294 NATURE AND OBJECTS OP EXCRETORY ACTIONS. 
to form water, and with, compounds of nitrogen to form urea. 
Hence there is no need of an organ to carry off the super¬ 
fluous oxygen; hut an organ to introduce it is rather required ; 
and this purpose, as we have seen, is answered by the Respi¬ 
ratory apparatus. But we find organs of excretion specially 
destined to carry off the carbon, hydrogen, and nitrogen, which 
are set free, under various forms, by the decomposition of the 
tissues. Thus the Respiratory organs, as we have seen, throw 
off carbon in the form of carbonic acid, and hydrogen which, 
has been in like manner united with oxygen so as to form 
water. The Liver has for its office partly to separate these 
same elements from the blood in a different form, throwing 
them off in the condition of a peculiar fatty matter, which 
consists almost entirely of carbon and hydrogen. But it has 
another function of no less importance in animals whose 
respiration is active ; for by its agency the hydro-carbonaceous 
matter circulating in the blood is brought into a state in 
which it readily combines with oxygen to form carbonic acid 
and water; and thus the liver may be said to prepare the 
pabulum for the combustive process. Lastly, the Kidneys 
have for their chief object to throw off the azotized compounds 
which result from the decomposition of the tissues ; these 
contain a very large proportion of azote or nitrogen, which is 
united with the other elements into the crystalline compounds, 
urea, and uric or lithic acid, the latter of which is usually 
thrown off in combination with soda or ammonia. And the 
kidneys further serve as the channel through which soluble 
matters of various kinds, which have found their way into 
the current of the circulation, and are foreign to the composi¬ 
tion of the blood, are eliminated from it. 
347. It is obvious that, when an animal has retained its 
usual weight for any length of time without change, the total 
weight of its excretions must be equivalent to the total weight 
of the solids and fluids it has taken-in. If these last have been 
no more in amount than was absolutely necessary for the main¬ 
tenance of the body during that period, all the azotized portion 
of the food was first appropriated to the formation of the 
azotized tissues ; whilst the non-azotized portion was used-up 
in maintaining the respiration (§ 157). Consequently, no 
part of the food would pass at once into the biliary and urinary 
excretions ; and these would have no other function than to 
