~Importance of the Mineral Salts.~--The way in which the mineral elements exist in the body and take part in its functions, has been graphically outlined by Sherman as follows. "(1) As bone constituents, giving rigidity and relative permanence to the skeletal tissues. (2) As essential elements of the organic compounds which are the chief solids of the soft tissues (muscles, blood cells, etc.). (3) As soluble salts (electrolytes) held in solution in the fluids of the body; giving to those fluids their characteristic influence upon the elasticity and irritability of muscle and nerve; supplying material for the acidity and alkalinity of the digestive juices and other secretions; and yet maintaining the neutrality, or slight alkalescence, of the internal fluids as well as their osmotic pressure and solvent power."[9] The above outline, showing the various ways in which the mineral constituents enter and take part in the various functions, as well as in the structure of the body, make it evident that the same close attention and study which was given to the other foodstuffs must be accorded to these substances. When the student realizes that the presence of certain salts dissolved in the blood assists in the regulation of the vital processes of the body such as the digestion, circulation and respiration; that they are responsible for the contraction and relaxation of the muscles; that they assist in controlling the nerves; that they are, in a way, instrumental in releasing the energy locked up in food--the value of these elements becomes very evident, and their importance in the dietary inestimable. Some of the mineral salts are more widely distributed in food than others, and the danger arising from their deficiency in the diet is not so great as is the case with others; hence attention is called to those found by investigators to be most often lacking or deficient in the average diet; _i.e._, calcium, phosphorus, and iron. A brief summary of the special parts played by these elements will be outlined here. ~Calcium.~--Physiology teaches that about eighty-five per cent. of the mineral matter of the bone, or at least three-quarters of the ash of the entire body, consists of calcium phosphates. It has long been known that this mineral salt is necessary for the coagulation of the blood, and science has demonstrated that "the alternate contractions and relaxations which constitute the normal beating of the heart are dependent, at least in part, upon the presence of a sufficient, but not excessive concentration of calcium salts in the fluid which bathes the heart muscles."[10]