A large number of salts occur in the tissues, generally in small quantity, in solution. In the teeth and in bone tissue salts exist in the solid form, and in much greater proportion than in any of the soft parts. Most of the salts are introduced into the economy with the food, but some, doubtless, are formed in the body itself. Our knowledge of the exact position occupied by the salts in the textures is very incomplete, as their amount is usually estimated from the ash of the tissue which remains after ignition, by which process some become altered, so that it is impossible to say what are the exact salts that are present in the body. They form chemical combinations with the complex organic compounds, which we do not understand, and probably have important functions to perform, such as. rendering certain materials (globulins) soluble, or otherwise facilitating tissue change. The salts pass out of the body in many secretions, largely in the urine, where they influence the elimination of urea, and therefore form an important constituent of that secretion.

Common Salt {Sodium Chloride), NaCl, is the most widely distributed, and is present in greater quantity than any other salt in all animal fluids and most tissues, except bones, teeth, red blood corpuscles and red muscle.

Potassium Chloride commonly accompanies sodium chloride in small quantity. In the red blood corpuscles and in muscle it occurs in greater amount than the sodium salt, while in the blood plasma but little is found in comparison with the sodium salts, and any excess seems to act as a poison to the heart.

Carbonates and phosphates of calcium, sodium, potassium and magnesium occur in small quantities in most tissues. The earthy part of bone is chiefly composed of calcium and magnesium phosphate and calcium carbonate, together with some calcium fluoride.

Sulphates of sodium and potassium, probably formed in the body from the oxidation of the sulphur in the complex proteid materials, occur in most tissues, and are removed from the body by the kidneys.

Finally, we find two of the elements free in the textures. Of these Oxygen plays by far the most important part. It is widely distributed among the fluids of the body, from which it can be removed by reducing the pressure of oxygen of the atmosphere by means of an air pump. Oxygen is introduced into the body by the lungs, where the blood takes it from the air. In the blood only a small quantity of that which can be removed by the air pump is really free; the remainder is chemically combined with the coloring matter of the blood. It is absolutely necessary for life, as it alone can enable the chemical changes of the tissues, which are mostly oxidations, to go on. It is, in fact, the element necessary for the slow combustion which takes place in the nutrient material after its assimilation.

Nitrogen also occurs in the blood, but in insignificant quantity. It is absorbed from the atmosphere as the blood passes through the lungs. So far as we know, it has no physiological importance in the body.