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.
This is the genuine alterative operation of the medicine; as its remedial influence is exerted with little or no obvious disturbance of the healthy functions. Nevertheless, a close observation will sometimes detect evidences of its action, even in health. There is not unfrequently some uneasiness in the stomach and bowels, occasionally amounting to slight griping pains; and, if the stools are examined, they will be found in general of a deeper or brighter yellow than in their normal condition. it is evident that the alterative action is exerted chiefly on the liver, the function of which is somewhat augmented. in consequence of the increased secretion of bile, the peristaltic action of the bowels is moderately promoted; and it is from the presence of the same agent, probably, that the sensations above referred to are experienced. in some persons, the susceptibility of the liver to this influence is so great, that very minute doses, even a single grain of the blue pill, or one-quarter of a grain of calomel, will produce a sensible effect; and three grains of the former, or a grain of the latter will operate as a cathartic, with bilious stools. in others, less susceptible, there will only be a slight increase in the softness of the passages, and a somewhat greater tendency to alvine evacuation; while in others, again, no apparent effect whatever will be experienced. Why the liver should feel more sensibly than other organs this first impression of the medicine is quite intelligible. The absorption takes place, not through the lacteals, but the radicals of the veins which empty into the vena portae; and the medicine is, therefore, first distributed throughout the liver, coming into intimate contact with its secreting tissue, and acting upon this, before it can reach the system generally through the hepatic vein, and the ascending cava. A portion of it may possibly be carried out again with the bile into the bowels, and another portion be detained in the tissue of the liver; so that but a small part of the medicine administered may enter the general circulation. Hence, probably, it is, that no sensible influence is exerted upon the system at large. Nevertheless, experience has shown that other therapeutic effects, besides those dependent on a change in the hepatic function, are often experienced from this insensible operation of mercury; and some of the medicine must consequently escape the secretory and detaining force of the liver, and enter into the general system. if this explanation is correct, the external application of mercury should be attended with less of this preferable influence upon the liver; and such has been uniformly the result of my own observation. Few, I presume, would expect so energetic an alterative action on the liver from mercurial inunction, as from the internal use of calomel or the blue pill; and yet the sialagogue operation can usually be thus obtained with great facility.