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.
Astringents are remedies which produce contraction of the living tissues. I do not here refer to the visible contraction which takes place in muscles under the influence of the will, or other excitant agency; but to a certain shrinking or condensation of structure, which is not attended with visible movement, but is nevertheless very obvious in the result, as in that well-known state of the skin called goose-flesh, produced by exposure to cold. That astringents have this effect locally, is proved by the obvious diminution of bulk in any part of the surface to which they may be applied, and by the strong puckering sensation occasioned by them when taken into the mouth. There can be no doubt that they operate upon the mucous membrane of the stomach and bowels, when brought into direct contact with it, in precisely the same manner as upon the mouth and skin. It has been denied that their operation extends beyond the surface of application; but the effects obtained by their use in disease can be explained only by the admission, that they exert upon the system at large their characteristic influence; though it must be allowed that their general are much less than their local effects. It is not impossible that a portion of the constitutional impression produced by them may be the result of sympathy or nervous communication; in like manner as the similar effect of cold upon the skin is transmitted to certain internal parts. But this explanation is not necessary to account for the result. Experiment has satisfactorily proved that astringents are absorbed; and the probability is that they are conveyed everywhere with the blood, and thus act everywhere by direct contact.
The fact, recently ascertained through the experiments of Bernard, that one of the functions of the sympathetic nerve-system is to produce contraction of the capillaries, very much facilitates the explanation of the general action of astringents. It is highly probable that, reaching the sympathetic centres through the circulation, they excite these centres to increased action, and thus, in part at least, produce the contraction which is their most characteristic effect. This explanation does not in any degree invalidate the idea that they act also directly on the tissues through the blood. There is nothing in the least inconsistent in these two modes of operation, which may be in action at the same time, and thus conjointly produce greater effects than either separately.