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.
* At the time of the appearance of the second edition of this work, the result of recent experiments had not been favourable to the opinion, that medicines applied to the skin by means of baths, whether local or general, can, to any considerable extent, find an entrance into the system through absorption. For a condensed account of these experiments, the reader is referred to the British and Foreign Medico -chirurgical Review for January, 1850 (Am. ed., page 108). Numerous substances, mineral and vegetable, were employed in baths and foot-baths: and the uniform result, when care Wis taken to prevent any possible absorption through the lungs, was that neither the substances themselves, nor any of their constituents, could be found in the urine. The medicines thus experimented with were iodine, iodide of potassium, chloride of sodium, ferrocyanide of potassium, sulphuret of potassium, acetate of lead, alum, borax, carbonate of potassa, nitric acid and nitre, sulphate of magnesia, sulphate of quinia, digitalis, and belladonna. But I did not, at the time, consider the experiments as conclusive against the absorption, by means of baths, of many untried medicines, nor even, in some degree, of those tried; for. though not detected in the urine, some of them might have been eliminated by other emunctories, or have been retained in the system; and, if not absorbed in the instances submitted to trial, they might possibly be so in others. Certain substances had by former experimenters been shown to be absorbed in this way; and it would require very numerous experiments, with different individuals, and under various circumstances, to determine positively that there is any soluble substance which can never enter the system, when applied by means of baths to the surface of the body in a perfectly sound state. Bines that time, numerous experiments, performed with great care to avoid all sources of error, by Dr. Willemin, of Vichy, in France, have established, beyond reasonable doubt, the possibility of absorption, by means of baths, through the skin, not only of water, but of various remedial substances. (Archives Generales, 6e ser., ii. 5, 105, 177, 313). Nevertheless, it must be admitted, as stated in the text, that the epidermis opposes a very strong impediment to absorption: and that baths do not offer a very efficient method of medicating the system. (Note to the second and third editions).
Means of Absorption. The lymphatics and lacteals were formerly supposed to be the exclusive agents of absorption. Magendie proved that foreign substances are also received from without directly into the blood-vessels; and multiplied experiments have since shown that most medicines enter the system in this way. It is probable that the capillaries, in consequence of the extreme delicacy of their walls, are chiefly concerned in the process; and that medicines enter them upon the same principles of endosmose by which liquids pass through dead membrane out of the body. Attempts have even been made to establish the conditions based on the rules of physical endosmose, upon which medicines in contact with the capillaries will either enter the blood-vessels, or cause the extravasation of the liquid contents of the vessels themselves, and thus to explain the effects of a medicine, if not indeed to deduce a priori its probable operation from its physical qualities. But, admitting to a certain extent this principle of operation in medicines, there are so many circumstances, physical, chemical, and vital, which modify the result, that no previous conclusions can be relied on; and even explanations upon this basis must be received with great caution, lest they lead into serious error. It must be remembered that the condition of a living membrane is very different from that of the same membrane out of the body; and there can be little doubt that the vital forces have such a control over the tissues, as greatly to modify their endosmotic relations to the fluids on opposite sides of them. For example, in a certain condition of vital influence the membrane may be contracted and firm, in another relaxed and loose; and it is impossible to say that some modifying power may not be exerted on the fluids themselves, in and around it, by the life-forces, as we know that the electric forces modify chemical conditions out of the body. In the present state of our knowledge, therefore, it is quite premature to attempt to explain the phenomena of medicinal absorption in every case, or to anticipate the results in any particular case, on simple physical principles.
It is highly probable that the principle which governs the process of dialysis, discovered by Mr. Graham, may have some influence in the absorption of medicines. Crystallizable substances may thus find an entrance into the capillaries, while the uncrystallizable are excluded. For an explanation of this principle the reader is referred to the U. S. Dispensatory (12th ed., p. 896). The comparative facility with which saline substances and the active principles of vegetables, which are generally crystallizable, enter the circulation, may be in some measure thus explained.*
After the entrance of the medicine into the capillaries, it is carried forward in the course of the circulation, and sooner or later mingles with the blood in the heart, with which it is transmitted over the whole sys-tem, and consequently reaches the part on which it is destined to act.
But it may be asked whether medicines are not also taken up by the absorbents, and conveyed by them into the blood. It would seem to be the legitimate function of these vessels, to select from the alimentary liquid in the bowels, and from the disintegrating tissues throughout the body, those principles which may assimilate with the blood, and thus contribute to the sustenance of that fluid in its requisite state and quantity. In order that this function may not be interfered with, they have the power to reject in great measure noxious substances, and medicines among others. Tiedemann and Gmelin found that, of numerous colouring and odorous substances given to animals with their food, a large proportion imparted their colour and odour to the blood of the portal veins, but not one to the chyle. Of a number of salts tried in the same manner, though many could be detected in the veins, a few only had entered the thoracic duct. Drs. Lawrence and Coates, of Philadelphia, in a series of carefully conducted experiments, proved that ferrocyanide of potassium was taken up from the bowels both by the radicles of the vena portae and by the lacteals. (Philad. Journ. of Med. and Phys. Sci., v. 327.) It has been subsequently shown, in repeated instances, that poisons introduced into the alimentary canal, fail to produce their effects on the system when the vena port® is tied. (Pereira, Elem. of Mat., Med., 3d ed., i. 103.) From these statements it is fairly inferrible that, though the absorbents are capable of taking up a few saline substances, probably not altogether incongruous with the blood, they are not the ordinary agents by which medicines are introduced into the circulation.