There is another interesting point of inquiry in relation to this subject. How is it that the impression upon the stomach is conveyed to the nervous centres concerned ? I think there can be no doubt that it is, in some instances at least, conveyed by nervous communication; through the afferent fibres, namely, of the par vagum. Such a relation has been established between the gastric mucous membrane and the nervous centres referred to, that irritation of that membrane, from any cause, shall make itself felt in these centres, and thus give rise to nausea and vomiting. This is a wise provision of nature to get rid of irritating substances what way they expand the orifice is not so clear. To me the statement of the text seems much the more probable; viz. that the circular fibres relax, and the orifice is opened by the liquid forced into it by the contraction of the stomach. {Note to the third edition.)

But this is not all. Nausea and vomiting, being the direct result of a certain condition of the nervous centres induced by gastric irritation, must equally result from the same condition produced by any other cause. it is a familiar fact that certain diseased conditions of the brain, in a perfectly sound state of the stomach, very often occasion nausea and vomiting. Now if this state of the nervous centres can be induced by medicines, without directly affecting the stomach in any degree, they will equally produce vomiting, as though they operated through the stomach itself. Such is now well known to be the case. Various emetic substances will produce the effect when injected into the rectum, applied to the surface, introduced into the serous cavities, or thrown directly into the blood. Entering the circulation, they are brought with the blood into contact with the centres, and act on them directly, with the same result as though they operated primarily on the stomach. it may be said that they are carried to the stomach, and act primarily upon that organ still. This is not impossible. Entering into the mucous membrane with the blood, they may produce in it the same irritation as when applied directly to its inner surface, and this irritation may then react on the nervous centres so as to bring about vomiting. indeed, it has been ascertained that certain substances which, like arsenic, inflame the stomach directly, are capable of producing exactly the same effect in it, when injected into the circulation. But there are reasons for thinking that some emetic substances act directly upon the nervous centres especially; and that, even when swallowed, they still act rather through absorption than directly on the stomach. Thus, medicines often operate as emetics, which are not in themselves very irritating; and this emetic effect is in no degree proportioned to their irritant influence. One grain of tartar emetic, dissolved in three or four fluidounces of water, is incapable of producing any sensible irritation of the mucous membrane; yet it may vomit actively; and it will produce the same effect, in whatever way it may enter the blood. The probability is, therefore, that even when swallowed, it acts not by an impression on the gastric surface, but by absorption, and direct action on the organic nervous centres. Another consideration favours this view. Emetics do not generally operate at once. Time is required, and not unfrequently a considerable time, before their peculiar effect is experienced. it may be assumed that this time is expended in the process of absorption; and the effect is not produced until the blood has acquired a sufficient degree of impregnation for the purpose. if they acted by irritating the stomach, the effect should be more rapid; and this is the case with the emetics which really operate through their acrimony. Hence, an emetic dose of sulphate of copper, sulphate of zinc, or mustard, acts much more promptly than the same relative dose of tartar emetic, given with a view to its emetic effect.

From what has been said, it seems highly probable that emetics operate in three different methods; 1. by a direct and exclusive irritation of the stomach, as mustard; 2. by absorption, as tartar emetic; and 3. by both methods, as probably ipecacuanha.

Effect of Repetition. Most medicines lose their effect, in greater or less degree, on repetition; more being required, at each successive administration, if the repetition be so frequent as not to suffer the system entirely to recover from the effects of the preceding dose. But emetic substances have been said to be an exception to this rule; and the fact is, that, when frequently repeated, they often operate in smaller doses after some time than at first. This, however, is only an apparent exception to the general rule. Emetics irritate the stomach. Their speedy repetition may increase the irritation into a state bordering on, if not amounting to inflammation. in this condition of the stomach, the mildest articles will often vomit, even a drink of water; much more a portion of the irritant emetic substance. Besides, the principle of association often influences the action of the medicine. Even the thought or recollection of a substance, which has once vomited, will sometimes, through this principle, produce an emetic effect. But, if the emetic substance be given cautiously, beginning in such doses as not to irritate the stomach, and gradually increasing, as the organ is found to bear them, we may in this way enormously augment the quantity taken, without vomiting. Every one knows how insensible the stomach becomes to tobacco, though, on the first use of it, the smallest quantity produces sickness and vomiting. The stomach and nervous centres have gradually become accustomed to the impression, and consequently feel it less sensibly. in other words, the emetic substance obeys the general law.

Susceptibility to the influence of Emetics. This is very different in different individuals, and in different states of system. Some persons, through idiosyncrasy, resist large doses of a particular emetic, and others are extremely susceptible to the smallest doses. in some persons, from six to ten grains of tartar emetic will not even produce nausea; and I once had a female patient whom ipecacuanha vomited in doses of one-sixth or one-quarter of a grain. in disease, the difference is very striking. Some nervous diseases offer a strong resistance to the emetic action. in delirium tremens and mania, it is often extremely difficult to induce vomiting. Such, too, is the case often in amaurosis, tetanus, and narcotic poisoning. The cause is probably the diminished sensibility of the nervous centres. The case is reversed in inflammatory and febrile diseases, in which vomiting is in general easily induced. Of the influence, in this respect, of the nervous element in disease, we have a remarkable example in spasmodic or catarrhal croup, consisting of a combination of laryngeal inflammation and spasm of the muscles of the glottis. in a child with this disease, a much larger dose of tartar emetic is required, to bring on vomiting, than in a simple case of inflammation of the same passages, as in ordinary catarrh. Mineral poisons are generally, in this respect, the reverse of the narcotic Producing irritation of stomach, and not obtunding the sensibility of the nervous centres, they often vomit themselves; or, if not, favour the operation of small doses of emetics. An exception to this rule exists, when their action is so violent as to disorganize the stomach, and thus to destroy at once its susceptibility and absorbing powers.