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Occasion their effects—

I. By Stimulating the Cutaneous Capillaries.

A. By external application.

The Stimulus of Heat, Frictions, &c.

B. By Medicines which enter the circulation and stimulate the cutaneous vessels by contact.

Mercurials—Sulphur.

C. By Medicines which act on the surface sympathetically, through the medium of the Stomach.

Cold Drinks, &c.

II. By Increasing the General Action of the Vascular System.

Violent Exercise—Ammonia—Guaiacum—Alcohol—Warm Bath.

III. By relaxing the morbidly constricted mouths of the Perspiratory Vessels.

Antimonials—Cold Affusion—Venesection—Saline Diaphoretics.

The action of the cutaneous vessels may be augmented by heat, without necessarily increasing, at the same time, that of the heart and arteries; hence it is that heat is, of itself, often sufficient to produce sweating, while it generally accelerates the operation of a sudorific medicine. To this general proposition, however, there are some very important exceptions; and, indeed, in certain conditions of the cutaneous surface, the stimulus of heat will be even found to impede, rather than to promote, diaphoresis; thus in the hot stage of a continued fever, there would seem to exist a peculiar constriction of the perspiratory vessels, accompanied with extreme heat and dryness. In such a state, remedies of the third class must be applied, or conjoined with those of the former. The warm bath may be said to partake of all the qualities upon which our classification is founded; it will stimulate the cutaneous capillaries,—increase vascular action, generally, and, by its emollient powers, relax the morbidly constricted mouths of the perspiratory vessels. During the ardent heat of fever, the external application of cold is the most efficient sudorific, as the valuable reports of Dr. Currie have very satisfactorily established.

Although the external application of cold was not often employed in the hot stage of fever, until within the last thirty years, yet the administration of cold drinks appears to have been practised by the ancients, as an expedient to produce perspiration. Galen, and his immediate disciples, as well as the physicians of the sixteenth century, seem to have frequently administered cold water for the purpose of exciting sweat in fevers.^[162] Celsus also describes the beneficial effects which arise from copious draughts of cold water in ardent fevers, “fereque post longam sitim et vigiliam, post multam satietatem, post infractum calorem, plenus somnus venit, per quem ingens sudor effunditur, idque præsentissimum auxilium est.”^[163] Cold water, when introduced into the stomach in the hot stage of fever, must produce its diaphoretic effect through the sympathetic relation which subsists between that organ and the skin. Nauseating doses of Antimony, and of other emetics, occasion a relaxation of the surface from the same mode of operation, and in this latter case, if the force of the circulation be at the same time increased by tepid diluents, the diaphoretic effect is more certain and considerable.

Alcohol, Guaiacum, and other powerful stimulants, produce their effects by merely accelerating the circulation; but in employing such remedies for the purpose of exciting sweat, we must be careful to adapt them to the circumstances of the case, and to the degree of action which prevails. In all febrile diseases attended with much increased heat, or connected with local inflammation, diaphoretics of this description must be very cautiously administered, for by accelerating the circulation they might counteract any benefit which they would otherwise confer by relaxing the vessels of the skin. In the whole history of medical opinions there is scarcely a theory which has proved so fatal in its practical applications as that maintained by Van Helmont, and his disciples, viz. that acute diseases were to be cured by expelling some morbific matter, after its proper concoction—a theory which suggested the administration of the most stimulating sudorifics, together with high temperature^[164] in every grade of febrile exacerbation. The fatal effects of such a practice during the seventeenth, and early parts of the eighteenth centuries, are incalculable, and may be very satisfactorily contrasted with the beneficial results which have accrued, in the same diseases, in the present age, from the use of diaphoretics of the refrigerant kind.

Saline Diaphoretics, as they readily pass with the chyle, may be supposed to enter the circulation, and be thus brought to act, directly, on the cutaneous vessels; at the same time it seems extremely probable that such remedies may also occasion an impression on the stomach, which is sympathetically communicated to the vessels of the skin; they have undoubtedly little or no influence on the general vascular system, and neither augment the force nor the velocity of the circulating current.

It is not, however, in febrile affections alone that this class of remedies proves highly beneficial; the very intimate sympathetic connection which subsists between the functions of the lungs and skins, renders the use of such medicines particularly advantageous in the cure of the diseases incident to the former of these organs; a fact upon which we shall hereafter offer some remarks under the history of Expectorants.

So again, in the treatment of bowel affections, in consequence of the intimate relation which exists between the cutaneous capillaries and those of the internal organs, gentle diaphoretics offer a valuable resource in their cure. How frequently do Diarrhæa, Enteritis, &c. ensue from the sudden suppression of perspiration by cold?

From the influence which these medicines exert upon the extreme vessels of the skin, they are also highly serviceable in various obstinate cutaneous affections, as Herpes, Lepra, &c.

As evacuating the serous part of the blood must necessarily have an indirect effect in promoting absorption, Sudorifics have been occasionally exhibited in Dropsy, especially in that form of the disease called Anasarca. It has been already observed that cases too frequently occur in which the discharge of urine cannot be increased by art; upon such occasions practitioners have sometimes had recourse to a trial of Sudorifics,^[165] but from the great difficulty which generally exists in exciting sweating in such affections, the indication has rarely been fulfilled. Where however a sudorific does succeed, it is less liable to debilitate than the other alternative of a drastic purgative.

There is still another point of view in which the therapeutic importance of Diaphoretics may be considered. It is generally acknowledged that by cutaneous transpiration a portion of excrementitious matter is ejected from the system; hence by the failure or imperfect performance of this function, a deleterious fluid is retained which may give origin to disease; to such a cause may perhaps be attributed the generation of Calculi, and other diseases of the urinary system, as we shall have occasion to notice under the head of Lithonthryptics.

The increased efficacy which these medicines derive from combination with each other, will form a subject of interesting enquiry in the succeeding essay.