Although the source of the stimulus which causes the muscle-fibers to atrophy is not so clear as we should like it, we are acquainted with certain influences to which involution is susceptible. Of these none merits so much attention as the influence of the breasts. The intimate relation between the breasts and the uterus manifests itself in such a variety of ways and with such force that no one doubts its existence. Thus, if a nursing mother becomes pregnant her infant is usually deprived of sufficient nourishment or suffers some digestive disturbance; if not, and the mother, ignorant of her condition, continues with the breast feeding, she may jeopardize the newly begun pregnancy. Very likely she will be warned of the fact by the signs of threatened miscarriage. More frequently, but in quite the same way, we find that nursing causes uterine contractions in the early part of the lying-in period, when they are called after-pains. Women who experience them tell us they are more severe while the infant nurses; and they also say that the discomfort disappears after several days, a fact which indicates that involution has made notable headway. The physician is not dependent on such evidence, however; for a simple examination reveals at any time how far involution has progressed. By this means we have learned that nursing facilitates the involution process. On the other hand, it is found to be true, as we should naturally expect, that women who decline to suckle the infant recover from childbirth somewhat less rapidly than those who follow nature's plan. In this fact, therefore, is found a selfish motive, yet a very good one, which should impel mothers to perform this exceedingly important duty. Aside from the change in the mass of the uterus, notable results of involution relate to its mouth and to its ligaments, for these structures are also chiefly muscle. The mouth of the womb, lately stretched to permit the exit of the child, gapes widely for a time; but ultimately its lips are drawn together, the tissues which compose them stiffen, and the canal which they enclose is narrowed to almost microscopical dimensions. When involution is complete, the uterus has so far regained its virginal character that no trace of childbirth remains other than a few small fissures in the margin of its mouth.