This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
if, with the relaxed condition of the capillaries or small blood-vessels just referred to, there be a more rapid current of blood through the vessels, or a greater distension of them, the transpiration must be increased, because the proportion of liquid from which it proceeds in a given time is increased. Hence the night-sweats of phthisis and other states of debility are usually greatest, when, along with a certain amount of relaxation, there is a frequent pulse. if, then, one of the nauseating and relaxing diaphoretics be accompanied with some influence which shall increase the flow of blood through the vessels, or the bulk of liquid in them, its action will be greatly promoted. in this way, in part, may be explained the powerful diaphoretic influence of Dover's powder; the opium stimulating the movement of the blood through the capillaries, while the ipecacuanha relaxes their coats.
In the febrile state, the skin is usually hot and dry, while the blood flows through it more copiously, and more rapidly than in health. The tissue is excited beyond the point at which it can perform the function of perspiration. The effect is only in obedience to the universal law, that a moderate stimulation increases function, a greater deranges it, and a still greater diminishes or suspends it. The last condition is present in the state of fever alluded to. it is not only the cell-function that is suppressed, but the transpiratory also, in consequence of the tension of the vascular coats under the excessive irritation. Now all that is wanting, in such a case, is to reduce the excitement to the first condition above mentioned, that, namely, at which function is increased; and copious perspiration will take place. The follicles will secrete, and the vessels transpire, beyond the mean amount. Hence, refrigerating and relaxing substances are powerfully diaphoretic in the febrile state. Tartar emetic and citrate of potassa operate partly in this way; and a draught of cold water, or sponging the surface with cool liquids, by which the excitement is reduced, are often followed by a gentle perspiration. A warm bath, or sponging with warm water, has a similar effect by the direct relaxing influence of the moisture.
3. By the Two Methods Jointly. Some medicines combine the two modes of action above considered. They at once stimulate the cell-function by which they are eliminated, and relax the coats of the vessels so as to favour transudation. The antimonials and ipecacuanha may be supposed to act in this way. By a slight nauseating impression on the stomach, scarcely sensible to the patient, they sympathetically relax the vessels; while, by entering the circulation, and coming into contact with the sweat-producing cells, they stimulate them into greater activity. if now, some influence be added which will augment the rapidity of the flow of blood in the part, and fill the vessels beyond the normal degree, the effect is still further augmented. Such an influence is obtained by the addition of opium, the application of warmth, and the free use of water.
4. By Filling the Cutaneous Vessels. Little more need be said upon this method of producing diaphoresis than what has been above incidentally stated. No fact in physiology is better established, than that distension of the vessels favours the elimination of their contents. This condition is produced by the free use of drinks, which are absorbed almost as rapidly as swallowed. But it is the whole general circulation that is affected. if circumstances call some one eliminating function into especial action, it is through this that the liquid escapes. Warmth directs to the skin. Hence the drinking of warm or hot water is a powerful promoter of perspiration.
5. increased Rapidity of the Current. This also has been incidentally mentioned as among the influences promotive of the perspiratory function. More blood is offered in a given time to the cells, which are also excited by its presence; while the transuding process must go on with proportionate increase of rapidity, if the tonicity of the vascular coats is not augmented. Hence, simple stimulation, directed especially to the skin, often produces a powerful diaphoretic effect, if kept within due bounds. Beyond a certain point, it will derange or check the function. Hence the additional efficiency given to water by heat and gently stimulating substances, such as the milder aromatics. Hot herb teas are notorious for their powerful diaphoretic action. Balm, the mints, sage, catnep, chamomile, and eupatorium, given in hot infusion, while the patient is kept warm in bed, operate very efficiently in this way; and are often given with advantage at the commencement of certain inflammatory affections, particularly catarrh and rheumatism.