Magnesia and its carbonate act as direct antacids and local sedatives; also, when given in powder, as mechanical absorbents. A dose of 30 gr. and upward, given to an adult under conditions favorable for absorption, produces, after eight or ten hours, moderate semi-solid motions, of less than normal odor. It has been stated that the effect of these preparations, though less quickly produced, lasts longer than that of stronger cathartics, and is often more copious, 1 dr. of magnesia causing more evacuation than 1 oz. of Glauber's salt (Trousseau) - also, that their effect increases with continuous use, so that bloody and mucous stools may occur after some time. On the other hand, if the drug accumulates in the intestine, mechanical obstruction may be caused by it, and concretions of ammonio-magnesian phosphate, mixed with mucus and debris of food, have sometimes formed when large quantities of magnesia or its carbonate in solid form have been taken; as much as 2 pints (Gubler) and "several pounds" (Brodie) of such concretions have been found in the colon, and Dr. G. de Mussy required a mallet and chisel to remove one from the rectum (Medical Times, ii., 1879).

Peritonitis and even perforation have followed from such obstruction, and I have myself seen one fatal case of the kind. The patient was an actor, accustomed to hurried, irregular meals, and to consuming large quantities of magnesia (for constipation), and on making a post-mortem examination the large intestine was found blocked up by magnesian concretions.

(When a moderate degree only of such a condition is suspected, full doses of vinegar deserve a trial.)

The citrate of magnesia acts as a mild, but efficient and somewhat quicker laxative, and being more soluble than the oxide or carbonate, is free from risk of concretion.

The sulphate, in small doses, acts as a gastric sedative, and if not sufficient to purge, often exerts a diuretic effect, especially if the skin be kept cool: 1 to 2 dr. freely diluted, and taken on an empty stomach, will usually produce several watery stools without colic but with some distension, rumbling, and sense of chilliness. The bitter, unpleasant taste, if uncorrected by carminatives, may induce nausea and debility, but these symptoms subside when the purgative effect commences. The pulse and temperature are lowered by the action, and some malaise may be felt from it; more or less subsequent constipation will also be noticed: the biliary secretion is not increased by it (Rutherford).

Much larger quantities (1 to 2 oz.) are sometimes taken by ignorant or careless persons, and if given to the weakly may cause serious depression, amounting to syncope, with or without severe purging: Christison refers to a fatal issue from a dose of 2 oz. On the other hand, it has been stated that minute quantities (1 1/2 gr.), given by hypodermic injection, will induce characteristic serous motions (Luton: Gazette Hebdom., 1874), but Caville could not verify this result on dogs, and Professor Gubler's trials resulted only in local abscess, so that we cannot depend on Luton's method.

It is, however, ascertained that when even large doses are injected into the veins, no purgation is caused (Moreau, Rabuteau), but rather constipation, and this fact bears upon an explanation of the medicine's action. [Cl. Bernard has stated that the intravenous injection of soda sulphate causes as much purging as its internal administration (Lecons, p. 85), but later observers are very clear as to the contrary.]

Theory Of Action

Poiseuille, and also Liebig, taught that the purgative action of salines when taken by the mouth was due to osmosis of serum from the vessels into the intestine, and in support of this view Moreau found that on including a portion of intestine (of an animal) between two ligatures, and injecting into it a drachm of Epsom salts dissolved in a little water, afterward returning the intestine to the abdominal cavity, a large quantity of fluid was poured into the ligatured portion within twenty-four hours (Archives Gen., 1872). Vulpian corroborated these observations, but noted also intestinal catarrh, which others have not done (Gazette Hebdom., May, 1873). Dr. Lauder Brun-ton has recorded, in an interesting paper, results similar to those of Moreau (Practitioner, vol. xii.). By means of ligatures he made three loops of intestine, and injecting into the middle one a measured quantity of water with a few grains of magnesiae sulphas, and into the others the same amount of water only, found, after a few hours, that the middle one contained treble the quantity of fluid injected, while the others were empty. He experimented also with concentrated solutions passed into the middle loop, always with similar result, and suggests that it is produced by a direct stimulation of the intestinal mucous membrane. Rutherford and Vignal also consider the drug a pure stimulant to the same membrane; this, however, was not found congested in the experiments, and although Brunton thinks that the structure of the intestine renders the osmotic theory unlikely, I believe that it best explains the facts with which we are acquainted.

For instance, constipation occurs if sulphate of magnesia be injected into the veins, and it may be presumed that an osmotic current is then directed from the bowel to the blood, and thus the intestinal contents are deprived of ordinary moisture: also constipation results from taking (by the mouth) small doses of 5 to 10 gr., and these may be supposed to become absorbed quickly into the circulation, and to act as if directly injected into it (as just described): again the purgative effect of full doses (draining off fluid) is followed by constipation. (Rabuteau explains this last fact by supposing a small portion at first absorbed, and producing its physical (endosmotic) effect several hours afterward, but I think it more probably a natural reaction connected with loss of fluid and empty condition of intestine.)

Some eminent writers, chiefly German, have offered other explanations which require a brief notice. Radziejewski observed, in experiments on animals, that the faeces were quite liquid when passing from the small to the large intestine, and argued that saline purgatives simply hurried them through in this liquid state, and that the larger quantity of fluid in stools procured by purgatives could not come from the blood, or even the glands, because on analysis the proportion of albumen in them was found too little for such a source (Reichert's Archiv, 1870, 39, 77). Thiry, experimenting with the peculiar form of intestinal fistula devised by himself, and formed by a separated portion of bowel communicating with the external surface of the abdomen, found that although local irritation would excite secretion in it, saline purgatives, such as soda sulphate, would not do so, and concluded, with Radziejewski, that they simply increased peristalsis. Buchheim taught that besides this, on account of their low diffusion-power, they did not readily pass through the intestinal membrane, but, remaining in the canal, retained the water in which they were given, and also much of the natural watery secretion from liver, pancreas, and glands (a very large quantity, according to Kuhne), and so carried from the intestine a large quantity of fluid without necessarily drawing it from the blood by endosmosis, or from the glands by stimulation. These reasonings, although ingenious, seem to me answered or qualified by the later experiments of Moreau and of Brunton. Thiry's fistula disarranges normal structure too much to furnish a strong basis for hypothesis, while Legros and Onimus have satisfied us that peristalsis per se is but little increased by sulphate of magnesia (Journal d'Anat., Robin, 1869). The purgative action of saline is not, however, a mere physical occurrence, the same in any membrane, living or dead; it implies integrity of nerve-supply, for Moreau found that no endosmosis occurred in an intestinal loop if he divided its connecting nerves.