. On the theory and practice of midwifery . daptedto the wants of the economy in labour, that it has power, in a major part of these fourthpositions, to rotate the vertex from the right sacro-iliac junction to the right acetabulum,and thence to the pubal arch; and that without any assistance given by the accoucheur. It is true that this favourable rotation sometimes requires the aid of the hand, oreven of an instrument. It also occasionally happens, that neither the hand alone, norany instrument, can enable the surgeon to bring the vertex round to the front. In suchcase, it slides into the hol
. On the theory and practice of midwifery . daptedto the wants of the economy in labour, that it has power, in a major part of these fourthpositions, to rotate the vertex from the right sacro-iliac junction to the right acetabulum,and thence to the pubal arch; and that without any assistance given by the accoucheur. It is true that this favourable rotation sometimes requires the aid of the hand, oreven of an instrument. It also occasionally happens, that neither the hand alone, norany instrument, can enable the surgeon to bring the vertex round to the front. In suchcase, it slides into the hollow of the sacrum, and the labour is thenceforward renderedmore painful and more difficult. When, in fourth positions, the vertex can rotate first to the acetabulum, and thento the arch, the labour is not seriously retarded; but when the posterior fontanelle getsinto the hollow of the sacrum, and will not suffer rotation, then the flexion becomesgreater and greater as the fontanelle slides down along the point of the sacrum, along Fig. the face of the coccyx, and down the mesial line of the perineum, until having pusheaoff the perineum , the occipitofrontal diameter, the vertex slips over the fourchette,and immediately turns over backwards, in strong extension, which allows the forehead,eyes, nose, mouth and chin successively to emerge from underneath the crown of thepubal arch, to complete the birth of the head. The annexed figure of a head in anoccipito-posterior position, shows these truths clearly enough. This is the mechanism in all cases of birth in occipito-posterior positions, withoutrotation to the front; and the student will clearly understand that it must be so, sincethe length of the line from forehead to vertex is too great to permit it to be , — the Science and the Art. — Editor. 14 208 MECHANISM OF PARTURITION. second position ; but for the minute explanation we are indebted to 315. As to the comparative frequency of th
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