Most of the salts of the alkalies, and the soluble salts of the alkaline earths act as cathartics, when given in quantities sufficiently large. Some of them, however, have other properties which forbid their use in large doses; others are not sufficiently certain and regular in their operation for practical purposes; and comparatively few are actually employed as cathartics. I shall treat only of the sulphates of magnesia, soda, and potassa, the phosphate of soda, the bitartrate and tartrate of potassa, the tartrate of potassa and soda, and the citrate of magnesia.

Effects on the System

When one of the salts is administered in small doses, insufficient to act as a cathartic, and frequently repeated, it often appears to operate as an arterial sedative or refrigerant on the system at large, somewhat reducing the circulation and general temperature, and very generally increasing one or more of the secretions, especially the urine or perspiration. Under these circumstances, the salt is absorbed, acts through the blood directly upon the circulatory organs, and, being thrown off by the kidneys or skin, stimulates their excretory function.

In relation to the salts of the vegetable acids which pursue this course, it is believed that, in the process of absorption, or anterior to it, their acid is separated and undergoes digestion, and the base enters the circulation, and escapes by the emunctories, either in the form of a carbonate or a chloride, or in some other state of saline combination. The salts with mineral acids are absorbed unaltered, or undergo change through reaction with other salts or acids they may happen to meet with, according to the predominance of affinities.

More largely given, the salts act in general very promptly as cathartics, producing liquid evacuations even in small doses, and, in the full dose, operating usually more than once, with copious watery discharges, and little pain or discomfort to the patient. They produce the cathartic effect by increasing the serous exhalation from the mucous membrane; and, being mainly carried off with the contents of the bowels, are less liable to absorption, and consequently less disposed to produce a direct refrigerant effect, or to act upon other emunctories. Their chief influence, therefore, is depletory; but it can scarcely be doubted that they are partially absorbed, and in some degree operate as direct arterial sedatives, and secretory excitants. It is not merely water that they separate from the blood, but soluble animalized matters also, on which the circulating fluid depends in part for its nutritive properties. Through this combined influence, they have a powerful effect in reducing the quantity and character of the blood, and thus diminishing the force of the circulation, the heat of the body, and the general strength. Though thus energetic in modifying the condition of the system, they are usually mild in their manner of operating, and, if properly administered, are little apt to produce injurious irritation of the mucous membrane.

A result I have often noticed, after the free operation of one of the saline cathartics, is a diminution of the secretion of bile, as indicated by a deficiency or want of its colouring matter in the passages. This is obviously owing to the depletion from the portal system through its radicles; so that the blood goes to the liver diminished in quantity and lowered in quality, and for a time inadequate to the due stimulation of that organ.

The saline cathartics lose their power, upon repetition, less than any other medicines of this class. I have known small doses of sulphate of magnesia to be taken every few days for months, and somewhat irregularly even for many years, and yet to operate as readily in the end as at the beginning.

Mode of Operating

The most reasonable explanation of the purgative operation of saline substances appears to be, that, by a direct stimulant influence on the capillaries of the gastro-intestinal mucous membrane, they invite into them an additional quantity of blood, which excites them to increased exhalation. Some are disposed to explain the phenomena on purely physical principles. A strong saline solution outside of the blood-vessels causes, they say, an endosmotic current from the thinner serum of the blood, into the more concentrated liquid of the bowels. On the contrary, a very weak solution, being less dense than the serum, enters the blood through the same law. Hence a large dose of one of the salts purges, while a small one is absorbed, and acts on the system and the emunctories. But, unfortunately, the theory does not conform altogether with well-known facts. A saline solution is not most apt to purge when most concentrated; and very weak solutions, taken with a Large quantity of liquid, do often purge with considerable activity; as. for example, many of the cathartic mineral waters, which contain a relatively small proportion of saline matter.

The fact seems simply to be, that a very strong solution irritates so highly, as, upon principles already sufficiently explained, to impair the functions of the membrane, and, among others, that of secretion, and may even induce serious inflammation; a less concentrated solution, though still rather strong, simply irritates to the point of increasing secretion, and consequently purges; while a very weak one, if in moderate quantity, scarcely irritates at all, and is consequently absorbed. if the weak solution be taken in very large amount, as when glass after glass of a natural mineral water is drank, the distension by quantity, added to the slight irritant influence of the salt, is sufficient to stimulate the peristaltic movement. Some interesting experiments upon these points have been made by Dr. Joseph Jones, which tend, on the whole, to confirm the views here taken. (See Am. Journ. of Med. Sci., N. S., xxxi. 61.)

Therapeutic Application

The saline cathartics are especially adapted, by their peculiar properties, to cases of disease, in which, in connection with a purgative action on the bowels, there are indications for depletion from the blood-vessels, and a refrigerant effect on the system.

1. Hence, they are well adapted to acute inflammation, with or without fever; and, to a certain extent, are upon the same principles applicable to the chronic forms. They are, indeed, almost universally used in this class of affections, when a purgative is wanted. Very often, especially in the milder cases, they are alone sufficient for the treatment. I have frequently known a commencing inflammation, as of the fauces, for example, to be entirely set aside by a full dose of Epsom salt, with a low diet. But, in severer cases, it is customary to commence the treatment with some other more energetic cathartic, capable of a more powerful revulsive influence, as calomel and jalap, calomel and rhubarb, compound cathartic pills, etc.; and afterwards to trust the case to the saline remedies of the class, sometimes, when a more than usually effective impression may be required, combined with senna and manna, as in the black draught.

2. Fevers of a sthenic character are also very advantageously treated with saline cathartics, often, indeed, to the exclusion of all other medicines belonging to the class; though peculiar circumstances sometimes call for special cathartics, as when calomel is wanted to correct hepatic torpor and portal congestion; castor oil, in cases attended with irritation or inflammation of the intestinal mucous membrane; magnesia, to correct acid in the stomach or bowels; and one or more of the drastics, to create a powerful revulsion from the head towards the alimentary canal.

3. in cases of large feculent accumulation, producing obstruction of the bowels, the more energetic saline cathartics often operate happily, through the abundance of the watery secretion they occasion, which tends to soften and break down the consolidated mass, by insinuating itself between the mass and the sides of the bowel, or even into its very substance.

4. When a prompt and thorough evacuation of the bowels may be required, and castor oil is rejected, or not sufficiently active, the saline cathartics are often resorted to with advantage; being usually combined with magnesia or its carbonate, when excess of acid is at the same time present.

5. in the treatment of dropsy, whenever cathartics are indicated, the saline are among the most efficient, through their hydragogue operation, and the consequent promotion of absorption. it is to the febrile form of dropsy that they are peculiarly adapted.

6. They may be used generally when it is desired to evacuate the bowels, without reference to special indications; care, however, being taken that no existing contraindication be neglected.

7. Finally, they are often called for, to hasten the operation of other and slower cathartics, given with some special view.

The chief contraindication is a state of debility, in which no blood can be spared, and evacuation of the bowels, should it be necessary or desirable, must be effected by cathartics operating exclusively on the motor function. The salts are also contraindicated in local debility of the stomach and bowels, which they tend to aggravate, and in torpor of the liver, unless they are given in connection with a mercurial alterative.