Although it has long been known that many of the vegetable purgatives act as well when introduced into the circulation as when taken by mouth, it has generally been stated that saline purgatives are inactive when injected either subcutaneously or intravenously. It has even been claimed that they have the opposite effect, causing constipation. Claude Bernard,[22] however, states that sodium sulphate causes purgation when introduced into the circulation, although he gives no experiments to support the assertion. Buchheim[23] believed that intravenous injection of purgative salts produced no purgation. Rabuteau[24] stated that the injection of a large amount of sodium sulphate into the blood caused constipation in a dog, an experiment which he took to prove that the strong solution outside the intestine withdrew fluid from the lumen by its endosmotic power. Headland[25] advanced the view that all medicines act after being absorbed into the blood, and that saline cathartics are first taken up into the circulation and stimulate the intestine in being later excreted by the intestinal glands. Carpenter reported an experiment in which he obtained a purgative action by introducing magnesium sulphate into the stomach after this organ had been separated from the intestine by a ligature. It was further found by Vulpian[26] that small doses of magnesium sulphate, but not of sodium sulphate, acted as purgatives when injected subcutaneously. Hay[27] gives an excellent resumé of the literature on this subject. Although he considers it already proven that purgatives do not act when injected subcutaneously or intravenously, he has made a number of experiments to confirm this idea. He was unable to obtain any purgative effect in dogs and cats by the introduction into the blood of 10% Na_{2}SO_{4} or 20% MgSO_{4}. With the subcutaneous injection of these salts, however, a purgative action was sometimes produced. This he attributed to the local irritation of the injection. It will be noted that the conditions of these experiments are by no means ideal. Hay injected directly into the blood a solution of Na_{2}SO_{4}, which is approximately twice as strong as a solution of this salt isosmotic with the blood would be. Similarly a solution of MgSO_{4} isosmotic with the blood would be about 4% (m/6 Sol^_n_ MgSO_{4} + 7 H_{2}O = 4.1 g. in 100 c.c.). Hence the solution which Hay introduced directly into the circulation was five times as concentrated as an isosmotic solution. Although it is not clear just what abnormal conditions would be brought about by the injection of such concentrated solutions, it is certain that the normal action of the salt could not be expected under these circumstances. This is further shown by the fact that Hay did obtain a purgative action in some cases when he injected the salt solutions subcutaneously. Here the injury caused by the concentrated solution was only local, and the salt itself was absorbed in small quantities and in