The Animal Body. 227 



which it is exposed. And it is through this oxidation of the 

 nutrients in the cells of the body that heat and mechanical work 

 are produced. 



Elimination. As has already been noted, the undigested resi- 

 dues of food, together with certain excretory products eliminated 

 by way of the intestines, constitute the feces. 



The products, which result from the metabolism of the body 

 cells, or of the food consumed, are removed from the body by the 

 lungs, the kidneys, the skin and the intestine. The carbohyd- 

 rates and fats, which are oxidized in keeping up the animal heat 

 or in furnishing energy, are broken down into carbon-dioxide 

 and water and removed as such from the blood in the lungs, and 

 to a smaller extent by the skin. Water and salts are removed by 

 both intestine and kidney, while the perspiration may also serve 

 to carry considerable quantities of these materials. The elimina- 

 tion of the products of protein degradation in the tissues is al- 

 most entirely by way of the kidneys. The larger part of the 

 nitrogen is eliminated in the form of the simple body, urea. There 

 are other forms of nitrogen occurring in the urine, such as uric 

 acid, creatin, creatinin, ammonia, etc., but they constitute only a 

 small proportion of the total nitrogen eliminated. 



The sulphur of the protein molecule is also removed as sulphate 

 through the kidney, while the phosphorus passes out of the body 

 in the form of a phosphate by both the intestines and kidney; 

 by far the larger proportion is removed through the intestine in 

 the herbivora. 



The quantity of nitrogen in the urine is taken as a measure of 

 the amount of protein decomposition in the tissue. This may be 

 only partly true. It is now believed that a considerable part of 

 the nitrogen of ingested protein has not been built into body 

 tissue, but is eliminated from the protein molecule as ammonia 

 in the intestine, carried to the liver, and from there finally ex- 

 creted through the kidney as urea. The carbonaceous part of 

 the protein molecule from which this nitrogen has been removed 



