2. Other medicines of the class operate directly and mainly on the blood itself; not through the agency of the digestive process, but by intimate admixture with that fluid, into which they find admission, either by venous absorption, or through the lacteals or intestinal lymphatics. They may act either 1. by entering immediately into the composition of some one of the proximate principles of the blood, 2. by modifying the vital condition of its organized constituents, or 3. by favouring, through a stimulant influence on its vital properties, the physiological actions which are constantly going on within it, and thus contributing to its full development and maintenance. The blood, thus elevated in its constitution, performs its offices in the economy with more vigour, and operates with a tonic influence on all the functions. Upon this principle it probably is that the preparations of iron chiefly act; and I am disposed to ascribe the peculiar influences of cod-liver oil in disease, in some measure, to analogous modifications produced by it in the blood.

3. Most of the class probably operate directly upon the ultimate organic constituents of the tissues, entering the circulation either unchanged or more or less modified, and being distributed everywhere by the blood as a mere vehicle; though it is not impossible that they may operate also upon the vital properties of that fluid, through the same power by which they affect the similar properties of solids. How it is precisely that they affect the ultimate organic constituents of the body is conjectural. They may act merely by their presence, or they may enter into a sort of chemical union with the living matter, though I am inclined preferably to the former of these views. At all events, some of them have been found, on chemical investigation, in the midst of the tissues. It is probable that each distinct function is performed through the instrumentality of a special power in the ultimate organic cells, nuclei, or molecules of the organ, and tonics may operate simply by stimulating this power into a somewhat increased activity. But we may advance one step further, and adopt the very plausible opinion, that in all the tissues there is a certain vital cohesion which is essential to the due performance of the function, and that tonics are moderate stimulants to this cohesion. To this mode of action we may ascribe in part the greater firmness of the tissues, especially the muscles and bloodvessels; and to this perhaps also that condition of the nervous centres, resulting from the use of certain tonics, by which they are enabled to exercise their proper functions more energetically, and have greater power of resistance to all kinds of disturbing influences. Such an influence is exhibited in the control evinced by some of the metallic tonics over various nervous diseases, as chorea, epilepsy, and neuralgia.

Before considering the several tonic medicines, with their subdivisions. I propose to give a succinct account of various influences of a tonic character, which, though not strictly medicinal, are often very useful in disease, and must therefore rank among remedies.