Now, Tartar Emetic and Ipecacuanha do not act in this way. When injected into the blood elsewhere, in sufficient quantity, they are found to produce vomiting. They have also special actions on the heart and lungs, which are not possessed by merely irritant emetics. They seem to me to act specifically on the Vagus Nerve, which is supplied to these organs as well as to the stomach, and to cause vomiting by deranging its functions. By this action on the Vagus while in the blood, they excite, in a special way, the same reflex contractions which are produced, in the case of an irritant emetic, by irritation of the extremity of that nerve in the mucous membrane. They are thus Neurotics, or nerve-medicines. They are not gland-medicines; or, at least, there is no proof that they are excreted by the stomach, and thus they do not come under my definition of Eliminatives. All substances which touch the surface of the stomach cause it to pour out its

secretion.

Specific emetics cause nausea, even without vomiting, depressing the action of the heart by their influence over the Vagus nerve. Irritant emetics scarcely cause nausea, producing only a feeling of discomfort, arising from the inverted action of the stomach.

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 stage 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.)

That Tartar Emetic acts from the blood may now be considered as an established fact. Thus, if the oesophagus of an animal be tied above the cardia, and a solution of this salt introduced into it, efforts at vomiting follow. Such specific emetics must either act upon the Vagus nerve itself, or, which is much the same thing, on that part of the nervous centre with which this nerve is connected, the medulla oblongata. Whereas it seems that merely irritant emetics have the power of at once stimulating the ultimate fibres of the Vagus which are distributed to the stomach. But as it will be stated directly that some irritant purgatives are also, after their absorption, specific purgatives, so is it still a matter of doubt whether some of these irritant emetics may not be also capable of acting from the blood as specific emetics.

This may be said especially of the Sulphates of Zinc and Copper. Dr. Maclagan made some experiments upon them several years ago. He found that section of both nervi vagi deferred the action of these two salts until long after the time at which they had previously operated on the same animals with these nerves uncut. This sustains the view already expressed, of their operation by irritating the ultimate filaments of this nerve. But as in one instance the emetic dose did ultimately act after the section, the experimenter reasonably infers that this can hardly be explained but by supposing the absorption of the salt, and its subsequent operation on that part of the medulla where the Vagus takes its rise, from which an action might then be reflected along the motor fibres. But the sources of fallacy in such inquiries are so many and so great, that we cannot, of course, found any positive statement on the result of one experiment.

b. Stomach-Anaesthetics.-There is a class of medicines used in Gastrodynia which seem to act locally on the sentient nerves or surface of the stomach, in the same way as Aconite acts on the superficial nerves of the skin, or a plaster protects from irritation a painful ulcer. Although the majority of them are subsequently absorbed, yet, in order to exert this particular action, it is not necessary that these should pass beyond the substance of the stomach itself. For the anodynes thus used 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 TrisNitrate) of Bismuth, are the most useful of these medicines. The two first 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 3j, or more), and it is probable that its anæsthetic action is simply mechanical in nature, and depends upon its affording a soothing sheath to the irritable and painful surface of the stomach. So that it is only by covering this, and not at all by an influence on the stomach nerves, that it operates as a local anesthetic.

Monneret also believes its action to be purely local. So do Trousseau and Pidoux. Monneret has given as much as 3j daily, without ill effects. Lussanna also has prescribed large doses, but he considers that a part is absorbed, being rendered soluble by the acid fluid of the stomach. It is just possible that this might occur in some conditions of the system, and peculiar states of the gastric secretion; but I think even this very improbable. It covers the coat of the stomach, and hinders

the excretion of gastric juice. And all the cachectic symptoms described by this writer as having followed its long use may be explained by this obstruction to the digestive process. A strong argument against its absorption is derived from the fact that it can never be discovered in the urine. Lussanna accounts for this by supposing that as soon as it enters the blood, the Bismuth is precipitated by the alkaline chlorides in that fluid, and thus, being rendered insoluble, is incapable of excretion. But what would become of the patient if precipitations were thus to take place in his blood!-(Gazzetta Medica Italiana Federativa Toscana, 1852, p. 44.)

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. But whereas the chief action of an Emetic is to excite contraction, that of a Cathartic is to increase secretion, by which contraction may result as a secondary effect. I will not now enter into the theory of Elimination, which I shall have to consider afterwards, but I wish at present 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 pre

viously 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.*

It is to be inferred, both from analogy and direct experiment, that other resinous Cathartics, as Jalap, Scammony, and Gamboge, act also 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

*Pereira's Materia Medica, 2nd edit. vol. ii. pp. 1496, 1507, and elsewhere. See also above, p. 51.