Fourth of the begins the fourth step. The anterior shoulder remaining fixed under the symphysis pubis, and appearing first, the uterine contractions force the posterior shoulder (and the rest of the fcetus) from above downward, making it, in its de scent, sweep along the posterior wall of the excavation. At last, urged on by the contractions, it distends the perineum, passes the posterior commissure, and is followed by the axilla, the thorax, the hips, the breech; and then the shoulder fixed under the symphysis is disengaged in its turn, while the head remains in the uterus to the last. Properly speaking, there is no such thing as delivery of the fcetus doubled on itself, but a gliding along the antero-lateral portion of the pelvis executed by the posterior-lateral position of the fcetus, of which the superior part— the shoulder—remains fixed under the symphysis, and the inferior part does not come forth until the other side of the fcetns is delivered. Supposing, for example, that the left shoulder is fixed under the sym physis, we see successively evolved over the perineum the right shoulder, the right axilla, the right side of the thorax, the right hip, then the feet, the left hip, the left side of the thorax, and at last the left shoulder; the head alone remaining in the uterus (Fig. 234.) Fifth step.—External rotation of the trunk, internal rotation of the
head.—Then occurs the external rotation of the trunk, which corresponds to an internal movement of the head; the latter readjusting itself, and turning at the same time, in such a way as to put the occiput in direct contact with the superior border of the symphysis, and when this rota tion is accomplished, the— Sixth step—Dtsengagement of the head—is effected as in presentations of the pelvic extremity, by the sub-occipital diameters.
Prognosis.—The prognosis of delivery by the trunk is always very seri ous. Besides, we must endeavor in every case to recognize this presen tation before labor; for, as we will see, we can transform it into a vertex presentation by means of external manipulations. But, if labor has corn menced, and external version is impossible, spontaneous delivery should never be waited for, and recourse must be had to version by internal manipulations, which, however, is not always devoid of danger for mother and foetus. Hence the prognosis is always grave in proportion to the time elapsed since the onset of labor, as the difficulties of intervention increase part passu. Roederer, Bleinwiichter, and . some other authors, report a few rare cases in which the foetus in its passage was actually bent double, as shown in Fig. 235.