A manual of practical obstetrics . allypresented. The purpose of this manoeuvre is to facilitatedelivery of the shoulders. Their longest diameter is, ofcourse, the bisacromial—from one acromion process to theother. This diameter entered the brim and descended intothe cavity of the pelvis parallel with the oblique pelvicdiameter extending from the right acetabulum to the leftsacro-iliac synchondrosis. But having reached the inferiorstrait, the bisacromial diameter must rotate from its obliquedirection in the pelvis to the antero-posterior one. Hence the RIGHT 0CCIPIT0-P0STERI0R POSITION. 189 ri
A manual of practical obstetrics . allypresented. The purpose of this manoeuvre is to facilitatedelivery of the shoulders. Their longest diameter is, ofcourse, the bisacromial—from one acromion process to theother. This diameter entered the brim and descended intothe cavity of the pelvis parallel with the oblique pelvicdiameter extending from the right acetabulum to the leftsacro-iliac synchondrosis. But having reached the inferiorstrait, the bisacromial diameter must rotate from its obliquedirection in the pelvis to the antero-posterior one. Hence the RIGHT 0CCIPIT0-P0STERI0R POSITION. 189 right shoulder glides along the right anterior inclined planeto the pubes; the left one, along the left posterior inclinedplane to the sacrum. This rotation of the shoulders insidethe pelvis, causes rotation of the head outside of it. Theshoulder at the pubes usually fixes itself there, while theother one, at the perineum, swings round, describing a circularmovement (as the occiput did), and comes out first. (SeePig. 54.) Fig. Restitution. When the shoulders are delivered, the rest of the bodyusually slips out at once, without any special mechanism. Mechanism of R. O. A. Position (Occiput to RightAcetabulum). 1. Flexion, by which the chin tilts up, and the occiputdown, so as to get the long diameter of the head more or lessendwise to the pelvic brim. 2. Descent, by which the head descends, occiput first,through the brim, into the cavity, down to the inclined planesof the pelvic floor. 3. Rotation, by which occiput glides along right anteriorinclined plane, downward, forward, and inward to symphysispubis; and forehead glides along left posterior inclined planeto middle of sacrum. 190 LABOR IN HEAD PRESENTATIONS. 4. Extension, by which occiput escapes under pubic archand rises up, outside, toward mons veneris, while forehead,nose, mouth, and chin successively escape at perineum. Fig. 55.
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