32 PRINCIPLES OF FEEDING FARM ANIMALS 



the soluble protein compounds undergo chemical changes, appar- 

 ently through the action of the living cell walls ; the peptones, which 

 are abundant in the intestinal fluids, have disappeared in the blood, 

 and in their stead we find more complex protein compounds 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 circulation 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 the 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, respec- 

 tively. 



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 body. This can take 

 place only in the growing animal. In the case of an insufficient 

 supply of feed the glycogen of the liver is first oxidized, then the 

 fats, and, last of all, the body tissues. Oxidation of body tissues 

 takes place in the animal cells so long as the animal is living. The 

 final oxidation 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 

 products formed, and urea represents the most important and, 

 practically 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 



