"Labours with the nates or feet presenting, follow certain laws quite as much as those where the head presents, only that one more frequently sees deviations from them, both with respect to the manner in which the child presents at the time of labour, and its passage through the pelvis; but where, under a proper state of the other requisites for healthy parturition, no prejudicial result occurs." (Naegelé, _on the Mechanism of Parturition_, transl. § 19. p. 128.) "In every case, whether the nates have at first a completely transverse or oblique direction, they will be always found, on pressing lower into the superior aperture of the pelvis, to have taken an oblique position; and that ischium, which is directed anteriorly, to stand lowest. They pass through the entrance cavity and outlet of the pelvis in this position, which is oblique, both as to its transverse diameter as well as to its axis." Thus, if in the first species the left ischium were either originally directed more or less forward, (which is usually the case,) or had taken this direction in passing through the superior aperture, the nates descend in this direction into the pelvic cavity, with the left ischium during the whole time standing lowest; and this is the part, during the farther progress of the nates, which first passes between the labia as the os externum dilates. As they advance, and while the left ischium, which is directed forwards and always somewhat to the right, comes completely under the pubic arch and presses against it, the other ischium, which is situated in the opposite direction, and which has to make a much greater circuit, passes forwards over the strongly distended perineum, so that, when the pelvis is born, the abdomen of the child will be directed to the inner and posterior surface of the mother's right thigh. "The rest of the trunk follows in this position, and as the breast approaches the inferior aperture of the pelvis, the shoulders press through its superior aperture in the direction of the left oblique diameter; and during its passage (viz. the breast) through the pelvic outlet, the arms and elbows which were pressed against it are born at the some moment. But whilst the shoulders are descending in the above-mentioned oblique position, the head, which during the whole progress of the labour rests with its chin upon the breast, presses into the superior aperture in the direction of the right oblique diameter, (viz. with the forehead corresponding to the right sacro-iliac synchondrosis,) and then into the cavity of the pelvis in the same direction, or one more approaching the conjugate diameter. After this, it presses through the external passage and the labia, in such a manner, that whilst the occiput rests against the os pubis, the point of the chin, followed by the rest of the face, sweeps over the perineum as the head turns on its lateral axis from below upwards.