This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
The effects of medicines, in other words, the changes produced by them in the system, must be either organic or functional; that is, must consist in an alteration, either of the organization or structure, or of the function or actions of the body, or of some one or more of its parts. Some have denied that there can be any change of action, or any action whatever, in the system, without change of structure, and consequently that the effects of medicines can ever be exclusively functional. This denial is founded upon the assumption that, in every operation of a living system, there is necessarily a chemical action, an oxidation perhaps of some portion, however minute, of the part or tissue acting, by which it becomes disintegrated and thrown off; while its place, in the healthy state, is supplied with new structure. This may be true, but it has not been proved; and, in the mean time, cannot be received as the basis of a general theory of the action of medicines. But, even admitting its truth, it does not follow that in all cases of physiological action there must be an alteration of structure. Suppose an organ to be secreting. The general opinion now is that the function is performed by the agency of cells, which, abstracting the material of the secretion from the blood, elaborate it when elaboration is necessary, and then, breaking up themselves, are thrown off with the secreted matter; their place being supplied by new cells. There has not been necessarily in this case any change of structure. One or more cells have disappeared, and their place has been supplied by one or more new cells of the same character exactly. The organ is precisely as before. There has been change of matter, but no change of structure or organization.
Now it may be readily conceived that a medicine, affecting the secretory function of an organ, shall act simply by increasing or diminishing the rapidity of the cell-action; that, in the time required in health for the throwing off and replacing of a certain number of cells, twice the number may undergo the same process in the one case, or only half the number in the other; and yet the organ shall remain precisely as in health, and in no degree altered; the only appreciable difference, even in its condition, being the greater or less amount of blood contained within it, or passing through it in a given time, and the greater or less amount of the secreted product. Medicines may, therefore, change the action of an organ, as regards the degree of rapidity, without altering its structure; and the same may be said of the nature of the action, as indicated, for example, by the character of the secretion; for a cell may elaborate different secretory products, according to the quality of the blood, or of the foreign materials brought with it, without differing in the least in its characteristic form or structure from the normal cell.
But, throwing aside this refinement of discussion, we may assume as functional effects all that are not attended with appreciable structural change; and as organic, all that are attended with such change; and this is the meaning which I attach to these terms, as employed in the present work.
In the sense Just referred to. the effects of the great majority of medicines, as ordinarily used, are exclusively functional; and it is chiefly those employed externally, to inflame, vesicate, or cauterize, that can be said to operate essentially by a change of structure.