32 PRINCIPLES OF FEEDING FARM ANIMALS 



are excreted through the lungs and skin. Other nutrients, like the 

 mineral salts and soluble protein compounds, pass into circulation 

 and are brought to the parts of the body where they are needed 

 for building materials. The proteins are decomposed chiefly into 

 amino-acids in the process of digestion, and pass through the in- 

 testinal wall as such, or possibly in part, as groups of amino-acids ; 

 they appear to be synthesized through the action of the living cell 

 walls into more complex substances, from which the body is able 

 to build its various protein tissues or fluids. In the same way the 

 free fatty acids, and the soaps formed from these in the digestion 

 of fats, are changed in their passage through the intestinal wall 

 into neutral fats which enter the lacteals and pass into the circula- 

 tion through the lymphatics. 



The carbohydrates of the feed, as we have seen, are changed to 

 sugar in the process of digestion and enter the capillaries as such ; 

 from these the sugar passes into the blood circulation and enters 

 the liver, along with all other nutrients except the fats. In the 

 liver the sugar is changed into a carbohydrate of the same composi- 

 tion as starch, called glycogen or animal starch, and is deposited as 

 such in the cells of the liver. By this provision an accumulation of 

 sugar in the blood is prevented, and the body has a base of supply 

 of a readily available and oxidizable carbohydrate which can be 

 drawn upon as needed. The liver normally contains only about 2* 

 per cent of glycogen, but after heavy feeding with starchy feeds 

 the content may rise as high as 10 per cent. Aside from furnishing 

 material for production of heat and muscular energy, glycogen may 

 also serve as supply material for the formation of body fat and butter 

 fat, in the case of fattening animals and milch cows, respectively. 



The fats may be stored between the muscular fibers or deposited 

 as adipose tissue, or, in the case of females giving milk, may be 

 changed into butter fat. We have seen that the muscular tissues of 

 the body consist largely of protein substances, and that they are the 

 form in which protein is stored in the animal's body. This can take 

 place only in the growing animal. Oxidation of body tissues con- 

 tinues in the animal cells so long as life exists. The final oxida- 

 tion products of protein substances in the body are carbon-dioxide 

 and water (as in the case of carbohydrates and fat), and, in 

 addition, urea, which is excreted through the kidneys in the 

 urine. As there are no gaseous nitrogenous decomposition prod- 

 ucts formed, and urea represents the most important and, prac- 

 tically speaking, the only nitrogenous decomposition product in 

 the oxidation of protein substances in the body, it becomes a meas- 

 ure of the protein decomposition in the body. By determining the 



