But we have now to do with Cathartics that act by topical irritation. Which are they? I believe that the same resinous Cathartics which have the power of acting specifically, may have, especially when in large doses, a double action. It has been shown that resins are difficult of absorption; and whether they be absorbed or not, we know that they must irritate the intestinal surface, from the violent griping which often attends their operation. Thus Scammony does not act so well, and does not gripe, when the bowels are lined with mucus. Gamboge and Euphorbium are irritant in an extreme degree, and are therefore too dangerous for general use. The powder of Euphorbium resin has been used as an Errhine, for, by irritating the mucous membrane of the nose, it increases its secretion. This illustrates the operation of irritant Cathartics. A great part of the fæces is secreted by the mucous membrane of the bowel. This secretion a Cathartic may increase by mere contact and irritation; or, being also a specific agent, by absorption and elimination likewise.[29] Some Cathartics employed as Vermifuges, as the hairs of _Mucuna pruriens_, metallic Mercury, and Tin powder, cannot be absorbed at all, and must act solely and altogether by irritation. These Vermifuges, or Anthelmintics, are employed for a strictly local purpose--that of killing and expelling intestinal worms. Any powerful Cathartic may be used to expel them. But such an agent should generally be conjoined with a medicine that tends directly to kill the parasite; for after that it will be more easily dislodged. For this purpose the root of Male Fern, Kousso, and the bark of the root of Pomegranate, have been used with advantage in the case of tape-worm. Ascarides are situated low down in the intestine, and may be dislodged by the use of an enema, as of Salt. Turpentine is often very efficacious in cases of this kind, being at once a poison to the worms and a powerful irritant cathartic. I have already alluded to Nitrate of Bismuth, as seeming to be an astringent to the mucous surface of the intestine, although apparently incapable of absorption. It has been used with advantage in diarrh[oe]a, and is highly recommended by Dr. Theophilus Thompson in the diarrh[oe]a of Phthisis. But all other astringents are absorbed; and when they act on the mucous glands of the intestines, it is probably from the blood. It has been supposed by M. Poisseuille and others that the action of Opium in confining the bowels is to be attributed to a power of checking the process of endosmosis, said to be possessed by a solution of Morphia. I shall afterwards state my reasons for discrediting this explanation. (_Vide Chap. IV., Art. Opium._) Thus we have concluded the list of substances which seem to act locally on the mucous surface, without passing into the blood.