These old terms of the Materia Medica are here retained for purposes of convenience. By them is meant that certain substances exert a distinct influence on the metabolism of many of the bodily organs, and that they thus alter the nutritional state of the body, in part or as a whole, and that they have their chief use in therapeutics to so influence certain metabolic activities. Thus, it is known that arsenic affects the nutrition of the skin; it induces in large amounts characteristic nutritional disturbances. Iron has a profound part to play in the blood-composition and on the oxidizing functions of the body. Mercury has a peculiar action on certain pathological states, and exerts a specific activity in overcoming the effects of the syphilitic infection - how, it is as yet undetermined. Salts of iodine show similar properties. Other illustrations are numerous.

It is obvious to every reflecting physician that a class of remedies act as such by supplying some deficiency in the animal organism, the agent in such cases being either itself, the substance lacking, or its analogue, or by its presence restoring the deficient element or secretion. Iron acts in certain forms of anemia in which this ingredient is wanting in the red blood-corpuscles; phosphorus or the earthy salts behave similarly in conditions where the tissues are deficient in these necessary constituents. In view of the physiological action of remedies belonging to this class the term restoratives so aptly expresses their general character that no apology is needed for its adoption.

Alteratives, on the other hand, are unnatural to the system and can be administered without injurious results, as a rule, only in diseased conditions, in which the particular remedy combats in a specific or unknown way the prime etiological factor of the disease. These medicines when given as alteratives normally produce no symptoms, the patient being unaware of their action save by a recognition of his gradually improved condition. Should, in fact, symptoms occur, they should serve as a warning that the remedy is not indicated or that the dose is unsuitable.

Restoratives and alteratives belong for the most part to the general group of the metals and metalloids, and show certain general similarities of action.