294 NATURE AND OBJECTS OF 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 ; but 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 



