In describing the effects and the use of cathartics, instead of explaining them in the way of some therapeutical authors, a capite ad calcem, we shall first speak of their more immediate effects; and, from them, trace the more remote and distant. We shall afterwards enumerate those diseases in which they arc chiefly indicated, and in which they are most essentially useful.

The first and most obvious effect of purgatives is the discharge of the contents of the abdomen. The intestines are subject to various accumulations of hardened feces, undigested matter, or inspissated mucus. These substances impede not only the passage of the fresh nutriment, but prevent the absorption of the newly-formed chyle, and occasion general languor and debility. Laxatives, as we have said, also stimulate the orifices of the excretory ducts, and promote the circulation through the vessels of the glands. This is a very important effect, and renders these medicines useful in a variety of complaints.

By the evacuation of the contents of the intestines, it may be supposed that cathartics would relieve the stomach, and promote, in every instance, the discharge of its contents: but this effect is not constantly produced; and where a viscid mucus has accumulated, its weight carries it to the bottom of the larger curvature of that organ, and it is rarely discharged without the assistance of an emetic. The complaints of the stomach, supposed to be relieved by cathartics, are more often accumulations in the duodenum.

When the intestines are loaded, and particularly the small intestines, some pressure is made on the descending aorta, and the blood is determined in larger proportions to the ascending branches, occasioning pain in the head, languor, and general uneasiness. These symptoms cathartics commonly relieve; and, by taking off the pressure from the descending aorta, facilitating the circulation through the glands of the chylo-poietic viscera, and thus increasing the circulation in its branches, cathartics become the most useful remedies in diseases of the head.

The efficacy of cathartics as evacuants of particular fluids, which seem chiefly to depend on the solubility of the medicine, merits our attention. The saline and other lenitives act immediately on passing into the intestinal canal, and stimulate not only the excretories of the liver and pancreas, but of the mucous glands in the neighbourhood; and it is this class of cathartics on which we chiefly depend for the evacuation of bile. By Some accident, rhubarb has been considered as the chief evacuant of this secreted fluid; but we have never found it peculiarly adapted for this indication, except when combined with soap. Indeed the stimulus of rhubarb on the internal surface of the canal is so considerable, that even when it has passed the entrance of the ductus communis, the irritation may be communicated to it. Irritation is always readily conveyed along the surface of mucous membranes; for we find a stone in the pelvis of the kidney sometimes convey a peculiar sensation at the extremity of the glans penis.

The active cathartics alone stimulate sufficiently the muscular fibres of the colon; so as to evacuate the contents of its cells: and for this purpose calomel is the most effectual. Jalap, a medicine of considerable utility, seems also to combine a power of assisting the watery secretions; and thus becomes a serviceable addition to the lenitives, when employed with this view. Rhubarb seems also particularly useful in evacuating the contents of the larger intestines. On the rectum, sulphur and aloes seem to act exclusively; and the one is supposed to relieve, as the other is to induce, the haemorrhoids. The evacuation of the contents of the rectum is, however, in general, more properly trusted to clysters. The importance of this discharge is considerable in relieving any inflammatory diseases of the uterus or vesica urinaria; and, as injections reach so nearly the affected part, opiates are thus advantageously administered. But this is from our present subject.

The diseases in which cathartics are advantageously employed are numerous. We have detailed the advantages arising from the evacuation of the contents of the bowels; and it will be obvious, that, in every case of dyspepsia, of bilious accumulations, and worms, they must be of essential utility. For the discharge of worms, and the mucus which forms their nidus, the most stimulating cathartics are requisite. For dyspepsia the eccoprotics; and for bilious accumulations the laxatives, and of these rhubarb and calomel, are the most useful.

The infarctions of the different viscera require the constant use of cathartics; since, from the steady and frequently repeated stimulus to their excretories, we promote a more active circulation through their substance. Dissection discovers various indurations in different organs, whose excretories open into the abdomen; but we can distinctly ascertain the existence of one only during life, viz. infarctions of the liver. The small doses of calomel, which give a general activity to the circulation, are greatly assisted by the regularly repeated stimulus of gentle laxatives; and, indeed, without these, would be of little service. When no symptom leads to the suspicion of disease in any one organ, a general cachectic habit renders laxatives almost indispensable: indeed, in every instance of this kind they are highly useful. When the mesenteric glands are affected, we can scarcely style the lacteals their excretory ducts; yet the stimulus of cathartics on their orifices is highly useful. We shall find reason to conclude, that all the infarctions of conglobate glands are owing to a want of irritability in these vessels; and the disease is not only relieved by the stimulus of the cathartic, but probably by the absorption of those found to be most useful, viz. sea water, and the purging mineral waters. The latter are frequently the most effectual also in infarctions of the liver; and, when joined with a chalybeate, which, as we have said, is supposed to prevent the debilitating effects of purging, have been greatly celebrated. Cheltenham water is the most fashionable of this kind.