The saline cathartics are certain salts of sodium, potassium, and magnesium. In the study of salts it has been found that their power of penetrating animal membranes, or, in the intestines, their absorbability, depends on the nature of the ions of which they are composed. Of ready absorbability, the cations (positive ions) are ammonium, potassium, sodium, and lithium; and the anions (negative ions) are chlorides, bromides, iodides, nitrates, and acetates. Among those that are absorbed with difficulty are the cations, calcium, magnesium, cerium, aluminium, and the heavy metals; and the anions, phosphates, sulphates, tartrates, citrates, malates, and lactates. Of all these, magnesium among the basic ions, and citrates, phosphates, sulphates, and tartrates among the acid ions, tend to give cathartic properties to their compounds. To possess this property, the salt must be in solution in the intestines. (Leathes and Starling have found that the pleural endothelium absorbed solutions of magnesium sulphate and sodium sulphate just as quickly as solutions of sodium chloride, but this is not true of the intestinal wall.)

Preparations And Doses

1. Of magnesium - the oxide, a very light powder, and the heavy oxide (oxidum ponderosum), dose, 30 grains (2 gnu); the hydroxide, in the form of magma magnesice (milk of magnesia), dose, 2 drams (6 c.c.); and the carbonate, dose, 45 grains (3 gm.), are very mildly laxative. The laxative powers of these insoluble magnesium salts are presumably due to the formation of the soluble chloride in the stomach, or the soluble bicarbonate in the intestine. In some cases they fail to dissolve, and in such have been known to form intestinal concretions of dimensions large enough to cause obstruction of the bowels. The hydroxide is the favorite for children. The citrate (liquor magnesii citratis), dose, half to one bottle of 12 ounces (360 c.c.), and the sulphate (Epsom salt), dose, \ ounce (15 gm.), very soluble in water, are more vigorous.

2. Of potassium - the citrate, 30 grains (2 gm.); the effervescing citrate, 60 grains (4 gm.); the bitartrate (cream of tartar), 30 grains (2 gm.); and the sulphate, 30 grains (2 gm.).

3. Of sodium - the phosphate, 30 grains (2 gm.); the effervescing phosphate, 2 drams (8 gm.); the sulphate (Glauber's salt), 2 drams (8 gm.); and the citrate, 30 grains (2 gm.). Best finds 2 tumblers of normal saline an effective cathartic.

The potassium and sodium tartrate, KNaC4H4O6, is Rochelle salt, dose, 2 drams (8 gm.). The seidlitz powder is made by enclosing tartaric acid in a white paper, and a mixture of Rochelle salt and sodium bicarbonate in a blue paper. The contents of the papers should be dissolved separately in water, the two solutions mixed, and the liquid drunk as soon as the violent effervescence has ceased. It contains Rochelle salt, 2 drams (8 gm.), and some acid sodium tartrate formed during effervescence. Potassium bitartrate, Khc4h4o6, is soluble with difficulty in water, but it forms Rochelle salt in the duodenum.

The effervescent preparations are usually preferred, as the Co2 present renders them more palatable and less nauseating. They are the solution of citrate of magnesia, the effervescing citrate of potassium, the effervescing phosphate of sodium, and the seidlitz powder. The laxative mineral waters usually contain sodium sulphate or magnesium salts.

Pharmacologic Action

Skin And Mucous Membranes

Applied to the skin, solutions of these salts are practically inert, as they penetrate the horny epithelium with difficulty. Applied to mucous membranes, the concentrated solutions are rather irritant because of the abstraction of water.'

Stomach

Solutions of salts in fairly concentrated form, as they are administered for cathartic effects, have an unpleasant salt taste and are irritant to the stomach, hence they tend to be nauseating. If they he in the stomach, they promote transudation and secretion, and therefore their own dilution. The view of Otto (1905) that solutions of salts are retained in the stomach until they become isotonic with the body fluids has been in the main corroborated, and Hertz (1910) concludes that "even very concentrated solutions become almost isotonic before their evacuation from the stomach." Brown (1912) found that hypertonic solutions were markedly retarded in the stomach, and that isotonic and hypotonic solutions leave less rapidly than the very hypotonic tap-water. He agrees with Leven and Barrett that from an otherwise empty stomach 200 c.c. of water leaves in about twenty minutes. In his experiments he ascertained that the strong laxative mineral waters call forth considerable transudation in the stomach and some secretion of gastric juice, and strongly inhibit the motor functions. They are irritant and are capable of inducing an acute gastritis. In their administration, they should be properly diluted to bring them nearly to an isotonic condition. For example, Hunyadi and Friedrichshall should be followed by an equal amount of water; magnesium sulphate should be given in 7.5 per cent. solution (isotonic); sodium sulphate, in about 2 per cent., and Carlsbad salts in about 3 per cent., solution.

The amount of fluid added by the stomach may be quite large; for instance, by a high duodenal fistula Brown obtained 503 c.c. after 150 c.c. of Hunyadi water, and 250 c.c. after 150 c.c. of 50 per cent. Hunyadi water (almost isotonic). Cobet finds also much fluid added in the small intestine.

Intestines

Some years ago Hober, Wallace, and Cushny administered solutions of various salts to dogs. On analysis of the contents of the intestines they found that the salts which were cathartic were regularly the ones not readily absorbed, and that these acted as cathartics even when in solutions isotonic with the blood. By means of a cecal fistula they also measured the fluid that reached the cecum after the administration of isotonic solutions. After 100 c.c. of sodium chloride solution there was none recovered at the cecum in one hour: it had been absorbed. After 100 c.c. of sodium citrate, 75 c.c. were recovered, and after 100 c.c. of sodium sulphate, from 80 to 90 c.c. were recovered. They concluded that from 75 to 90 per cent of cathartic salts, with the fluid in which they were dissolved, was unabsorbed; and that the catharsis was due to the large bulk of fluid and not to any active stimulation of the intestinal wall. Boas found that, as the solution was more concentrated, it proved less cathartic and more prone to be absorbed and to produce systemic effects. He reports 10 cases of magnesium poisoning from concentrated doses of Epsom salts. Meltzer, Lucus, and Auer have pointed out that when magnesium sulphate is administered intravenously it reduces the irritability of the intestines and checks the peristalsis aroused by physos-tigmine or barium chloride. Magnus has shown that magnesium sulphate has no power of itself to stimulate peristalsis, and Cohn-heim placed it in the duodenum, with no effect on the motility of the bowel. These findings corroborate the belief that the bulk of unabsorbed or added fluid is the laxative agent.