The character and composition of the blood, and the part it plays in the animal economy, has been dealt with in that part of this work devoted to physiology. It has been shown that every part of the body depends upon the blood for the supply of those materials which maintain it in health and repair the waste that is always going on.

Every tissue, nerve, muscle, bone, skin-cell, and hair-follicle depends upon the blood-stream for its maintenance, every gland for its functional activity.

The nutritive material so constantly being given up by the blood for the sustenance of the body is renewed by the food in its passage along the digestive canal. The blood also serves as a vehicle to remove waste products from the body, chiefly by the lungs, kidneys, and skin, and through its medium the body is affected in various ways for good or ill. If from any cause it is deficient in the materials needed for repair or secretion, the body as a whole or some part thereof will suffer, as will also be the case if some deleterious matter be allowed to remain in the blood-stream in excessive quantity, or beyond the time when it should be separated and passed out.

It is intended by the foregoing remarks to show how the nutrition of the body may be affected by introducing into the blood agents which, being carried to all parts of the system, exercise some influence upon its tissues.

The preparations of iron in forms not detrimental to digestion, together with suitable diet, appear to increase the production of red corpuscles and improve the general health.

It is sometimes desirable to employ substances which, instead of inducing the red blood corpuscles to give up oxygen freely, will arrest combustion. Where the body-heat has risen above the normal standard, and a condition of fever prevails, we employ for this purpose quinine, alcohol, salicine, and some other agents, the effect of which is to reduce temperature.

Medicines also act upon the blood by increasing the amount of albumen, salts, fat, phosphorus, etc.