The foetus generally comes into the world during the tenth revolution. When it is born at the eighth or ninth it lives, and these deliveries are not looked upon as miscarriages, because the child, although not so perfectly formed, is still sufficiently so for the purpose of life. It has been pretended, that examples have been seen of children born at the seventh and even at the sixth revolution, that is, at five or six months, which have lived. There is, therefore, no difference between a birth and a miscarriage but what is relative to the living powers of the infant. In general the number of miscarriages in the first, second, and third months are very considerable for the reasons we have given ; and the number of deliveries of the seventh and eighth months are also very great, in comparison with the miscarriages of the fourth, fifth, and sixth months, because in this middle period the product of generation has received more solidity and strength, and having resisted the action of the four first periodical revolutions, a more violent force than the preceding is required to destroy it. The same reason subsists, with additional force, for the fifth and sixth months. But the foetus, which till then is weak, and can act only by its own feeble strength, begins to get strong, and move with vigour; and at the eighth revolution the foetus, uniting its efforts with those of the matrix, facilitates its exclusion, and it may come into the world in the seventh month, and be capable of living, especially if it happens, as is sometimes the case, to have more than ordinary strength for that period. But if it comes into the world only through the weakness of the matrix, which could not resist the action of the blood in this eighth revolution, the delivery would be regarded as a miscarriage, and the child would not live. But these cases are very rare, for if the foetus has resisted the seven first revolutions, only particular accidents can prevent it from resisting the eighth. The foetus, which has acquired this same degree of strength and vigour only a little later, will come into the world at the ninth revolution; and those which require nine months to obtain this same strength, will come at the tenth revolution, which is the most common and general term; but when the foetus has not acquired in nine months this degree of perfection, it may remain in the womb till the eleventh, and even till the twelfth revolution; that is, till the tenth or eleventh month, as we have many examples. This opinion, that it is the menstrua which is the occasional cause of delivery at different times, may be confirmed by many other reasons. The females of every animal which have no menses, bring forth at nearly the same terms, and there is but a very slight variation in the duration of their gestation. We may, therefore, suppose that this variation, which is so great in women, comes from the action of the menstrual blood, which is constantly exerted at every periodic return.