This name I give to remedies which, applied to a diseased surface, protect it from the contact of the air, and of irritant agents generally. I do not insist upon the name, but know no other word in our language which would better express the idea; and I am averse to the invention of new words when not essential. in relation, however, to the class itself, there seems to be a necessity for it in a system of arrangement, which is to embrace all therapeutic agencies. Other classes may include substances which act on this principle; and the operation of certain classes necessarily implies a certain amount of protective influence, as of the demulcents and emollients; but there are substances used for this purpose which cannot be introduced elsewhere, and, in regard to the two classes mentioned, the protective influence is but partial.

The therapeutic effect of the protective operation is to aid in the cure and prevention of inflammation. This is its characteristic aim and object. That the measure has such an influence, and one of no little efficacy, is shown by daily experience and observation. We constantly witness the efforts of nature to avail herself of this therapeutic principle. She covers small sores with a scab, upon the falling off of which, the surface is found to have healed beneath it. in extensive vesications, if, upon the escape of the liquid, the epidermis lies undisturbed in contact with the surface, the inflammation generally ceases, and a new cuticle soon forms; whereas, if the protection of the cuticle be removed, the denuded surface is very apt to become still more inflamed. it has already been stated that the best dressing for a blister, which it is desired to heal, is the cuticle as nearly unbroken as possible. When such protection is impracticable, nature endeavours to supply the place, either by the production of pus, which answers the purpose partially, or by throwing out a layer of coagulable lymph, which produces a still greater protective effect. We every day see slight inflammation, spontaneous or accidental, yield to the simple application of a piece of court-plaster, which, without such protection, would often advance and prove very annoying. in recent times, we have had abundant opportunities of witnessing the same effect, from the application of collodion and similar agents.

Upon what principle do these agents operate? So far as concerns the protection they afford against mechanical injury, as from friction, sand, dust, etc., or known dynamical irritants, or even changes of temperature, their influence is intelligible. They obviate the effects of these irritant causes, and, in cases of existing inflammation, enable nature to effect a cure. But this is not all they do.

It is obvious that they act advantageously also by excluding the atmospheric air; for, if this is admitted, the exclusion of all other agents often proves quite unavailing. But the air is not in itself irritant. We are always surrounded with it; we are constantly inhaling it even down to the delicate vesicular tissue of the lungs; it is probably never entirely absent from the alimentary canal; it has often found its way into serous cavities and the areolar tissue; and yet inflammation is not observed to result from it, under these circumstances. The air is perfectly bland. Some have explained the result by reference to the drying influence of the atmosphere, which produces a condition of things in denuded, wounded, or ulcerated surfaces, which strongly tends to excite or sustain inflammation. There is, no doubt, truth in this explanation. The protectives certainly do good in this way, in diseased and uncovered surfaces. But it very often happens that such surfaces may be kept abundantly moist, so as entirely to obviate the drying effect of the air, yet without improving; whereas, if completely protected against the contact of the air, they quickly begin to amend, and soon heal entirely. The same effect, moreover, is observed from the protectives in diminishing inflammation upon surfaces with the epidermis or epithelium sound, and when moisture or dryness forms but a small part of the consideration in the case. There must be some other method of accounting for the effect. The following appears to me to be the most satisfactory explanation.

That the air is bland, in its ordinary influence in health, is owing to the existence of a due balance between its dynamic properties and the requisitions of the system. The air is always performing important functions, not only in the lungs, but upon the surface of the body, and probably in the alimentary canal. It is always supplying oxygen and receiving carbonic acid. This is certain in relation to the air-passages; has, I think, been proved in relation to the skin; and is fairly inferrible to be true also in the alimentary canal. In the healthy state, it imparts enough oxygen, and receives enough carbonic acid, to meet the wants of the system; and nothing more happens than the regular maintenance of the systemic functions. But, when inflammation is established in any portion of the great surfaces with which the air is in contact, as it consists in, or is attended with an elevation of the vital functions, more air is required to support the excited process. More oxygen is consumed, more carbonic acid formed, and more heat evolved. A portion of this oxygen the inflamed surface receives from the contiguous air. If, therefore, we exclude this air, we cut off one source of the pabulum necessary vol. ii.-54 for the support of the inflammatory process, which consequently diminishes or ceases. it may be said to die for want of aliment. This is a simple explanation, and it will, I think, apply to all the problems that can be presented in the case. The presence of the air supports the inflammation; remove it, and the inflamed parts must then depend for their supply of oxygen solely on that brought with the blood from the lungs; and, though it does not necessarily follow that the inflammation must cease, when the local supply of air is cut off, especially after fever has set in, by which the movement of the blood is accelerated, yet one influence is certainly added to those which favour the reduction and resolution of the inflammatory state.

Nor is this influence confined to inflammations entirely superficial. it enters, for a considerable depth, into the tissues; and subcutaneous inflammation, as in the cellular tissue, muscles, and lymphatic glands, is often much relieved.

There are two objects in the use of protective measures; one to prevent, the other to cure inflammation. When apart is liable to be injured by friction, or by irritating liquids, we cover it with the view of obviating injury. Thus, bed-sores are prevented by lead-plaster, applied over the parts exposed to injury, and surfaces liable to the action of irritating secretions are guarded by covering them with some stiff unctuous substance. But the therapeutic effect is that for which the class is mainly used.

The protective agents are of two kinds; one operating of themselves, the other through changes produced in the surface of application. The former are purely mechanical, simply serving, by their presence, to exclude irritating agents; the latter at first act dynamically or chemically on the tissues, the surface of which they so alter as to render it insensible to irritants, and impervious to the air; and a portion of the tissue itself is thus made to protect the remainder. The one kind are direct, the other indirect Protectives. Court-plaster and collodion are examples of the former; nitrate of silver and iodine, of the latter. I shall consider them in these two divisions.