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 
