Presentations of the nates, although perfectly natural as far as labour is concerned, are far more dangerous for the child than those of the face, for when the head enters the pelvis, if every thing be not favourable for its passing rapidly through it, the cord is so long compressed that the child is almost certainly lost. The natural position of the foetus in utero is admirably adapted for its safe passage through the pelvis under these circumstances, and is what we ought to maintain, as far as possible, during labour. The legs are turned upon the abdomen, the arms are crossed upon the breast, the chin rests upon it, the head being bent forwards, so that the whole forms an oval mass. So long as the child advances gradually, the fundus presses firmly upon the head, and keeps the chin close upon the breast; the head therefore enters the pelvis in the most favourable position possible, and the uterus, not having been suddenly emptied of a part of its contents, continues to act briskly, and presses the head so rapidly through the pelvis, that the child is born without having suffered from any serious pressure upon the cord. As however the body of the child diminishes from its pelvis up to the axillæ, it is very apt to be rapidly expelled as soon as the nates have passed the os externum; and if not, it is but too frequently _assisted_, as it is called, at the very moment when it ought rather to be supported and prevented from advancing too suddenly. When this is the case, the fundus ceases to press upon the head, the chin quits the breast, and as a space is thus left between them, the arms slip into it, and then turn upwards, so that the head not only enters the pelvis in a most unfavourable position, but, to make matters still worse, it has an arm on each side of it: at this critical moment the uterus, from having been suddenly emptied, ceases to contract, and the head remains so long in the pelvis that the child has no chance of escaping with its life.