This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Rhubarb combines purgative with tonic and astringent properties. in very small doses, insufficient to operate on the bowels, it acts like the simple bitters, increasing the appetite, and promoting digestion, and is thought to exercise a somewhat astringent power, shown in the checking of mild diarrhoea. With a little increase, however, of the dose, it becomes laxative, and I have often known Russia rhubarb to act gently on the bowels, producing one soft feculent discharge, in the dose of three grains. in larger doses it is actively purgative, operating probably in chief, if not exclusively, upon the peristaltic function, and evacuating the liquid contents of the upper bowels, along with the more concrete matter of the colon and rectum. There is no reason to believe that it promotes, to any considerable extent, secretion or extravasation into the bowels. Though it produces not unfrequently more or less griping, this probably arises from the energy of its action on the muscular coat, and not from any direct irritation of the alimentary mucous membrane. it does not appear to be capable, in any dose, of producing serious inflammation of that membrane; in this respect differing strikingly from the drastic cathartics. The stools are coloured by it of a deep brownish-yellow. it is rather slow in producing its purgative effect, and is thought to leave behind a disposition to costiveness, which is attributed to the tannic acid contained in it.
It is doubtful whether rhubarb acts on the muscular coat through an original impression upon the inner surface, or by means of absorption. That its colouring matter, at least, is taken into the circulation, is sufficiently proved by its imparting a yellow colour to the urine, and, as has been asserted, to the perspiration also, especially that of the axilla.* if it be true, as stated by writers, that it has produced a purgative effect when sprinkled over the surface of large ulcers, and that the milk of nursing women who have taken it operates on the bowels of the infant, it must be admitted that its purgative principle also is absorbed; and that it may possibly exercise its ordinary influence upon the bowels through the channel of the circulation.
In its influence on the system at large, rhubarb is supposed by some to be gently excitant, somewhat increasing the pulse and general warmth, especially when administered during the existence of a febrile condition. There is no reason whatever to believe that it has any special action on the liver, as was at one time supposed.