By this process is meant the displacing or prevention of one affection by the establishment of another in the seat of it. It is a general, though by no means universal pathological law, that two powerful diseases, or forms of abnormal action, cannot exist in the whole system, or in any one part of it at the same time. If, therefore, we can produce a new disease, or new mode of abnormal action, in the exact position of one that may be existing or expected, we may possibly supersede the latter; and, if the new disorder subside spontaneously without injury, we cure our patient. The operation of numerous remedial agents may be explained in this way. It is thus, for instance, that mercury has been supposed to cure syphilis. But we have better examples in the powerful influence of certain antiperiodic remedies, such as quinia and arsenic, in the cure of intermittent diseases. They establish their own morbid impression in the absence of the paroxysm; and the system, being thus occupied at the moment when the disease was to return, is incapable of admitting it. In the same way may be explained the effects of blister. opiates, emetics, or indeed any violent impression from any source, in the cure of paroxysmal diseases, if caused to be in full action at the time of the expected recurrence of the paroxysm.

Mental influences are sometimes very powerful in the superseding not only of intermittent diseases, but of continued disease also, when of a merely functional character. The excitement of any strong emotion may have this effect; and the pre-occupation of the nervous system resulting from a strong faith has often exhibited a wonderful influence.

The same law holds in cases of purely local diseases. It is probable that many cutaneous eruptions, and diseases of the mucous membrane of the alimentary canal and urinary passages, yield, upon this principle of supersession, to certain applications made to them directly, or, in the case of urinary diseases, through the route of the circulation.