammonia resemble the preceding salt very closely in their physio- 

 logical relations. They are formed in very much smaller quantity 

 than the urate of soda, and appear like it as ingredients of the urine. 



The substances above enumerated closely resemble each other in 

 their most striking and important characters. They all contain 

 nitrogen, are all cry stall izable, and all readily soluble in water. 

 They all originate in the interior of the body by the decomposition 

 or catalytic transformation of its organic ingredients, and are all 

 conveyed by the blood to the kidneys, to be finally expelled with 

 the urine. These are the substances which represent, to a great 

 extent, the final transformation of the organic or albuminoid in- 

 gredients of the tissues. It has already been mentioned, in a pre- 

 vious chapter, that these organic or albuminoid substances are not 

 discharged from the body, under their own form, in quantity at all 

 proportionate to the abundance with which they are introduced. 

 By far the greater part of the mass of the frame is made up of 

 organic substances: albumen, musculine, osteine, &c. Similar 

 materials are taken daily in large quantity with the food, in order 

 to supply the nutrition and waste of those already composing the 

 tissues; and yet only a very insignificant quantity of similar 

 material is expelled with the excretions. A minute proportion of 

 volatile animal matter is exhaled with the breath, and a minute 

 proportion also with the perspiration. A very small quantity is 

 discharged under the form of mucus and coloring matter, with the 

 urine and feces ; but all these taken together are entirely insuffi- 

 cient to account for the constant and rapid disappearance of organic 

 matters in the interior of the body. These matters, in fact, before 

 being discharged, are converted by catalysis and decomposition into 

 new substances. Carbonic acid, under which form 3500 grains of 



