_Third Movement, Rotation of the Shoulders._--When the head is delivered the shoulders come next, which also require to turn round a little, to adjust themselves to the long diameter of the lower strait; and as they turn within the Pelvis the head also turns, to correspond, and thus the back of it is brought opposite the middle of the mother's left thigh. Immediately this movement is effected, the shoulders rapidly escape through the external opening, the right one being in front, a little to the left of the symphysis pubes, and the left one behind, a little to the right of the os coccygis. The body then curves upwards, to accommodate itself to the curved axis of the Pelvis, and speedily follows the shoulders. These curious movements cause the child to pass in a spiral direction, so that each part may pass through the Pelvis in the most favorable position. Sometimes all these movements are not effected, and yet the delivery may occur, though not so speedily, or safely, as when they are. The shoulders do not always fully rotate, but may nevertheless pass the opening, if the parts be large, and well relaxed. It is questionable however, if the head can ever pass the lower strait without rotating, when it enters the Pelvis diagonally, the occipit on one side and the forehead on the other; as it is necessary for either one or the other of these parts to pass under the pubes. MECHANISM OF DELIVERY IN ALL OTHER POSITIONS OF THE HEAD. _The right posterior occipital position._--This position, called the _second_ by some authors, and the _fourth_ by others, is exactly the reverse of the one just described, the head lying in the same direction, but the back of it being behind, to the right of the Sacrum, and the forehead to the left of the pubes. Precisely the same movements are gone through in this position as in the other, excepting that the head has to rotate considerably further. In the former position the back of the head is only a _little_ to the left of the pubis, and therefore has not far to turn to pass under it; but in this position the back of the head is _behind_, and therefore has to turn very far round to reach the same position. The rotation is therefore more difficult, not so soon effected, and sometimes likely to be dangerous to the child. In the other movements there are no difference worthy of notice, but it must be borne in mind that they all occur the reverse way, to what they do in the first position, because the occiput is on the _right_ side instead of the left. _The right anterior occipital position._--This position is precisely the same as the first, but on the other side. The back of the head is in front, but to the _right_ of the pubis instead of the left, while the forehead is behind, to the left of the Sacrum. This will be apparent enough by observing Plate XXXI. PLATE XXXI. This is the second most frequent position; the left anterior occipito iliac being the _most_ frequent.