It appears to be a law in nates presentations, that whatever may be the direction of the child (first or second position) at the beginning of labour, it will always, if not interfered with, be found with its anterior surface turned towards one or other of the sacro-iliac synchondroses, when the thorax or the shoulders are beginning to pass through the outlet of the pelvis. When the nates have once passed the os externum, the position of the child frequently varies a good deal, the abdomen turning first to one side and then to the other. This is especially the case in the second position, where it is more or less forwards; nevertheless, as labour advances, it will almost invariably turn obliquely backwards, and be born in this position. Dr. Collins is, as far as we know, the only English author who has distinctly noticed this fact. "It is very desirable," he observes, "the child should be delivered in this position (viz. the back of the child towards the mother's abdomen,) as it renders the getting away of the head much less difficult; yet where there has been no interference by the attendant in the previous part of the labour, he will rarely find it necessary to alter subsequently the child's position, the breech naturally making the turn above alluded to in its passage." (_Practical Treatise on Midwifery_, by Robert Collins, M. D. p. 41.) It sometimes, although rarely, happens in these presentations, that the head does not rest with the chin upon the breast, but the occiput is pressed against the nape of the neck, as in presentations of the face. The passage of the trunk through the pelvis follows, as above-mentioned, as far as the head: this enters the brim with the occiput in advance, and vertex towards one or other ilium. As it advances through the brim into the cavity of the pelvis, it gradually turns more and more backwards, so that when the body is born, the vertex is turned towards the hollow of the sacrum, and the under surface of the lower jaw behind the symphysis pubis. The _diagnosis_ of nates presentations is not difficult. The pointed and more or less moveable coccyx, bounded at its broader end by the hard uneven sacrum, and in the contrary direction by the anus, will scarcely admit of a mistake. The tuberosities of the ischia may easily be mistaken, for the malar bone of a face presentation, or even a shoulder, can scarcely be distinguished from them, and the external organs of generation become too much swollen and pressed together to give any certain diagnosis; nor indeed can they be examined in this state without considerable risk of injury. The direction of the sacrum, like that of the forehead in face cases, points out the exact position of the child.