There is much uncertainty on the occasional causes of delivery, and we do not perfectly know what obliges the infant to quit the womb. Some imagine, that the foetus having acquired a certain size, the matrix is too confined for its longer stay, and that the constraint felt by the foetus, obliges it to use every effort to quit its prison; others say, and it is nearly to the same purport, that the weight of the foetus becomes so great, that the matrix is forced to open to free itself from the burthen. These reasons do not appear satisfactory; for the matrix must always have capacity and strength to contain and sustain the weight of a foetus of nine months, since it often contains two, and it is certain that the weight and size of the twins of eight months are more considerable than the weight and size of a single child of nine. Besides, it often happens that a child born at nine months is smaller than the foetus of eight months, although it continues in the womb. Galen pretends, that the child remains in the matrix till it is able to receive its food by the mouth, and that it only forces its escape from the need of nutriment. Others have said, that the foetus always receives its nourishment by the mouth from the liquor of the amnios; but which becomes at length so contaminated, by the transpiration and urine of the foetus, that it becomes disgustful, and obliges the foetus to use every exertion to quit its confinement. These reasons do not appear better than the first; for it would from thence follow, that the weakest and smallest foetuses would remain longer in the womb than the strongest and largest, which never happens; besides, it is not food that the foetus seeks immediately after it is born, for it can stay some time without it; on the contrary, it seems most desirous to disembarrass itself from the nutriment it took when in the womb of its mother, and to return the meconium. Other anatomists have supposed that the excrement accumulated in the bowels of the foetus, gives it great pain, and causes it to make such efforts, that the matrix is at length obliged to give way, and to open a passage for its escape. I acknowledge I am not better satisfied with this explanation than the rest; because, why cannot the foetus void its excrements in the amnios, if it was pressed so to do? Now this never happens; it appears, on the contrary, that this necessity of voiding the meconium is not felt till after the birth, when the motion of the diaphragm, occasioned by that of the lungs, compresses the intestines and causes this evacuation; for the meconium has never been found in the amnios of a foetus of ten months who had not respired, whereas a foetus of six or seven months voids this meconium a short time after respiration.