MECHANISM OF LABOR IN PRESENTATION OF THE TRUNK.
Presentation of the trunk always calls for the intervention of the obste trician; its spontaneous termination should never be waited for. Never theless, in some cases, Nature may expel the fcetus unaided, and the validity of the law regarding the unity of the mechanism of labor is shown, in that we here find the mechanism identical with that of the other pres entations, comprising the six steps which we have thus far encountered in all presentations.
But there is another mode of termination in presentation of the trunk; it is rare, very rare, but is attested by different authors, and consists in the spontaneous transformation of the presentation of the trunk, with out intervention on the part of the obstetrician, into either a breech or a cephalic presentation. It is known as spontaneous version.
Spontaneous Version.
In general, the conditions under which spontaneous version may take place are: the beginning of labor, when the contractions are still of little vigor and infrequent, integrity of the membranes, a notable quantity of liquor amnii, a not too voluminous foetus; in one word, all the factors which give to the foetus a certain mobility. Under such conditions we can under stand how the foetus, either by spontaneous movements on its part, or under the sole influence of uterine contractions, may be displaced and undergo a change of presentation. We see the shoulder recede from the superior strait, and the head or the pelvic extremity take its place at the level of the superior strait; this is called spontaneous version—cephalic, when it is the head; podalic, when it is the pelvic extremity which is substituted. for the shoulder. There is hardly an accoucheur who has not seen simi lar cases. We ourselves have observed cephalic spontaneous version in a, case of fibrous tumor of the uterus. But, what is still more extraordinary is that this spontaneous version has taken place, in some rare instances, after rupture of the membranes, engagement of the shoulder, and even prolapse of the arm and hand into the vagina. The head or breech being thus substituted for the trunk, the delivery proceeds in one of the phases regularly observed in one or the other of these presentations.
Spontaneous Evolution.
But in some cases it has been observed, that, while the shoulder re mained the presenting part, the labor terminated spontaneously, and what is still more remarkable, sometimes resulted in the birth of a living child.
It is easy to understand how, normally, when the pelvis is roomy, the child small, and the contractions are vigorous, the dead foetus can be thus expelled; but living ! and yet Denman and P. Dubois enumerate some incontestable instances. Of course, in these cases, the labor is always prolonged, but we will see the classical six steps reproduced therein.
First step.—Reduction in size. (Pig. 231).—The uterine contractions being uniformly vigorous, and the membranes ruptured long since, the uterus applies itself closely to the foetus compressing it, rolling it up, and bending it, until the head and breech seem disposed to approach one another. As a consequence, the prominence formed by the shoulders becomes, as Mme. Lachapelle remarks, still larger, and the shoulder appears to be fixed at the level of the centre of the superior strait.
Second step.—Engagement.—Propelled by the violent uterine contrac tions, this shoulder engages in the excavation, but its descent is naturally dependent on the length of the neck and hence limited. The head remain ing at the border of the superior strait in the iliac fossa, the neck is not long enough to reach the full length of the excavation, and as soon as the neck is stretched ad maximum, the descent of the shoulder is necessarily arrested.
Third step.-1nternal rotation.—After the lapse of some considerable time, during which the fcetns usually succumbs, we observe—although we have some difficulty in understanding it—a movement of rotation, which brings the presenting shoulder under the symphysis pubis. This movement, communicated to the shoulder, being transmitted to the head, the latter glides along the border of the superior strait and becomes fixed above the symphysis pubis (see Fig. 233). Hence the fcetus occu pies the following position: immediately above the symphysis pubis is the head, fixed there by the right or left sub-maxillary region, according to the original position of the presenting shoulder. The neck, as in face presentations, is stretched to the height of the symphysis, against the posterior wall of which it is applied. The anterior shoulder is fixed ender the symphysis pubis, and the posterior shoulder, with the remainder of the foetus, is situated in the posterior portion of the pelvis (see Figs. 232, 233).