ANOMALIES IN THE MECHANISM OF LABOR.
First Period.—Flexion.—This movement rarely fails, but, when it is absent, it may transform a physiological into a pathological labor. Or dinarily, the anomalies consist in an exaggeration or in a diminution of this flexion. These are the anomalies which constitute, according to the German view, occipital or frontal positions. Generally, however, labor is simply retarded, flexion being completed in the pelvis or on the pelvic floor, and the position being regulated, after a time, by the uterine con tractions. The situation of the fontanelles, as regards the centre of the pelvis, will suffice for the recognition of these anomalies.
Second Period.—Engagetnent or Descent.—The anomalies are here rather physiological varieties. Two conditions govern these, viz: the dimensions of the canal and of the foetus, on the one hand, and the in tensity of the pains, on the other. The varieties consist in varying degrees of engagement, the vertex being at the superior strait, a little below it or low in the pelvis.
Third Period.—Internal Rotation.—Anomalies are most likely to occur in this stage. Rotation may be absent, incomplete or exaggerated. It may, particularly in posterior positions, occur before the head reaches the pelvic floor, the first three stages occurring simultaneously. When rota tion is exaggerated, the occiput, after its arrival beneath the symphysis, instead of becoming fixed in that position, passes to the other side, trans forming, thus, a left into a right position, or the reverse. After a time, however, the occiput returns beneath the symphysis, and labor is com pleted as if the original rotation had been regular. When rotation does not take place, if the head is small, labor may yet be spontaneously com pleted, even in primiparre, in spite of the feeble pains which usually occur in such cases. In multiparte, the rotation is of less importance, as the soft parts offer less resistance. If rotation is absent, the head escapes in an oblique position, the occiput resting upon one of the ischio-pelvic rami. Labor is much protracted, in these cases. The trunk sometimes fails to participate in the internal rotation of the head. In this case a genuine torsion of the neck occurs, and, consequently, incomplete rotation of the head. The head is expelled, as in the former case, obliquely, and it is in this case that restitution really occurs, the neck being forcibly un twisted, when the head is expelled. But the most serious anomaly is the perversion of rotation. In certain cases, rotation, instead of occurring in a takes place in a backward direction, the occiput turning into the hollow of the sacrum, so that the position becomes occipito-sacral (Fig. 202). This anomaly, which ordinarily makes intervention a necessity, may yet allow of a normal termination of labor. The labor is prolonged for a reason first given by P. Dubois. The occiput must, as all accoucheurs admit, pass along the whole posterior surface of the pelvis, which is much longer than the anterior wall. But, moreover, the foatus represents, in this case, a straight and rigid branch or stem, 5.06 to 5.46 inches long, and, consequently, longer than the pelvic diameters. This rigidity will persist until the occiput shall have escaped from the pelvis, which it will only do after passing over the whole posterior pelvic and perinea' surface.
When posterior rotation occurs, labor may be completed in different ways. Generally the head becomes forcibly flexed, the occiput looking backward and the forehead resting against the pubes until the occiput reaches the posterior commissure of the perineum. It then escapes first,
and the neck having become fixed against the commissure, one sees the successive expulsion of the bregma, the face and the chin.. According to Cazeaux, the head remains oblique. Exaggerated flexion occurs, as before assumed, but the forehead reaches the inferior strait first. The anterior frontal eminence becomes engaged beneath the pubic arch, and fixed there, and the occiput, traversing the postero-lateral wall of the pelvis, is first liberated by the perineal commissure. The descent of the occiput causes the frontal eminence to ascend behind the pubes, and it is not until the escape of the occiput over the perineum, that extension is produced and the head expelled. The bregma escapes first, then the forehead, the face and the chin. Tarnier and Chantreuil state that: " while the occiput traverses the perineum, forcibly distending it, the region of the anterior fontanelle, and the upper part of the forehead, are the parts which first descend to the vulva. The occiput has hardly reached the perineum at the moment when the whole forehead is being expelled anteriorly. When the occiput has once escaped, the head makes a movement of extension, having for its centre the perineal commissure, which supports the sub-occipital region, and expulsion occurs by the suc cessive engagement of the sub-occipital diameters. The chin escapes last beneath the pubes." Guillemot has seen the position O. R. P. spontane ously converted into a face presentation at the inferior strait, i.e., the movement of extension exaggerated, and the chin first expelled beneath the symphysis. This mutation seems very hard to reconcile with the normal pelvic diameters, for it is necessary that the occipito-mental diameter, which is 5.26 inches, should swing in a pelvis with a diameter of only 4.68 inches. Charrier has seen one example in a normal pelvis, but there was central perineal laceration, and the engagement of the oc ciput in the laceration allowed of the swinging movement. Cazeaux further says, that he has seen one such case. The failure of rotation is one of the most frequent causes of intervention (ride article Dystocia).
Fourth Period.—Expulsion.—The anomalies are dependent on those of the third stage. Expulsion occurs, according as rotation is incomplete or absent, obliquely, or over the commissure of the perineum.
Fifth Perind.—External Rotation.—It is dependent upon the move ment of the shoulders. If rotation of the trunk does not take place, external rotation fails. If it is extreme, external rotation is exaggerated. If, on the contrary, rotation of the shoulders occurs in a direction the reverse of what it should have been, the head follows this movement on the exterior, and, in the left positions, for example, rotation tikes place as in the right positions and the reverse. But, most frequently, it must be admitted, this rotation, which seems irregular, is perfectly normal, and is considered irregular only because one has wrongly diagnosticated the position.
Sixth Period.—Finally, the expulsion of the shoulders may take place obliquely or even transversely. We will see, in the article on " Dystocia," that their exaggerated size, and the absence of their rotation may, in certain eases, create difficulties not always easy to surmount, and which demand rapid and active interference.