EXCRETION OF MATTER FROM THE BODY 415 



In herbivora, when suckling or when fasting, the urine is 

 acid, but when on their normal diet it is alkaline. 



The alkalinity is due to the presence of alkaline carbonates 

 formed from the citrates, inalates, and tartrates of the 

 vegetable foods, and also from the acetates, etc. , produced by 

 the decomposition of cellulose in the rumen and intestine. 



Urine in carnivora is normally transparent; but when it 

 has stood for a few hours, a cloud of a mucin-like substance 

 is seen floating in it. In herbivora, as the urine cools, it 

 rapidly becomes turbid and throws down a white precipitate 

 composed chiefly of carbonate of lima 



The smell of urine is characteristic, and it may be modified 

 by the ingesfcion of many different substances. 



II. Composition 



Since the relative amounts of water and solids vary within 

 such wide limits, the percentage composition of urine is of 

 little moment. Of the solids, rather more than half are 

 organic, rather less than half are inorganic. Since water and 

 solids are derived from the water and solids taken by the 

 individual, the amounts excreted depend upon the amounts 

 taken, and must be considered in connection with them. 

 Thus if an animal takes little fluid, it will pass little water in 

 the urine. If it takes little food, a small quantity of solids 

 will be excreted by the kidneys. Since excretion and ingestion 

 must be studied in relationship to one another, it is convenient 

 to compare them during a definite period of time, and the 

 natural division into days of twenty-four hours is generally 

 adopted. 



I. Nitrogenous Substances 



A. Urea. The chemistry and mode of formation of urea 

 have been discussed on p. 390. Since it is as urea that, on 

 an ordinary diet, nearly 90 per cent, in the dog, and probably 

 about 80 per cent, in the horse, of the waste nitrogen is 

 eliminated in the urine, the amount excreted affords a measure 

 of the amount of proteins taken in the food. 



When the urine is allowed to stand, certain micro-organisms 



