This is often very important in the treatment of disease. Upon careful examination, however, of its effects, it will be found in general to act upon some one or more of the principles already considered. The following are the different modes in which this kind of influence may be remedially employed.

Position may be made to favour or counteract, through the agency of gravitation, the entrance of blood into a part. Thus, when fainting is threatened from a want of the due pressure of the blood upon the brain, by placing the patient in a horizontal posture, the pressure is favoured, and the apprehended result prevented. Much more frequently, however, the object is to diminish congestion or inflammation in a part, by diminishing the access of blood; and this is accomplished by elevating the part affected above its usual position. Thus, in an inflamed limb, the extremity should be raised, instead of being allowed to retain its ordinary dependent position of health. It is clear that the remedy operates, in the latter of these cases, upon the principle of local depletion, and in the former, upon that of local repletion. Position is occasionally useful in other modes, as in obviating intussusception of the bowels, in favouring the passage of calculi, in relieving painful pressure or tension, etc.

Compression is another useful mechanical process, which may be made to diminish or increase the quantity of blood in a particular part of the body, and thus to accomplish in some degree the same object as the former remedy. Thus, the access of blood to a part may be lessened or cut off by pressure upon the arterial trunks which supply it; or the capillaries themselves may be emptied by direct and equable pressure made upon them. In the latter mode especially, much good is often done in obstinate inflammation and passive congestion. An accumulation of blood may be produced by pressure upon the veins, and not upon the arteries, as when the tourniquet is applied not very tightly. This process may sometimes be useful by abstracting temporarily a quantity of blood from the general circulation, without its ultimate loss. It is a mode of general depletion. Compression upon nervous trunks has been used as an anaesthetic agent in surgical operations. Other advantageous effects of this agency are to promote absorption, and to afford mechanical support to relaxed parts, as in varicose veins of the legs, and to the abdomen after the operation of tapping.

Distension sometimes also operates usefully by stimulating a part to increased action; as when large fluid injections are thrown up the bowels. It may, however, be carried so far as to produce paralysis of the muscular fibres, and thus to prevent all contraction. This is an important therapeutical fact. Distension is used to enlarge passages, strictured or otherwise, as by means of bougies; and substances are also used for this purpose which swell when they become moist, as compressed sponge, s|ippery-elm bark, and gentian root Friction may be considered as a mechanical remedy. It acts partly by compression, partly by stimulation. Employed for the latter effect, it is often a powerful agent in rousing and supporting the system in low disease, and in exciting the part itself when enfeebled; but it is more frequently and usefully employed for its effect in producing revulsion from within outwardly.

The covering of surfaces, so as to protect them against irritating substances, and the contact of the air, is another useful mechanical process. Thus, demulcents protect inflamed mucous surfaces; and collodion, cataplasms, plasters, cerates, and thin layers of gutta-percha and caoutchouc, are applied for the same purpose to the skin. It is not improbable that the effect of nitrate of silver and iodine, in subverting superficial inflammation, may be partly owing to a chemical change in the epidermis, rendering it less pervious to the air. How the exclusion of the air proves useful it is not easy to determine. Perhaps it may be partly by maintaining the moisture which would otherwise be evaporated; perhaps, as suggested by Dr. Latour, by diminishing calorification to which the presence of the air may contribute, if it be not essential {Archives Generates, 4e ser., xxvii. 231); or, possibly by preventing any direct influence which the atmospheric oxygen may have in supporting the inflammatory process.