Biassed by the satisfactory observation which he had made of the modus operandi of particular medicines, and misled by the insufficiency of his knowledge, each of these writers was tempted to apply the view which was applicable to a certain set to all remedies alike. Once persuaded of its sufficiency, he easily found arguments by which to fortify both himself and others against any subsequent objections.
The right course lies in a combination of these various theories, embracing what is true and discarding what is erroneous in each of them, and supplying what may seem to be wanting in the whole. None of these ideas being by itself perfect, the sounder reasoners of the present day are driven to suppose that there are various different ways in which medicines may counteract, and thus cure, different diseases. This counteraction is distinct from contrary action; it may be direct or indirect; and it allows of any action in a medicine, tending to restore health, except an effect similar to the disease. Such a view was adopted by Dr. Cullen, the well known Nosologist, who lectured at Edinburgh towards the close of the last century. He discarded all special and confined views of the operation of medicines, believing that they acted in many and various ways, all of which tended to the same end—to counteract the influence of the disorder. This is well exemplified in his admirable directions for the treatment of fever, in which he enjoined the use of a number of different remedies, varied according to the nature of the case, and progress of the symptoms.[16]
Dr. Pereira, the most learned and acute of living English writers on this subject, appears, like many others, to prefer a wide explanation of this description.
On such views my own statements are based. I suppose that a disease in the blood is to be met by agents in the blood, which directly or indirectly counteract it there; that disorders, generally temporary, which depend on nervous derangement, are to be benefited by remedies which affect the nerves; and in the same way that a laxity of muscular fibre, or a failure in a secretion, should be treated by agents which especially possess the property of restoring to a right condition such parts or functions.
Concluding, then, that it is impossible to account clearly for the actions of most medicines on Mechanical or on Chemical principles, we are led to infer that their influence must for the most part be vital in its nature—that it must be such as could only be exerted in the living body. Even then we are unable to fix upon any single rule or formula which shall be capable of accounting for the actions of all at once. So it seems that the only general explanation which we can offer of the modus operandi of medicines in the cure of diseases, is to say that they operate by various counteractions.
This, then, introduces my Third Chapter.