Cathartics are not only of use to remove feces, but owing to the physiological relation of the intestinal tract to the other organs, they are often of great service in a number of different pathological conditions. With a knowledge of their mode of action we can select on a rational basis the proper cathartic for the different indications. Cathartics are indicated: (1) _To remove dropsical effusions._ In disease of the liver, heart, kidneys, etc., there are sometimes accumulations of serum in the serous cavities and subcutaneous cellular tissue. For this purpose those which stimulate secretion and cause a profuse watery discharge are best suited. When the effusion is due to obstruction in the liver so that the serum leaks from the congested postal vessels, they are most prompt in action, because they drain the water from the postal vessels and lower the high portal pressure. Magnesium sulphate, elaterium, and gamboge are efficient here. (2) _To remove urea, etc., from the blood._ Sometimes the kidneys are so diseased that they cannot excrete the waste matter from the body, and its action, when abnormally accumulated, causes serious symptoms. For this purpose use those which cause watery stools, so as to wash the waste out through this channel. If the case is urgent as in uremic convulsions or coma, croton oil is the best to use, because it can be dropped on the tongue and does not have to be swallowed, and, besides, acts in an hour or two. As it is a violent irritant, it should not be continued. (3) _To lower high systemic pressure._ High arterial tension often increases the disease as sometimes occurs in cerebral hemorrhage, in meningitis, and at the beginning of many acute diseases. Those drugs are of use which cause great dilatation of the intestinal vessels, draining the blood from the other organs, or which cause a profuse watery discharge from the blood. In this way croton oil is of use in cerebral hemorrhage, calomel in meningitis, etc. (4) _To depress the liver when it is excessively active._ In that condition known as “biliousness,” where there is congestion of the liver, and enormous quantities of bile are poured into the intestine, chologogue cathartics are needed, because they remove all this bile from the duodenum, and give the over-active liver rest. (5) _To deplete the mucous membrane in gastro-duodenitis._ When persons of a certain predisposition over-eat there is congestion or even inflammation of the gastro-intestinal mucous membrane. This extends up the bile-duct, and the swelling occludes it, so that the outflow of bile is obstructed, and soon it diffuses into the blood, and the person becomes jaundiced. The salines, specially sodium salts, are here of use, because they drain away the water without stimulating the mucous membrane, and hence lessen the hyperemia.