Page:Popular Science Monthly Volume 7.djvu/177

Rh character, whose deviation from physiological and approach to pathological development have been less decided, or extremely slight, it is evident there exists between the two kinds of growths no well-defined line of demarkation; physiology and pathology run gradually into each other. It is not possible to say where one ends and the other begins. That this must be the true state of the case is unmistakable when we consider that the change of external conditions from natural to unnatural may be in any—the slightest or the most extreme—degree. Thus we frequently observe modifications of structure induced by exceptional conditions that have been brought to act upon the body, of so trivial a character that they can hardly be called diseases, while at the same time they are in some measure anomalous deviations from the typical standard of the species; but in these, as in the higher grades of structural modification, it will be seen that the conservative purpose of adaptation is carried out. For example, the muscles in the right arm of the blacksmith, those of the leg-calf in the limbs of the dancer, and the crural adductors of the jockey, undergo a process of increased growth (a physiological hypertrophy) by which they become adapted to the increase of function imposed on them. So the thickened epidermis of a laborer's palm adapts the hand—by protecting the softer tissues underneath from being bruised—to the rough handling of manual instruments; an adaptation altogether wanting in one unaccustomed to labor, as evidenced by the inflamed and blistered condition of his hands when first beginning to practise such exercises. By the same kind of thickening and induration the finger-ends of the violinist become adapted to sustain without inconvenience prolonged pressure upon the strings of his instrument. When the main artery of a limb has been obstructed, or tied by the surgeon's ligature, we find the nutrition of the tissues beyond is supported by the blood finding its way through the smaller anastomosing arteries, and in time we observe these smaller arterial branches to become considerably enlarged, thus adapting themselves to the increased amount of blood they have been called upon to transmit. In cases where obstruction to the arterial circulation is more general, so that it requires an increased heart-force to pump the blood through its channels, we observe the heart itself to become larger, and thus its increase of structure (like the blacksmith's arm) is adapted to the required increase of function. The head of the thigh-bone, when irreducibly dislocated, becomes surrounded in its new position with fibro-ligamentous and muscular structures, which so far resemble an articulation as to permit the patient to walk about. Similarly in ununited fractures, we find the ends of the broken bone, when the muscles attached to them cause