Cathartics are medicines which tend to increase the secretion from the inner surface of the bowels, and promote the natural expulsion of the contents of the intestinal tube. Of these two operations the first only is an action of elimination, and the second is an accompaniment to it. The first can hardly take place without being followed by the second; but in some few cases the second action alone may be produced. The subject of the application of Purgative medicines is so extensive, that it is impossible for us now to inquire into it at any great length. It should however be observed, that they are the most powerful and the most useful of all Eliminative medicines. The fæces consist partly of the undigested matters of the food, and partly of a secretion which is poured out by the inner surface of the bowel. (_Vide infra._) The majority of Cathartics increase this secretion. Whatever notion we may adopt as to its physiological purpose, it appears that we can act upon this intestinal function with ease and certainty in the great majority of cases. The surface of the intestine, covered as it is with a closely packed glandular apparatus, forms in the aggregate the largest secreting organ in the body. From the measurements made by Meckel, it appears that it covers a space of 1400 square inches. By the administration of a medicine of this sort, we are enabled to act upon this surface, producing simply an increase of the fæcal secretion, or causing, when the action is violent, an outpouring even of the fluid part of the blood. When this secretion is stopped, we may cause it to reappear; when another secretion is repressed, we may be enabled to replace it by this; and in the treatment of plethoric or inflammatory disorders, we find among Cathartics the simplest and readiest of antiphlogistic or evacuant medicines. For they possess these great advantages,--that they act with certainty, and produce a notable effect.