These are medicines calculated to remove worms from the alimentary canal. The term might be so extended as to embrace also measures fitted for the destruction of those parasitic entozoa which reside in the solid structures of the body; as the Strongylus in the kidney, the different species of Filaria in the areolar tissue, the eye, etc., Distoma in the liver, and the different Hydatids almost everywhere; but, so far as means have been discovered of treating the effects of these parasites, they arc almost exclusively surgical; and no medicine has yet been found capable of destroying them by operating through the system. it is not impossible, however, as the parasites are not the same in theft susceptibilities as the human body, that substances may some time be discovered capable of reaching these animals through the circulation, and acting poisonously upon them, without injuriously affecting the system itself; and the probability that, in sulphurous acid and the sulphites, we possess means of destroying parasitic life in even a more dangerous form, should encourage us to be on the watch for detecting agents that may operate on the visible parasites. indeed, it is a point, worthy of consideration, whether these very substances, employed freely and long enough, may not exercise the influence which is so desirable. Anthelmintics effect the discharge of worms in several ways.

1. By the increased peristaltic movement which they sometimes excite, they forcibly expel the worms. But when these parasites are of ordinary health and vigour, they have the power to maintain their hold in the bowels against any force which can be brought against them, through the contractility of the bowels themselves. Still, active purgatives often do bring away worms; and we may infer that, under these circumstances, the parasites are off their guard, or debilitated.

2. Another and frequent method in which they probably act is by killing the worm. This they sometimes do mechanically, as by the sharp spicula of cowhage, which have been shown to have the power of killing them out of the body. Another method is no doubt by poisoning the worms, the susceptibilities of which are happily not the same as those of the body they inhabit; for some substances are very fatal to these animals, which have little or no effect on man. in the dead state, the worms, if in the stomach, are digested; if in the bowels, pass away with the feculent matter, or are subsequently expelled by purgation.

3. When they do not destroy, they probably often sicken the worms, and, thus disqualifying them from maintaining their place, enable purgatives, simultaneously or subsequently given, to expel them.

4. Sometimes it is probable that the sensibilities of these animals are offended by substances swallowed, and they allow themselves to be carried off by the movement of the bowels, as if to avoid the offensive vicinity, or even make voluntary efforts to escape.

5. Some medicines combine a true anthelmintic virtue with a purgative property, at the same time poisoning or injuring the worm, and causing its expulsion; as large doses of the oil of turpentine, the bark of pomegranate root, and koosso.

6. Another anthelmintic measure is to put the stomach and bowels into a condition unfavourable to the development and support of the worms, and thus ultimately destroy them, or at least co-operate with other methods for their extermination. Though it is probable that some worms are capable of living and nourishing in a healthy state of the alimentary canal, and will be developed whenever the germs find an entrance, as the tapeworms for example; yet there are others, which seem to require for their support a morbid state of the digestive function, and perish in the germ when this condition does not exist. We have no other means of accounting for the almost total absence of these parasites in certain conditions of the digestive organs, and their frequent presence in others, when the exposure to their cause, so far as can be ascertained, is the same in both. Thus, the roundworm is so frequently found in all parts of the world, that their germs must be widely diffused; and probably there may be no individual, living beyond a certain age, into whose system they have not found access. Yet in some they undergo rapid and vigorous development, in others die, or at least remain quiescent; and, in the same individual, it often happens that at one time circumstances shall be favourable to them, and at others unfavourable. Children are much more apt to be affected than adults; and the difference can be explained only upon the supposition, that there is a difference in the condition of the stomach and bowels, in the former congenial to the worms, in the latter adverse to them. What it is that constitutes the difference is not certainly known. Some ascribe it to a more abundant production of mucus in the one case than the other, serving as food for the animals; but in most cases any evidence of this superabundance is quite wanting, and, when it exists, the condition is probably the result of an irritation produced by the worms, and not the cause of their development. The probability is that debility of the digestive process, permitting the accumulation of half-digested crudities in the alimentary canal, is the main favouring influence; and consequently that the best anthelmintic measure is to invigorate the digestion, and thus prevent the accumulation of this nutriment for the parasites. Wholesome and digestible food, exercise, a regular condition of the bowels, and the judicious use of tonic measures, whether medicinal or otherwise, are probably efficient anthelmintics under such circumstances, both aiding in the extirpation of the worms when existing, and preventing their birth and growth when only in the egg. The bitters and chalybeates have probably acted as anthelmintics in this way.

It does not belong to therapeutics to treat of the origin, characters, and varieties of worms. These subjects belong rather to pathology, and have been treated of in my work on the Practice of Medicine. I shall now, therefore, proceed to the consideration of the individual anthelmintics; taking it for granted that the reader has made himself acquainted with the several species of these parasites, known to infest the human bowels.

There are two methods of exhibiting anthelmintics; one, to give them in full doses, either in connection with purgative medicines, or followed by a cathartic in a few hours, should the anthelmintic itself not operate on the bowels; the other, to administer relatively small doses, morning and evening, for two, three, or four days, or longer, and then a brisk cathartic. I do not know that either of these methods has an absolute superiority; and one or the other may be adopted, as may seem most appropriate in any particular case. Not unfrequently the two plans are partially conjoined; that is, the anthelmintic and purgative are given together, in moderate doses, repeated once, twice, or thrice daily, so as to exercise a steady influence on the worms, and at the same time always be ready to expel them when they become sickened or debilitated.