216 Agricultural Chemistry. 



a dog eating exclusively a meat diet, probably a greater part of 

 the protein eaten is not stored but is used as fuel. We see from 

 this that the proteins can serve most of the requirements of the 

 animal, a statement which cannot be made of any other food 

 constituent. They are the true tissue builders. 

 H An animal, even when not increasing in weight, will always 

 require a certain constant supply of protein in its food to replace 

 the waste of nitrogenous tissue, which is always going on even 

 during rest. The cell proteins are constantly undergoing de- 

 composition and reconstruction. 



-"When the nitrogenous tissues of the animal, or the proteins 

 consumed as food are decomposed in the body, the nitrogen they 

 contain is largely excreted in the form of a simple nitrogenous 

 substance, urea. This is eliminated by way of the kidneys in the 

 urine. There are small quantities of other nitrogenous products, 

 such as uric acid, creatin, creatinin, and in the case of herbivora, 

 hippuric acid, voided in the urine, but they constitute but a 

 small proportion of the total nitrogen eliminated. The urea pro- 

 duced is rich in nitrogen, containing about 46.6 per cent. It 

 represents about one-third the weight of the protein oxidized. 



The amides and amino-acids consumed as food are burned in 

 the body and their nitrogen excreted as urea. It is very prob- 

 able that they can, in part, take the place of proteins as tissue 

 builders. In addition, by their combustion, they serve as sources 

 of heat and force. 



The fats are free from nitrogen. Those contained in food are 

 similar to those found in the animal body. It appears possible 

 for a vegetable fat to become deposited in the animal without 

 essential change. Small deposits occur in every organ and cell. 

 The fat reserves vary much in size, depending on nutritive con- 

 ditions, so that no definite statement can be made regarding the 

 fat content of the individual organs. The fat of the food is 

 either burned in the animal system to furnish heat and mechan- 

 ical energy or is stored up as reserve material. With their larger 

 content of carbon and smaller proportion of oxygen, fats are less 



