The Sulphates of Zinc and of Copper; common Salt, and among vegetables, Mustard and Horse-radish, are used as irritant emetics. They cause, by contact and irritation, a large quantity of the gastric juice to be poured out. This, together with the emetic, and any contents of the stomach, is rejected. The process is not followed by much inconvenience.
Such emetics are chiefly used when we wish to unload the stomach of any irritating or poisonous matters; but not when our object is to cause nausea, depression of the heart's action, or relaxation of the muscles. The violent action which they produce may possibly act on remote parts on the principle of counter-irritation. Thus emetics of various kinds are often administered in the early stages of inflammatory disorders, and have been known sometimes to cut them short. But such an effect is much more likely to be produced by a specific emetic, which adds to this counter-irritant action the production of nausea, by which the force of the heart is powerfully depressed, and the pulse reduced. Tartar Emetic, the most powerful of these specific agents, must doubtless be absorbed to a certain extent before it can produce its effect. Thus the important difference between the modes of operation of irritant and specific emetics is, that the former do not produce vomiting when injected into the blood, but act locally; whereas the latter act from the blood on the nerves. (Vide Prop. VIII., Specific Sedatives.)
b. Stomach-Anæsthetics.—There is a class of medicines used in Gastrodynia which seem to act locally on the sentient nerves of the stomach, in the same way that Aconite acts on the superficial nerves of the skin. Although the majority of them are subsequently absorbed, yet, in order to exert this particular action, it is not necessary that they should pass beyond the substance of the stomach itself. They do not seem to have any special or peculiar tendencies towards the stomach nerves. But if introduced into the blood elsewhere, they would not pass the nerves of the stomach in so concentrated a form as when coming directly from the mouth, and thus would not be so useful as stomach-anæsthetics. Thus this action depends upon local contact, and is so far a local action.
Hydrocyanic acid, Creosote, and Nitrate (also called Tris-Nitrate) of Bismuth, are the most useful of these medicines. The first two are subsequently absorbed, and pass into the blood. The third is a very insoluble salt. It acts also as an astringent on the mucous surface of the intestine, and is probably the only astringent which is not absorbed. Being insoluble, its action is quite confined to the mucous surface. It may be given safely in very large doses, (as ʒss, or even ʒj.) and it is likely that its anæsthetic action may be in some part mechanical in nature, and depend upon its affording a mechanical sheath to the irritable and painful surface of the stomach. Hydrocyanic acid and Creosote,—general sedatives,—act locally as anodynes to the nerves of the stomach.
c. Irritant Cathartics.—As there are two kinds of emetics, differing in their mode of operation, so also do there seem to be two kinds of Cathartics. But the distinction between them is not exactly the same. As with the Emetics, one kind seem to act by topical irritation, exciting an outpouring of the intestinal secretions, and causing an expulsion of the contents by exciting peristaltic contraction. But Specific Emetics, which act from the blood, seem to produce their effect by influencing the nerve of the stomach; so that they are Neurotic medicines. Specific Cathartics act differently. They are truly Eliminatives. They exert no influence over nerves, but they operate by passing out of the blood through the intestinal glands. Like Specific Emetics, they must be first absorbed. I will not now enter into the theory of Elimination, which I shall have to consider afterwards, but I wish merely to distinguish between local action on a surface, and specific action on a gland, exerted from the blood. Many substances used as purgatives are capable of absorption, and are absorbed. But it seems that they are not fit to remain in the blood; and after passing round in the circulation, they are expelled by the depurative force at a point near to that at which they previously entered by the laws of absorption, namely, the mucous surface of the bowels. They are most commonly expelled by the glands of this surface somewhere in the lower or fæcal portion of the intestinal canal, which is more engaged in secretion, but less active in absorption, than the upper part. The increased secretion which they excite causes peristaltic action, which expels both it and them, so that they cannot again be absorbed.
Such a specific Cathartic would be capable of acting thus if introduced into the system at any point. Castor Oil and Croton Oil, whether received into the stomach, or injected into the veins, or introduced into the system at any part, equally produce purging. So also do Rhubarb, Aloes, and Senna. The principles of these medicines have been detected in the blood by Tiedemann, Gmelin, and others. Colocynth and Elaterium have also been proved to act specifically.[28]
It is to be inferred from analogy that other resinous Cathartics, as Jalap, Scammony, and Gamboge, must also act from the blood. It has also been shown that both from actual experiment, and from a consideration of the laws of the process of absorption, we must conclude that saline Cathartics are absorbed into the blood before they cause purging.
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.