It may be said, in general, that all positions of the head are favorable to both mother and child, and may terminate spontaneously. It is seldom that anything more than ordinary assistance is required in any of them, and they could in general terminate without any at all, though sometimes with difficulty. The worst cases are those in which the head does not turn round, but remains across, or where the back of it turns behind, instead of coming to the front. In these cases there is great danger of the perineum, or external lips, being much lacerated, or even of an artificial passage being torn through the perineum, leading to the most serious after results. About one child also, out of every fifty, is lost in these unfavorable positions. CHAPTER XII. MECHANISM OF DELIVERY IN PRESENTATIONS OF THE LOWER EXTREMITIES. This presentation includes the feet, the knees, the breech, and also the hips, there being no difference in the delivery for all these parts. It is the same also whether there be one foot, or knee, or both feet and knees. There are but two positions worthy of notice in this presentation, and they are determined by the child's sacrum. If the sacrum, or posteriors, are to the right of the mother's Pelvis, it is called the _right sacro iliac position_; but if they are on the left side it is called the _left sacro iliac position_. The direction in which the child's pelvis is placed, is analagous to that of the head, the sacrum answering to the occiput. Thus most frequently the sacrum is on the left side, a little to the left of the pubes, (_left anterior_) while the abdomen faces the right side near the sacrum. When on the right side however, it is most usually nearest the sacrum, with the abdomen facing the left side near the pubes. The lower extremities present most frequently next after the head, but still they are but seldom met with. M. P. Dubois tells us that out of _twenty thousand labors_ he only met with _eighty-five_ such cases. In these eighty-five cases the breech presented _fifty-four_ times, and the feet _twenty-six_, the knees being found _but once_.