The operation of this most useful remedy has occasioned numerous disquisitions and eager controversies. It is fortunate that the calm attentive practitioner has steadily pursued his path, and contributed to relieve or save his patient, without being influenced by the surrounding contests. As the principal application by. which we excite blisters is the Spanish fly, the nature of this insect has contributed to keep alive the controversy, or to add to the difficulties. It will, however, make no part of our enquiry, and we must consequently refer to that article for some account of its peculiar qualities. See Cantharides. ,

Blisters, when applied to the skin, first produce a tingling heat, a redness, and afterwards the cuticle is elevated, and a portion of fluid resembling the serum of the blood is inclosed, as in a bladder. When this is evacuated, a redness continues,. the serum gradually thickens, at last becomes a whitish curdly substance, under which the new skin is again formed, or assumes a truly purulent appearance, and the blistered part contracts until the whole wound is healed.

From this very simple and confined operation, it is not, a priori, probable that extensive benefit should be produced. The first effects are pain and irritation; and it was once supposed that blisters were only useful by their stimulant power. The evacuation followed; and others then thought that from this source only they were beneficial, and that their first effects were injurious. They were then antispasmodics from some unknown influence; they coagulated or thinned the blood according to the fancy of the pathologist; but the manner in which they really operate is still uncertain, notwithstanding the labours of Tralles in his closely printed quarto, entitled, Usus Vesicantium.

The first effect of blisters is undoubtedly stimulant; yet this stimulus is local, and seldom communicated to the whole system. In irritable skins, however, when the pain is considerable, when restlessness and want of sleep are the consequence, they are certainly for a time injurious from their stimulant power, but in general they relieve more pain than they give; they lessen previous irritation or uneasiness, and dispose to sleep. These are their effects in fevers and inflammations, where we might chiefly dread their stimulant power. It may be asked if they are never used as stimulants ? Undoubtedly, but chiefly as local ones, and where we come near the affected nerve; and, indeed, from the moment of their application, they must be considered as such, though the external stimulus, relieving the internal, renders the former an object of little comparative importance. The great difficulty arises from considering the benefits derived from so small an external inflammation, when the internal, which it relieves, is so extensive and violent. Various havebeen the modes of resolving the question, and numerous the discussions which the various solutions have occasioned. The effects are undoubtedly disproportioned to the cause, but it is probable that the smallest relief given to the internal over distended vessels, gives nature an opportunity of exerting her powers, and the turgid arteries of propelling more effectually their contents. We shall not encumber this comprehensive account with the various theories of Inflammation, or the different explanations of Derivation, but refer to these articles, q. v.

The stimulus of a blister seems also of service in lessening the excessive action of the nervous power. We well know that the tone and the sensibility of the nerves, and the consequent irritability of the muscles which they supply, are intimately connected with the state of the circulation in their extremities. We can easily see, therefore, the means by which this excessive action may be mitigated. In some peculiar circumstances, however, we have thought that diseases more, purely nervous have been relieved by this remedy, and have suspected that there may be a balance between the excitement of the internal and external nervous power, as there more evidently is of the circulation. We need not enlarge on the subject, but leave this hint to suggest future enquiry. We may, however, add, that if blisters ever act as antispasmodics, it must be from this or a similar effect.

The discharge, in many instances, gives a greater permanence to the benefits derived from blisters, and in some cases seems to be the chief source of their advantages, particularly in dropsies, in humoral asthmas, the more decidedly serous apoplexies, and a few other diseases. It is continued, however, with some difficulty, as in many constitutions the blister rapidly heals, whatever be the application. The sabinc ointment now generally supplies the place of the blister ointment, which is inconvenient by its effects on the neck of the bladder.

Though, as we have said, the inflammation is confined and slight, and the discharge inconsiderable, yet it probably has more effect on the constitution than we might suspect from the absolute quantity; for in many constitutions the continued discharge from blisters produces considerable debility: in some they can scarcely be borne for even the period of two or three days. We might attribute this to the quality of the discharge; but M. Margueron, who has analysed it (Annales de Chimie, vol.xiv.), found that it very nearly resembled the serum of the blood, containing only a little less of the albuminous portion. It is seemingly darker coloured from the tinge of the plaster, whose peculiar smell it retains. He found it the same when the blister was applied in putrid fevers, as when the person was in health.

Blisters have on many constitutions a cordial and exhilarating effect, generally on those of full habits, and probably of languid circulation, by relieving the over distended vessels. A gentleman, once highly distinguished at the bar, and of brilliant convivial powers, always applied a blister when he wished to shine in either sphere, and the effect was produced as soon as the warmth in the part began. We have heard also many, who even felt the pain of blisters acutely, declare that the relief of the languor they previously experienced, counterbalanced all their sufferings.