These are sometimes called vesicatories, and not unfrequently, by an elliptical mode of expression, blisters. They are characterized by the special property of blistering; that is, of producing an extensive elevation of the cuticle, with serous liquid beneath. Their first effect is to inflame the part to which they are applied; and the subsequent vesication is the result of the inflammation, the distended vessels relieving themselves by effusion. The particular phenomena will be more appropriately detailed under the individual articles of the class. I shall here treat only of the general principles upon which they operate as curative agents.

1. Through the sympathy of the system with the inflamed parts, they produce a general excitement, evinced by an increased frequency of pulse, and heat of skin; and, if the surface affected be extensive, they may even induce a general state of fever, in the same manner precisely as fever is generated by internal inflammations, arising from cold or other morbid causes. This symptomatic excitation is greater, during the rubefactive stage, than after vesication is fully established; as the emptying of the vessels has the effect of moderating the inflammation, upon which the general phenomena depend. Epispastics are, therefore, primarily local, and secondarily general stimulants.

With a view to their general stimulation, they are used in low states of disease, requiring an excitant influence to support the actions of the heart, and those of the nervous system. it is not on the circulation only that they operate, but also on the nervous centres; and the latter necessarily, before they can affect the former; because it is mainly, if not exclusively, through these centres, that the local inflammation influences the heart. The debility of exhaustion is not the condition to which they are most applicable; for, though they excite the nervous centres and heart, they have no direct stimulant effect on the digestive, blood-making, and nutritive functions, which most need support in this condition of the system. But, when the depression is sudden, arising from some sedative but temporary influence on the nervous system or circulation, or when, if more protracted, it is sustained by a similar influence more persistent, but still of limited duration, there is an indication for these with other stimulant remedies, in order to arouse the great functions from the torpor in which they may have been left, or to sustain them until the depressing agency shall cease.

A condition of the first kind is presented in the prostration sometimes existing at the commencement of severe fevers, and that produced by a sudden and severe shock, as from a blow on the head, a fall, an overwhelming mental impression, the action of certain violent poisons, etc. in such cases, these external stimulants are often even more applicable than the internal; because, as reaction and high fever or inflammation may follow the state of depression, it is important to employ agents which shall not continue to stimulate after reaction has taken place; and, again, as the head is peculiarly apt to suffer in the reaction, it is advisable to use means least liable to add to the cerebral disorder. These contraindicated effects can be much more certainly guarded against, under the use of external remedies, which can be removed at the moment that they may be no longer wanted, and the general excitement produced by which has no special local direction. Besides, by means of the local impression which may remain after the subsidence of the sympathetic excitement, a revulsive influence is exerted, which, instead of increasing any internal inflammation, would tend to counteract it.

The condition of system, of the second kind above referred to, exists in low febrile diseases of the typhoid character, in which the system is depressed by some sedative morbid agency, whether a distinct poison floating in the blood, or a depraved state of the blood itself. it is important, in these cases, to seize the proper period for the application of the blister. it should not be employed in the highest state of reactive excitement, nor postponed to the last condition of prostration and debility. in the former it might add injuriously to the fever, in the latter it might endanger sloughing in the blistered part, in consequence of the very low state of its vitality. in doubtful cases of the latter kind, the seat of application, instead of the extremities, where the vital actions are feeblest, should be some part of the chest; or, if it may still be thought proper to apply the blister to one or more of the limbs, the thigh, or arm above the elbow, should be preferred to the leg or forearm, which, under other circumstances, is usually selected.

2. By the general impression or shock produced by blisters on the nervous centres, they may be made to supersede other diseases. Thus, if applied so as to be in full operation at the period of an expected paroxysm of some regular periodical disease, they will not unfrequently supersede the paroxysm, and thus interrupt, and probably set aside, the affection. intermittent neuralgia, intermittent fever, and both affections also in the regular remittent form, may often be interrupted in this way. Before the use of quinia in remittent fevers was so well understood as at present, blisters to the extremities were often employed, with the view of breaking the succession of the paroxysms; and they may still be resorted to, with the same object, when circumstances may forbid the use of quinia, as in active cerebral congestion or inflammation, or when that remedy may have failed. it will be remembered that the blister must be in full operation, at the time for the expected paroxysm.

3. Blisters are powerful revulsive agents, and are more employed for this effect than any other, or all others combined. The principles on which they act have been sufficiently explained in preceding parts of the work. (See vol. i. p. 49.) They are, through this mode of action, most important remedies in internal inflammations, and in irritations, whether purely nervous, vascular, or mixed.