[11] When applied to the skin of the abdomen, it acts chiefly by exciting action, on the principle of the sympathy of equilibrium.
Blisters likewise act by simply abating the action of the part; but differ from cold, in requiring to be applied, not to the part which is affected, but to some other, with which it exhibits the sympathy of equilibrium; as, for instance, to the integuments of the thorax, in pulmonic inflammation; to the skin of the knee, in affections of the joint, &c. It is, however, necessary, when a general disease, or fever, is induced, that bleeding be fully employed, before we have recourse to blistering; because, if it be not, the inflammation, excited by the blister, co-operates to increase the fever, alongst with the original disease, which it has not had time to overcome, or lessen. Blistering likewise acts more effectually, when the local action has been already diminished, by previous bleeding. The size of the blister should be proportioned to the probable extent of the diseased action; at the same time, we must set bounds to this magnitude; because, if too large, they may not only produce, or keep up a general disease, but also, by lessening the action of the internal parts too much, and too quickly, they may prevent recovery[12]. It is therefore better to apply them of a moderate size, and renew them frequently, than to apply one too large at once. It must, however, be remembered, that inflammations of every part are not equally readily overcome in this way; and, therefore, one will require a larger blister than another; thus, the same quantity of inflammatory action in the brain, will be more difficultly subdued, than in the breast; and, therefore, we must apply, in that case, a larger blister. As it is the inflammatory action, induced by the blister, and not the discharge, as was once supposed, which is useful, it follows, that the same blistered place should not be kept too long from healing, or in the state of an issue, but that we ought rather to apply a succession of blisters; and this succession should be pretty rapid. There is indeed one case, in which issues are admissible, namely, where, from the nature of the inflamed part, or the peculiarity of the inflammation, if it be specific, or scrophulous, the progress of the action is very slow. In these cases, a rapid, and continued succession of blister would, doubtless, be most useful, but, from the duration of the treatment, would scarcely be submitted to: Issues, which are less painful, and less troublesome, are, therefore, generally preferred. We have an instance of this in many diseased joints.