On the other hand, a theory propounded by Aubert (1852), that the salts had to be absorbed in order to act on the intestine, received some corroboration by the work of J. B. MacCallum (1904). He found that laxative salines (sodium citrate and sulphate) administered intravenously where cathartic. This has not proved, however, to be regularly the case, and investigators have considered the theory untenable. However, Hertz (1910), after numerous studies with the x-rays, has revived the theory. He discovered that in two patients with fistula at the end of the ileum the soluble purgative salt traveled no faster than the heavy bismuth salt given with it, so he assumed that it was fair to judge by x-ray pictures and by auscultation of the cecal sounds. The x-rays showed that though a watery stool was passed one and a half hours after the saline was taken, the bismuth given with the saline did not reach the cecum for four hours. He showed further that in the watery stools from sodium sulphate there was no increase in the sulphates; that half the salt was excreted in the urine in eight hours; and that the greater part of the salt of the feces appeared the next day after the liquid stools had ceased. He concluded that the salt must have been absorbed, that it acted through the blood as a stimulant both to secretion and to peristalsis of the colon, and that it acted independently of its own appearance in the colon.

Sodium sulphate administered intravenously may be slightly laxative, but magnesium sulphate administered by mouth but prevented from reaching the colon, or administered hypoder-matically or intravenously is not cathartic; indeed, Auer says that an intravenous or hypodermatic dose definitely checks peristalsis. MacCallum attributed the failure of the intravenous dose to too rapid excretion by the kidneys, and believed that only through the intestines could asufficient concentration of the salt be absorbed for cathartic effect. He suggested that these salts are purgative by precipitating the calcium salts in the tissues and so neutralizing their depressing action. And, as a matter of fact, the cathartic compounds are, for the most part, the ones that precipitate calcium (citrates, tartrates, sulphates, and magnesium), and calcium tends to inhibit their cathartic action.

Meltzer's summing up of the intestinal action of magnesium sulphate is as follows: The salt is partly changed in the intestine to sodium sulphate and magnesium carbonate, so that these two salts with some unchanged magnesium sulphate are present. Since peristalsis consists of a coordinated excitation (contraction) above and inhibition (relaxation) below, it is promoted by an increase of the irritability of the excitation phase by the sodium sulphate, and an increase of the irritability of the inhibition phase by the unconverted magnesium sulphate, while at the same time the magnesium carbonate attracts fluid and probably stimulates peristalsis.

It is usual that in one or two hours the dose results in one or more watery stools, which consist of - (1) the salt and the water in which it is dissolved; (2) some of the gastro-intestinal contents of which absorption is prevented by the salt; (3) some of the feces already formed in the colon; and (4) liquid added by transudation and secretion. Bayliss and Starling, Magnus, Cannon, and others have shown that the passage of liquids along the intestine is different from that of solid or pasty matter. Solids stimulate peristalsis, whereas liquids simply generate rhythmic intestinal segmentations (Cohnheim). The result of this is that, while the liquid passes along, more or less of the solid contents of the intestine are likely to be left behind. Hence a saline cathartic may not be so thoroughly cleansing as the ordinary more slowly acting stimulants of peristalsis.

In connection with saline cathartics, Moreau's loop has become a classic experiment. It is a loop of intestine tied off without injury to the vessels and nerves of the mesentery. Into such loops different salt solutions are injected, and they show that - (1) An isotonic solution remains almost unchanged at the end of three hours; (2) a hypotonic solution loses in volume, that is, is absorbed, and (3) a hypertonic liquid gains in volume. It is of interest that in the latter case there is no protein or other evidence of inflammation. The gain in volume is due either to secretion or to osmosis. However, as the loops prevent peristalsis and segmentation, the results of such experiments are not at all conclusive as to the action of saline cathartics.

Of saline cathartics as a class it may be said that -

1. They irritate the stomach and are prone to produce nausea, an effect which may be largely overcome by administering them as effervescent drinks.

2. Their stools contain much liquid, but no inflammatory products.

3. They are often not thoroughly cleansing.

4. They act most rapidly and best if taken fasting, as before breakfast, and with a large volume of water. Their action comes on in an hour or two.

5. Their catharsis is the effect of the increased bulk and fluidity of the colon contents, and this is chiefly due to the prevention of absorption.

6. They do not induce irritant griping; but accompanying their rapid passage through the intestines there may be some griping, much gurgling of the intestines, and more or less faint-ness and nausea.

7. If they are not evacuated, they produce no inflammation and are absorbed.

8. When absorbed, they pass out by the kidneys and act as diuretics.

9. In moderately hypertonic solutions they tend to remove fluid from the body. This may not, however, be the case if the dose is repeated daily, and especially if the patient is on a "dry" diet, as in dropsy. In such cases the salt may be absorbed and only add to the work of the kidneys.

10. Violent purging results in nausea, lowered blood-pressure, and prostration.

11. Small doses taken at night tend to promote and soften the morning stool.

Therapeutics Of Salines

They may be employed:

1. In acute constipation or food-poisoning as a rapidly acting non-irritant cathartic. 2. In habitual constipation for a short period only. 3. In intestinal putrefaction. 4. After a dose of calomel. 5. As an occasional purge.

Their use in dropsy and obesity and to lessen the secretion of milk in nursing mothers is dependent upon the power of salines to decrease the fluid in the body. For this purpose they are administered daily, a diet low in liquids being prescribed. But they usually very soon cease to carry out excess of liquid, and when profuse watery catharsis does not result, should be stopped. They probably have no influence on obesity; at any rate, of themselves alone they are unable to cause the body to lose fat.

Moderate doses make the stools soft and non-irritant, so have been advised in hemorrhoids, fissure of the anus, etc.; large doses cause such sudden expulsion as to be harmful in these conditions.

Objections to the habitual use of salines in chronic constipation are - (1) That they accustom the intestines to a greater bulk of contents than usual so that the intestines lose their sensitiveness to the usual bulk of intestinal contents; and (2) that they activate the intestine for one or two hours only, and allow it to remain "fallow" for the rest of the twenty-four hours.

Poisoning By Magnesium Sulphate

Magnesium sulphate in very concentrated solution does not induce peristalsis, is absorbed, and is poisonous. The toxic symptoms are: marked depression of respiration and a curare-like action on the junctions of motor nerves with striated muscle (Meltzer and his associates and Barbier). The salt is eliminated in the urine and gives this a very high specific gravity, even 1070 or 1080, which of itself is suggestive of magnesium sulphate poisoning. The antidotes are calcium or physostigmine (Meltzer and Joseph). (See Magnesium Sulphate, under Anesthetics, page 313.)