336 FOOD AND DIGESTION 



also serves as a source of fuel in the body, particularly when 

 dextrose and fat are not present in sufficient amount. 

 The products of protein oxidation, however, in addition to 

 carbon dioxide and water, are nitrogenous compounds, 

 largely urea. The protein compounds can also be split 

 up in the body into a nitrogenous and a non-nitrogenous 

 residue. From the non-nitrogenous residue carbohydrates 

 can be formed and there is some evidence to show that fat 

 may also be formed. The nitrogenous residue is eliminated 

 as waste material. 



(d) Inorganic Elements. — These constituents in mineral 

 form give rigidity to the skeleton. They serve also as neces- 

 sary constituents of protoplasm, sulphur and phosphorus, for 

 example, and are combined organically for this purpose. 

 Finally, these elements in the form of inorganic salts are 

 present in the fluids and tissues of the body, having an 

 influence upon the activities of the muscles and nerves, 

 supplying an alkaline or acid reaction as may be necessary, 

 and regulating the osmotic pressure of the cells. 



240. How to Express Food Value. — As has been stated 

 (Section 237, c), not all of the food taken in by the animal is 

 absorbed, and of course, that portion which is not digested is 

 of no use to the animal. It is customary to express the 

 amount of digestible material in each constituent in per- 

 centage. These "digestible coefficients," as they are called, 

 are determined by analyzing the original food and also by 

 analyzing all excreted waste products. The difference 

 between the two sets of results gives the percentage of food 

 digested, and hence serves as an indication of the amount 

 of tissue building or energy material in the food eaten. 

 The results, however, are not absolutely correct for several 

 reasons. In the first place the excreted material contains 

 protein and ether soluble material called fat, derived from 

 the intestinal juices and waste cells of the intestines. 

 In the second place, unless there is an analysis of all the 

 gas eliminated from the body there is a waste of some 

 material in this way that is unaccounted for. In the third 

 place, if it is desired to determine the value of the food for 

 the production of work or milk or fat, it is necessary to 



