easily answered one, whether other tissues diminish in consequence of a failure to exercise them in their office.
A physiological proof that the smooth muscles are strengthened by exercise is wanting. The adaptation of the eye to near vision diminishes from childhood to age according to a regular law, notwithstanding the constant exercise of the faculty; but it does not follow from this that Brücke's muscle does not gain strength, for its gain may be more than compensated by the growing stiffness of the tissue and the diminished elasticity of the crystalline lens. The fact that men see imperfectly at close range what their occupation gives them little occasion to regard, indicates that Brücke's muscle loses strength when it is not used. The uterus has no occasion to be exercised, for it is active only after long pauses, and gains a portion of new fibers every time for that purpose. We know nothing of the movements of the muscle-maw of the bird, which forms a transition to the cross-striped muscles. On the other hand, such pathological facts as the hypertrophy of the muscles of the bladder and the pylorus under circumstances of extraordinary resistance leave no doubt that the smooth muscles, like the cross-striped ones, are strengthened by labor. Thus an empirical basis is given to Herr Rosenthal's supposition that the immunity against cold conferred by cold-bathing depends upon the exercise of the smooth muscles of the skin and their vessels, which are intrusted with the lowering of the co-efficient of cooling of the body in the cold. Cold washing and bathing are the gymnastics of the smooth muscles.
The young blacksmith, of whom we spoke a short time ago, had gained another advantage from exercise besides greater strength in the muscles of his arm: he ceased to burn his fingers. Every one knows that the epidermis thickens on those parts of the skin that are frequently subjected to pressure, rubbing, and the touching of hot things and caustic fluids. Handling of tools, rowing, vaulting on the rack and bars, produce a callus chiefly at the ends of the middle-hand bones or in the palms; glass-blowing produces callus on the inside of the fingers. Recurrent blisters often result in callus. Under the pressure of hard shoes the form of skin-thickening known as corns takes the place of callus. Callus and corns have been histologically investigated, yet we can not tell why the useful callus is formed here, the painful corn there, to say nothing of our having a theory of the processes. They fall in the category of what Herr Virchow calls formative stimulation of the cell-complex, and regards, like the nutritive stimulation, as the result of a general and fundamental property of the elementary organisms. An increased supply of matter, immediately conditioned on an increased flow of blood, also takes place here. Since we can not well predicate a vis a fronte, enlargement of the vessels remains the only yet possible step toward an understanding, and with this we reach a closed gate before which many other problems are
