This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
* These experiments were made chiefly in Germany. Begun by Oesterlein, they were repeated and varied by Eberhard, Kolliker, Meyer, Donders, and Mensonides, all with more or less confirmatory results. (London Med. Times and Gazette, March 13, 1852, p.270).
9. Another modifying influence, exerted over the absorption of medicines, is probably their own power of altering the condition of the intervening membrane, either by a physiological or chemical action upon it. Thus, it may be readily conceived that the contraction produced by tannic acid, and the chemical reaction with solution of nitrate of silver, might interfere with the absorption of these medicines in an unchanged state.
Changes in Medicines before and after Absorption, and their Elimination. Of the organic medicinal substances none are probably taken into the system in the exact state in which they are furnished by nature. It is only certain proximate principles contained in them that are absorbed, such as the volatile oils, vegetable alkaloids, bitter and colouring principles, etc.; the residue being left behind when they are applied externally, and either digested, or thrown off by the bowels, when they are swallowed. Thus, in the instance of garlic, the gum, starch, sugar, and albumen are probably digested, the lignin passes off with the feces, and the volatile oil alone is absorbed. In like manner, Peruvian bark probably yields its quinia, cinchonia, and other alkaloids, rhubarb its colouring matter, and aloes its bitter purgative principle to the blood; while the remaining constituents are evacuated or destroyed.
A great number of medicines undergo various changes in the stomach and bowels before absorption, in consequence of the chemical reaction between them and the acids, salts, and various animal principles, introduced or secreted, which abound in those passages. Alkalies, alkaline earths, and many metallic oxides are neutralized by acids; metals are oxidized and form salts; metallic salts form combinations with albumen or other proximate organic principles, or undergo decomposition with sulphuretted hydrogen or other acids, or by reaction with other saline compounds; carbonates are decomposed, with the extrication of carbonic acid in the stomach and bowels; salts with vegetable acids yield their acid constituent to the digestive process, while their bases appear to enter the circulation as carbonates; acids often combine with bases and form salts; iodine and chlorine become acidified and then neutralized; and, by some one of the above, or by other chemical reactions, insoluble substances are rendered soluble, and consequently more capable of absorption.
Some medicines appear to be absorbed unchanged, as ether, alcohol, chloroform, hydrocyanic acid, various proximate organic principles previously isolated, as the volatile oils and vegetable alkaloids, and many saline bodies in aqueous solution.
It should be observed that medicines are not confined to the bloodvessels in their course through the system. By the principle of diffusion, many of them escape from the capillaries into the neighbouring tissues; and they may thus penetrate into the interior of structures containing no blood-vessels, cartilages for example. Dr. H. Bence Jones detected traces of lithia in the superficial parts of the crystalline lens, thirty minutes after the swallowing of chloride of lithium; and in two hours and a half it was found throughout the lens.*
After entering the circulation, many medicines undergo chemical change, through the agency either of principles they meet with in the blood, or of the constituents of the various tissues with which they come in contact. The character of these changes is not well understood, and for the most part is merely conjectural. It is possible that some soluble substances may become insoluble, and by a mechanical operation modify the state of the capillaries. It would seem that certain metallic compounds are reduced; as mercury and silver are asserted to have been found in the metallic state in the tissues. Some medicinal substances, as alcohol, probably serve in part the purpose of nutrition or respiration; being, in the latter case, oxidized and thrown off from the lungs, in the forms of water and carbonic acid.
Sooner or later almost all the substances absorbed are eliminated from the system, cither unchanged, or variously decomposed; the change being produced either in the circulation, or in the process of elimination. Examples of such change we have in the oil of turpentine, which imparts not its own, but a violaceous odour to the urine, and in benzoic acid, which, when taken into the stomach, is eliminated by the kidneys in the form of hippuric acid. The elimination is usually effected, for solid or non-volatile matters, by the great secreting organs, especially the kidneys and the skin; and so frequently are the former emunctories the avenue of escape, that, when evidence is required of the absorption of any medicine, it is almost always sought for in the urine. Volatile substances escape not only by these organs, but in general abundantly also by the lungs.
The period required for elimination is very various with different substances. With some the process begins almost immediately, and is completed in a short time; with others, it is in various degrees protracted; and, with a few, many months elapse before the system is entirely freed from them. Thus, A. F. Orfila, having administered bichloride of mercury, sulphate of copper, acetate of lead, and nitrate of silver to dogs, for a sufficient length of time to impregnate their systems with those metallic poisons, found, upon killing the dogs, and submitting their bodies to a rigid chemical examination, that mercury disappeared from their systems in a period of from eight to eighteen days, silver sometimes so early as six weeks, but in other instances not until the expiration of six months; while lead and copper were found in the liver eight months after they had ceased to be administered. (Gazette des Hopitaux, Jan. 24, 1852.) With perhaps the single exception of silver, it has not been demonstrated that any substance given medicinally remains permanently in the system, and in reference to this, only in a few rare instances, in which it has been given in large quantities, and for a long time. Under such circumstances, it sometimes leaves a permanent dark stain of the skin, probably from the deposition of its oxide in the substance of the cutis vera.
* Dr. Jones experimented upon patients operated upon for the extraction of cataract, giving the salt of lithium at various times before the operation, and examining the cataract afterwards by spectrum-analysis. Upon exhibiting the carbonate of lithia, he found lithium in every particle of the cataract in 3-5 hours, traces still remaining in 4 days, and its entire disappearance in 7 days. (Med. T. and Gaz., Sept. 1866, p. 246.) - Note to the third edition.