222 A Manual of Veterinary Physiology. 



We can readily understand why it is that the food taken 

 into the body should contain the same principles as the 

 organism. 



These principles are constantly undergoing change, the 

 nitrogen being oxidized into urea and other products of 

 nitrogenous metabolism, and the fats into carbonic acid and 

 water. 



The Income of the body is the food taken into the diges- 

 tive canal, and the oxygen taken in at the lungs. 



The Expenditure consists of the Carbon in the form of 

 carbonic acid expired at the lungs, the small and unknown 

 quantities given off by the skin, and the carbon excreted 

 by the urine ; the Nitrogen excreted in the form of urea, 

 hippuric acid, and other substances given off by the kidneys ; 

 the Inorganic Salts excreted by the kidneys, through the 

 skin, and mixed up with the various digestive secretions ; 

 the Water given off at the lungs by transpiration, through 

 the skin, by the kidneys, and an amount got rid of by 

 ordinary secretions. 



Besides these we have the excretion of faeces, containing 

 those portions of the food which the s}-stem has been 

 unable to assimilate, and mixed up with them are certain of 

 the secretions found in the intestine. The f;oces do not con- 

 stitute an expenditure, though, in ascertaining what the 

 system has assimilated, it is clear that they must be 

 deducted from the food ingested. 



There are other sources of loss besides those mentioned 

 above, such as the production of milk, wool, and semen ; 

 but as these have only special application, the general 

 statement made above is not affected. 



The material supplying the income of the body consists 

 of so much carbon, hydrogen, oxygen, nitrogen, sulphur. 

 and phosphorus ; and the matter forming the expenditure 

 also consists of the same elements. When the body is in a 

 state of equilibrium, the amount of these elements excreted 

 is the same as the amount ingested, and this is the principle 

 on which all observations on the wear and tear of the body 



