The Mucous Membranes absorb more rapidly than the skin, and this tissue is more or less permeable according to its thickness, its density, as the epidermis which covers it is thick or thin. Absorption is therefore very rapid in inoculation—that is, when the substance to be absorbed is introduced into the substance of the tissues. Wherever the point of absorption may be, it takes place by the lymphatic vessels, and especially by the veins. The veins absorb a greater number of substances than the lymphatics, and carry them more quickly into the circulation; they charge themselves especially with the materials which are to be rejected from the economy, while the lymphatics absorb from preference those that can still be assimilated. The veins and the chyliferous vessels, which are a variety of the lymphatics, draw from the mucous membrane of the intestine the useful products of digestion; but the lymphatics take possession of fats, while the veins prefer fluids, albumen, sugar, and salts.
It is well known with what rapidity certain substances taken into the alimentary canal, or into the lungs, pass into the other organs, and exhale, or are eliminated. Thus the presence of the ferro-cyanide of potassium has been recognized in the urine within one minute after its ingestion into the stomach. Indigo, gallic acid, and other colouring matters, or those possessing a characteristic odour, pass in the course of fifteen or twenty minutes through all the windings of the circulation.
As has already been stated, absorption takes place much more rapidly by the skin when it has been deprived of its epidermis. Five or six minutes are generally a sufficient time for the alkaloids of opium or belladonna to manifest their action on the nervous system, and in some cases this action is produced in a few seconds. Other substances, especially sulphate of copper, are almost as rapid in their effect on the stomach as chloroform and several gases when placed in contact with the mucous membrane of the lungs; they produce phenomena which develop themselves with terrible rapidity. Medicine derives great assistance from this absorbent property of the tissues; thanks to which, humanity is daily spared a vast amount of suffering.