If there be but little liquor amnii, or the membranes have been ruptured prematurely: if the head be firmly pressed against the os uteri, and we examine when it is not more than two-thirds dilated, we feel a puffy oedematous swelling upon that part of the head which corresponds to the os uteri. This will therefore be found to be situated upon the sagittal suture, nearly equidistant from the anterior and posterior fontanelles; it arises from the circulation in the scalp being obstructed by the pressure of the os uteri upon the head. If the remaining portion of the labour be rapidly completed, this will be the situation of the swelling with which the cranium is born; if, however, it follows a more gradual course, and the head passes slowly through the os uteri into the vagina, as it thus advances deeper into the pelvis, and alters its position more or less, the swelling upon the sagittal suture disappears in part, and forms on that portion of the head which is advancing under the pubic arch, and is now tightly encircled by the external passage: we shall, therefore, find that this second swelling is situated upon the posterior and superior quarter of the right parietal bone, and this is precisely the situation of the swelling of the head, which the child is usually born with. From these facts we may deduce the following simple law respecting the mechanism of parturition, where the head presents: viz. that the head enters, passes through, and emerges from, the pelvis obliquely; and this is the case not only as to its transverse diameter, but also as to the axis of its brim; the side of the head being always lowest or deepest in the pelvis. This shows the beautiful mechanism of the process, for, on account of its oblique position, there is no moment during the whole labour at which the greatest breadth (still less length) of the head is occupying any of the pelvic diameters; even at the last, when the head is passing under the pubic arch, the complete obliquity of its position, in order that it should take up the least possible room, is very remarkable; for the ring of soft parts, by which the head is now encircled, passes obliquely across it, running close behind the left, and before the right parietal protuberance. The head never advances with the occiput, forwards, under the pubic arch, as is stated in works on midwifery, still less with the sagittal suture parallel to the antero-posterior diameter of the pelvis; for the direction of the right lambdoidal suture, as also of the posterior fontanelle, and the position of the cranial swelling, or _caput succedaneum_, as it has been called, completely prove the inaccuracy of such a theory, the sagittal suture crosses the left labium at an acute angle, the right lambdoidal suture being parallel with the left descending ramus of the ischium.