We are concerned, in this treatise, only with matter relating to regional, topical, or local applications of the agent under discussion, and we can well begin the consideration with the identification of the agent itself and the different forms under which it is most commonly used. For all practical purposes, we can confine the discussion of the agent itself to that of the four forms, or preparations, of iodine in almost universal use by practitioners of veterinary medicine and surgery. When, in veterinary medicine, allusion is made to iodine, it is almost, without exception, to one of the following preparations: 1. Tincture of Iodine. 2. Ointment of Iodine. 3. Aqueous solutions of Iodine. 4. Oily solutions or mixtures of Iodine. Only in rare cases, and then under specific reference, is iodine used in other forms, or in its elemental state, in veterinary medicine. Iodine is a very active agent, chemically as well as therapeutically, and is not readily compatible with other agents. It is for this reason, that combinations of iodine with other drugs and chemicals are not common, and therein lies a distinction for iodine that not many other therapeutic agents can claim; namely, that beneficial effects resulting from iodine medication are almost, without question, due to it alone; it is hardly ever applied in combination with synergists which might obscure the activity of individual ingredients. This remarkable therapeutic activity of iodine is such that, when properly applied in some of its forms, its presence can be demonstrated in the underlying tissues. After prolonged courses of topical application, its action is occasionally appreciated, both subjectively and objectively, in the evidence of more or less clearly defined constitutional or systemic indications of its presence within the animal organism. From this, it is apparent that, in iodine preparations of a particular class, we have an agent whose topical remedial effects are, in some slight measure, due to systemic action; in part at least, this action being the effect of great physiological activity exerted in the limited area of its topical application. In some degree this activity of certain preparations of iodine can be explained by reference to the chemical properties inherent in iodine as elemental matter, and in its well-known affinity for certain elemental constituents of the tissues of the animal organism.