On ascertaining this, we are immediately struck with the remarkable fact that all the tissues and fluids of the body, muscles (or flesh), bone, blood--all, in short, except the fat--contain nitrogen, and, consequently, for their building up in the young, and for their repair and renewal in the adult, nitrogen is absolutely required. We therefore reasonably infer that the nitrogenous substance is necessary for this purpose. Experiment has borne this out, for men who have been compelled to live without nitrogenous food by dire necessity, and criminals on whom the experiment has been tried, have all perished sooner or later in consequence. When nitrogenous substances are used in the body, they are, of course, broken up and oxidized, or perhaps we ought to say more accurately, they take the place of the tissues of the body which wear away and are carried off by oxidation and other chemical changes. Now, modern science tell us that such changes are accompanied with manifestations of energy in some form or other, most frequently in that of heat, and we must look, therefore, upon nitrogenous food as contributing to the energy of the body in addition to its other functions. What are the substances which we may class as nitrogenous. In the first place, we have the typical example of the purest form in _albumin_, or white of egg; and from this the name is now given to the class of _albuminates_. The animal albuminates are: Albumin from eggs, fibrin from muscles, or flesh, myosin, or synronin, also from animals, casein (or cheesy matter) from milk, and the nitrogenous substances from blood. In the vegetable kingdom, we have glutin, or vegetable fibrin, which is the nourishing constituent of wheat, barley, oats, etc.; and legumin, or vegetable casein, which is the peculiar substance found in peas and beans. The other organic constituents--viz., the fats and the starches and sugars--contain no nitrogen, and were at one time thought to be concerned in producing animal heat.