_a._ The Acetabulum, or socket of the hip joint. _c._ The cut edge of the Womb. c. The Foramen Ovale. _d._ The top of the ilium bone, called the _crest_. _h._ The Symphysis Pubes. _i._ The ischium. K. K. The pubic bones. [Illustration: Plate XXXI. Head in the _right_ anterior occiput iliac position.] The mechanism of delivery is precisely the same, and all the movements occur in the same order and manner, as in the first position, but the reverse way. The rotation, for instance, being from right to left, instead of from left to right, and so of all the others. _The left posterior occipital position._--Delivery is effected the same in this as in the left anterior, or first position, excepting that the rotation is more extended, owing to the occiput being behind, as explained in the right posterior occipital position. GENERAL REMARKS ON THE DIFFERENT POSITIONS OF THE HEAD. In all the other positions, and their varieties, there is nothing that calls for special notice, or that is material in practice, the delivery being nearly the same in them all. No matter what position the head is placed in, the back of it nearly _always_ comes to the front, under the pubes, even though it have to turn half round to do so. The cause of this is supposed to be the peculiar form of the parts, which give it a screw like motion, in its descent, and the shape of the external opening, which, being longest from before to behind, can only allow the long diameter of the head to pass through in the same direction. Sometimes, it is true, the occiput passes behind, instead of coming to the front, and then the _chin_ comes under the pubes, while the occiput presses on the coccygis. This is very seldom observed, and when it occurs the labor is more difficult and tedious, though it may still terminate spontaneously. The resistance of the soft parts, externally, appears to be the chief cause of the head turning; for when they are much relaxed, and the child's head small, it will sometimes pass without, or in the anterior posterior position. And sometimes, when the head is large, so that it distends the parts very much, the shoulders will pass cross wise, there being room enough for them without turning. In some females, formed large, whose organs are excessively relaxed, and whose children are small, the delivery takes place without _any_ of the movements being effected, the child passing straight through in whatever position it may happen to be: this is rarely seen however. When there are twins they do not usually both present by the head, but one by the feet; and frequently the parts are so relaxed by the passage of the first, that the second is delivered without rotating at all, though in general it follows precisely the same movements. It sometimes happens however, that the second birth does not take place till some hours, or even days, after the first.