This, when gestation goes on regularly, is accomplished gradually and slowly by the extension and formation of fibres. In abortion, no fibres are formed; but muscular action does all, except in those instances where the action of gestation goes on irregularly and too fast; in which case the cervix distends, sometimes by the third month, by the same process which distends the fundus. But much more frequently the cervix only relaxes during abortion, as the os uteri does in natural labour, and yields to the muscular action of the fundus, or distended part. The existence and growth of the fœtus depends on the fœtal portion of the ovum. The means of nourishment, and the accommodation of the fœtus in respect of lodgement, depend on the uterus; and these circumstances requiring both fœtal and maternal action, are intimately connected. The condition of the uterus qualifying it to enlarge, to continue the existence and operation of the maternal portion of the placenta or ovum, and to transmit blood to the ovum, exactly in the degree correspondent to its wants, constitutes the action of gestation. When this condition ceases, then muscular contraction begins, provided the cessation be universal in the uterus. This is necessary, for as the fœtal and maternal actions are dependent on each other, the fœtus would suffer if it were not expelled. The injury, indeed, will not be immediate; otherwise, in labour, the child would die before it could be born, because labour implies a cessation of the action of gestation. On the other hand, the loss of action in the fœtal part will soon influence the maternal part, and stop its action. In labour, and at other times, when the action of gestation ceases, the circulation is still kept up in the maternal vessels of the placenta, until either separation and expulsion take place, or the vessels suffer so much as to cease to transmit blood. The cessation of action then does not necessarily immediately affect the fœtus. As long as it, and the fœtal portion of the ovum connected with it, remain stationary, the same quantity of blood will do. But the uterus cannot now increase its actions along with those of the fœtus, so as gradually to enlarge and send more blood. This is one cause of disagreement. Another is, that, in consequence of cessation of action in the uterus, the maternal portion of the placenta or ovum ultimately suffers, and flags or decays, whilst the fœtal portion must sympathize with it. From this it results, first, that even in tedious labour the child does not die: secondly, that when the action ceases in the early months of pregnancy, the fœtus does not instantly die, nor abortion immediately take place: thirdly, but it invariably happens, that, at whatever period the action ceases, the fœtus will, if not expelled within a certain time, perish.