Ultimately, the amniotic fluid assists in dilating the mouth of the womb, which remains closed until the beginning of the process that terminates with birth. The uterine contractions at the onset of labor compress the fluid; in turn the fluid attempts to escape but is held in check by the amniotic membrane, which it drives into the canal leading from the uterine cavity to the vagina. Acting like a wedge, the fluid gradually pushes the mouth of the womb wider and wider open, until it is large enough for the child to pass. The sac usually ruptures when that point is reached, the fluid escapes, and in due time the child is born. This is followed within half an hour by the extrusion of a mass of tissue--in reality the collapsed fetal sac-- which in every language, so far as I know, is named the After-Birth. An examination of this tissue at the time of delivery repays the physician, for it is important to ascertain that none of it has been left in the uterus. Our interest at present, however, is to learn how the after-birth has assisted toward the growth of the child. THE PLACENTA.--The after-birth has puzzled scientists as well as the laity, and not until comparatively recent times have its origin, structure, and use been satisfactorily explained. Its meaning profoundly interested primitive men and stimulated their imagination scarcely less than the mystery of conception. Some uncivilized tribes believed that the after-birth was animated like the child; consequently they spoke of it as "the other half," and often saved it to give to the child in case of sickness. But generally the after- birth was buried with religious ceremony, and was occasionally unearthed later to discover whether the woman would have other children; the prophecy was made according to the manner of disintegration or some other equally absurd circumstance. The after-birth consists of a round, fleshy cake, the placenta, to which two very essential structures are attached. One of these, running from one surface of the cake, is a rope-like appendage, the umbilical cord, which links the placenta with the fetus. The other, attached to the circular edge of the cake, is a thin veil of tissue, in some part of which a rent will be found. Now, if we lift the margin of the rent, we shall see that the veil and the cake together form a sac which we are holding by the opening. This aperture through which the fetus passed, and it was really made for that purpose, was formerly placed over the mouth of the womb; the sac itself, distended by the fetus and the amniotic fluid, was fastened everywhere to the inner surface of the womb.