When received into the stomach, it is absorbed, and passes in the circulation to distant parts. Its action consists in a power of causing the contraction of muscular fibre when in contact with it; and this power appears to depend mainly upon a chemical property that it possesses of coagulating albumen and analogous compounds. Capillary vessels, and the small ducts of glands, contain in their walls certain fibres of unstriped muscle; and Tannic Acid, by causing this muscle to contract, is able to diminish the caliber of these tubes, and thus to moderate or arrest the flow of blood in the vessel, or to lessen the amount of the excretion of the gland. Thus, in all cases of hæmorrhage from the small vessels, or of inordinate secretion from any glandular organ, Tannic acid is appropriate. It is the best and safest of all Astringents for internal use, excepting only Gallic acid. I have already stated as worthy of remark the fact that Gallic acid is of little or no use as an Astringent when applied to external parts. Given internally, it is very efficacious. From this alone it would seem that there must be something in the blood which enables it to take effect. Reasons have also been alleged for supposing that Tannic acid is not a simple substance. When it is taken into the system, Gallic acid is secreted in the urine; and it appears that Tannic must actually consist of Gallic acid combined with certain other elements, or with some other definite substance; and that the astringent property which is so valuable is confined to the compound body thus produced. Gallic acid alone has not the power of precipitating albumen. What then is this other substance? It must be something which is contained in or formed in the blood, which added to Gallic, will be capable of forming Tannic acid; and which, when united to it, will enable Gallic acid to precipitate Albumen. These various requirements are fulfilled by the group of elements that constitutes grape-sugar (C_{12}H_{12}O_{12}.) This compound is continually forming in the blood during the blood-processes that are connected with respiration. It has been observed that a solution of gum (C_{12}H_{11}O_{11},) added to one of Gallic acid, will enable the latter to coagulate Albumen. And the elements of Tannic acid may be constructed out of those of Gallic acid and grape-sugar. Tannic acid = C_{18}H_{6}O_{10} + 2 aq. Gallic acid = C_{7}HO_{3} + 2 aq. Three equivalents of anhydrous Tannic acid amount to six of Gallic acid and one of grape-sugar. 3(C_{18}H_{6}O_{10}) = 6(C_{7}HO_{3}) + C_{12}H_{12}O_{12}.