Measured by ordinary time relations of scientific progress through the centuries, it is not long since Liebig was describing the blood as essentially a solution of protein and inorganic salts in which pigmentcontaining corpuscles are suspended. From a strictly quantitative standpoint, such a description still is entirely warranted. During the last twenty or thirty years, however, attention has been focused on other constituents of the blood that are involved in important physiologic functions, although present in relatively minute amounts. Some of these less conspicuous organic components are nitrogenous, others non-nitrogenous. Biochemists have succeeded in demonstrating in the blood all those components of the urine ordinarily classed as "waste products." With limited exceptions, perhaps, the kidneys serve to eliminate, not to manufacture, them. Urea, creatinine, uric acid, hippuric acid, and similar substances are included in this category of blood constituents. The circulating sugar glucose represents a blood component that is conserved