These are substances of a bland unirritating character, soluble in water, and capable of imparting to that fluid more or less viscidity or adhesiveness. They generally consist of gummy, saccharine, or farinaceous substances, or of two or more of these combined.

Therapeutically they operate in three ways; 1. by covering over and sheathing irritated surfaces, and thus protecting them, in some degree, from the contact of acrid or irritating materials; 2. by retaining the surfaces in a moist state essential to a proper performance of their functions; and 3. by mingling with acrid substances, and obtunding their acrimony.

1. So far as concerns their first method of action, they correspond with the class which I propose to call Protectives; and I shall postpone a full consideration of the principles by which their therapeutic results are produced, till we come to the consideration of that class. Examples of their effects in this way are afforded in the relief of cough, obtained by holding gum arabic in the mouth, and swallowing it as it slowly dissolves in the saliva; and in the alleviation of inflammation of the conjunctiva, by mucilage of sassafras pith introduced into the eye. in the former case the mucous membrane of the fauces, and in the latter the conjunctiva, are protected against the influence of the air, which tends to sustain the inflammatory condition.

2. The second principle of action, so far as the demulcents are concerned, is more efficient. inflammation of an exposed surface is generally attended with more or less diminution of secretion, and with a corresponding degree of dryness. The pliability and softness of the tissues, produced by a due degree of moisture, is essential to the performance of their functions. The movements necessary everywhere for the performance of function, cannot be duly effected in a rigid and hardened condition of the parts. But attempts at these movements are always made, and, without accomplishing their intended object, they excite irritation and sustain inflammation, possibly by friction, possibly by rupture of the minute and rigid fibrils of the tissue. Suppose, with a diseased and stiffened condition of the skin in the ham, or the flexure of the elbow, we should nevertheless move these joints in the performance of their respective offices; we should have, in the cracks, the hemorrhage, the inflammation, which would certainly follow, an illustration, upon a large scale, of what may be supposed to take place in the invisible molecular movements of a tissue, deprived of its due amount of moisture, and consequent mobility. Hence, an inflammation in the exposed mucous membranes and skin has a tendency to sustain itself by the condition of .the tissue it produces; and, if we can artificially correct that condition, we remove one great source of irritation, and favour the influences which tend towards health. Every one knows how much relief is obtained in the dry skin of erysipelas, and the dry mouth and throat of angina, by keeping the parts constantly moistened. Demulcents enable us, in a good measure, to accomplish this object, either through their own essentially moist nature, as in the instance of glycerin, or by the tenacity with which they hold on to the water in which they are dissolved, or which is incorporated with them, as in the mucilage of flaxseed or slippery elm. it is not by any immediate influence of its own, that the gummy substance gives relief. it merely acts the mechanical part of retaining the requisite moisture.

There is another method in which water acts in the relief of inflammation, besides that of maintaining the necessary physical condition of the tissue; it is by the direct sedative influence which it exerts when in excess, and which has been already fully discussed under the general head of sedatives.

3. The third principle of action, with the demulcents, is the influence they have upon the irritating cause. We all know that, by the mixing of an acrid substance with gum or sugar, it loses its sharpness of impression on the palate in much larger degree than could be explained from the mere dilution. its particles, becoming mingled with those of the viscid matter, have less facility of movement than before, and come into contact by fewer points with the impressible membrane. So it is also with other tissues besides that of the organ of taste. The influence of an irritant is almost universally diminished and obtunded by demulcent admixture. Acrid medicines, when swallowed, not only affect the taste less when mixed with demulcent matter, but the stomach also. Besides, when demulcents are swallowed, they mingle with the acrid contents of the stomach, and possibly of the bowels, and moderate the irritant influence these contents exercise on the mucous membrane. injected into the bladder, the vagina, the rectum, the ear, the nostrils, they produce the same effect in obtunding the acrimony of the morbid secretions in these parts, and diminish their injurious reaction on the tissues that produced them, or over which they pass.

The therapeutic deduction from the above considerations is obvious. Demulcents are extremely useful as remedies in inflammation and vascular irritation; and it is for their beneficial effects in these conditions that they are used as medicines. They relieve inflammation or irritation, by protecting the diseased part, in some degree, against the irritant action of the air and other external agents; they enable us to keep the parts moist, and obviate the mischievous effects which result from the opposite condition; and they diminish the irritant action, upon an inflamed part, of substances the contact of which they cannot wholly prevent.

Incidentally, they have another advantage in the treatment of inflammatory diseases; that they afford, namely, precisely the kind of food which the circumstances of the case require, in the highest state of excitement. Gum, sugar, and starch are exactly the articles of diet suitable in the early stages of inflammations and inflammatory fevers.

We may conveniently recognize three types of demulcent matter, and treat of the varieties under these respectively. They may be often mixed in nature; but we shall find that they are sufficiently distinct to enable us to make this arrangement. The demulcents, then, will be considered under the heads of mucilaginous, saccharine, and amylaceous substances.