When there is more than one child each usually has a separate placenta, and they are all intimately connected. In some rare cases however, one placenta only exists with twins. Each child has also a separate amnion, and waters; but there may be only one chorion and decidua for the whole, or each may have a complete set of membranes itself. Instances have been known however, of two children being enclosed in the same amnion, and surrounded by the same waters, having but one placenta between them. These possible diversities show the accoucheur how necessary it is for him, in any case of multiple pregnancy, to be sure that he has abstracted _all_ the after birth; and they also caution him not to proceed to unnecessary manipulations merely because it is not the same as in other cases. THE UMBILICAL CORD. This is composed, as already stated, of an artery and two veins, which twist round the artery, like the strands of a rope. These are all enclosed in a sheath, and surrounded by a thick kind of mucus, called the _Gelatine of Wharton_. Its thickness is about that of the little finger, though it may be much larger, having been seen as thick as the child's body. Its usual length is about eighteen or twenty inches, but it has measured as much as five feet. Then again it has been found so small that the vessels in it could not nourish the child. These exceptional variations however, are very rare. The veins may also be enlarged, or full of knots, and the circulation may be so much impeded thereby as to cause abortion. The various accidents which may result from anomalies in the cord will however, engage our attention in another place. SECTION IV. THE MECHANISM OF DELIVERY IN ALL THE DIFFERENT PRESENTATIONS AND POSITIONS OF THE FŒTUS. CHAPTER X. PRESENTATIONS AND POSITIONS OF THE FŒTUS. The child may present several different parts of its body, at the commencement of delivery, and they may be in different positions relatively to the different parts of the Pelvis. All these require to be known. Different authors have made different classifications of the presentations and positions, and have differently named them; but this is of little consequence, since they are still the same, no matter how they are named. I shall follow the arrangement of M. Chailly, because I think it equally perfect, and much more simple and practical, than any other yet proposed. PRESENTATIONS. The fœtus may present at the mouth of the Womb either by the _head_, the most usual way--by the lower extremities, which is the most frequent way after the head--or by various parts of the trunk, which is the least frequent way of all.