Before going into a general description of nitrogen metabolism it is of the utmost importance to get a general idea of the newer facts known in regard to protein bodies. Protein bodies represent a highly complex organic combination which has become better known through the studies of Hochmeister, Kossel, and especially of Emil Fisher.
The researches of Emil Fisher, Abderhalden, Bergel, Bona and others show that the end-products of the proteins, are the amino-acids obtained by hydrolysis, and are bodies of the normal fat and aromatic series in which the H atom is displaced by an NH2 group. Of these acids many varieties have been described, of which the better known are glycocoll, alanin, tyrosin, valin, leucin and isoleucin. Whereas the carbohydrate (glycogen) of the body represents combinations of a definite chemical group, the glucoses, and the fats a combination of glycerin and fatty acids, the proteins on the