Pressure, applied to a part, increases in particular the functions of absorption and deposition. If moderate, these functions are moderately increased, and the structure of the part continues either the same, or it augments in size, as we observe, in the effects of walking, on the skin of the feet; but, if the pressure be greater, then these functions are morbidly increased, the particles are deposited imperfectly formed, and are as quickly taken up. The structure is therefore destroyed, and a vacuity formed. These effects are produced more easily upon diseased than healthy structures; because their powers of acting, and sustaining action, are less. Friction is in this respect similar to pressure. From these remarks, we may understand the mechanical utility of pressure in the cure of ulcers; for, when in a proper degree, it causes the absorption and destruction of the callous edges, or diseased substance, and likewise makes, if moderate, the diseased granulations be taken up, and more healthy ones be formed. We may likewise perceive, that, if the degree be too great, the action will become of an inflammatory nature, and injury will be done. We are therefore frequently under the necessity of counteracting this hurtful effect; for, the degree of pressure which is requisite for answering the first intention in old sores, or inducing action, in consequence of bringing the granulations or particles into the natural state of being in contact with some body, or covered by it[54], is often attended with such mechanical irritation, that the one effect would destroy the other, unless we kept the action within due bounds, by applying cold to the part. In healthy ulcers, the pressure necessary to produce its first set of effects, or to accelerate healing, by producing contact, is so trifling, that no counteracting effect takes place by the production of the second set of effects; and, therefore, no cold requires to be applied: But, in old ulcers, the pressure must be greater; and, therefore, cold water must be constantly applied to the bandage over the sore, by means of a sponge. We thus indeed lessen the effects of pressure on the absorbing system, and, therefore, the callus will be longer of being destroyed; but we, on the other hand, prevent the action of the granulations from being rendered morbid.