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The latest histologic experiments of Unna have proved that the digestive power of pepsin and hydrochloric acid combined penetrates the horny layer of the epidermis and that this combination may even be used for carrying other chemical agents through the horny layer. A large number of chemicals can thus be caused to be absorbed by the surface of the skin; this has hitherto been impossible to accomplish, as, for instance, in the case of epinephrin. Naturally, in the first place it is the pepsin and hydrochloric acid itself which penetrate the intact horny layer. When this combination has penetrated to a certain depth it will affect certain diseases of the cutis and subcutis, as I shall try to show.
Our experiments were first tried on keloids and scarring after burns, cicatrices, with the intention of testing the possibility of digesting the fibrous (collagenous) tissue, of which these scars chiefly consist.