Fourth Period.—E.rpulsion of the Head.—When the chin has reached the symphysis, the expulsion of the head is easy. The mental diameters do not now engage, but the sub-mental ones. The sub-mental region, situated under the symphysis pubis, is immobilized, .like the sub occipital region in vertex presentations, and it is around this region, act ing as a centre, that the head is expelled, in describing the arc of a circle around the symphysis pubis. But the head is extended, and as the sub mental region and the chin are thus withdrawn from the influence of uterine action, all the force of the contraction is transmitted to the fore head, which is lowered, more and more, by a movement of flexion. The face is then expelled by the successive liberation of its sub-mento-frontal, sub-mento-bregmatic, sub-mento-occipital and sub-mento-sub-occipital diameters, the chin tending to ascend, in proportion as the face is liberated, as we have seen the occiput rise during the expulsion of the head, in vertex presentations.
Fifth Period.—External Rotation.—Sixth Period; Expulsion of the Trunk. —These two periods occur exactly as they do in vertex cases. The shoulders turn within the pelvis, the head turns outside, and the chin returns to the side which it occupied. It thus looks toward the right thigh of the mother, in the position M.I.R P. The expulsion of the shoulders and of the rest of the trunk occurs just as in vertex presenta tions.
Second Position M.L R.A. (Fig. 208)—The chin here looks forward and to the right, the bregma backward and to the left. The head occupies the right oblique diameter, and the nostrils consequently look forward and to the right. The circumference M. B. is in relation with the inlet, the diameter M.B. with the right oblique diameter, and the bi-malar diameter with the left oblique diameter. The periods are almost identical with those of labor in the position M.I.R.P. The only difference consists in the fact that the chin, being in relation with the right anterior half of the pelvis, has a much shorter distance to travel than in the preceding case. The rotation is, hence, easier, and it occurs, as in the position M.I.R.P., from right to left.
3. Position M.I. L.A. (Fig.209)—Its relations are the same as those of the position M. I. R. P., excepting that the chin looks forward and to the left, and the forehead backward and to the right. The face is in the left oblique diameter of the pelvis.
4. Position M: L L.P. (Fig. 210)—Same relations as in M.I.R. A. except that the chin is behind and on the left, the forehead in front and on the right, the face occupying the right oblique diameter of the pelvis. The third period is longer, since the chin is behind. Rotation occurs in this case, as in M.I.L.A., from left to right.