THE MECHANICAL PHENOMENA OF LABOR term mechanical phenomena or mechanism of labor, designates the ensemble of the movements imparted to the foetus by the uterine contractions, and which are intended to facilitate the expulsion of the foetus from the maternal parts. These different movements are described by accoucheurs under the heading " periods of labor," five of which are usually recognized. We make, with Tarnier and Chantreuil, a sixth period, which is the second part of the fifth period of other authors. Be fore entering upon the details of the mechanism of labor, let us, with Pajot, admit that: "All labors, from a mechanical standpoint, are subject to the same law, and that there is really only one mechanism of labor, no matter what the presentation or the position, provided only that expul sion occur spontaneously, i.e., without the intervention of art, and at term, as abortions do not result in regular expulsion." In all labors, therefore, there are six periods. In the first period the foetus, under the influence of uterine pressure, will tend to accommodate its form to that of the uterus, and to that of the canal through which it must pass, and must, therefore, reduce its volume. This period is called the first period, or period of moulding. In the second period, the foetus becomes engaged, and descends into the pelvis. Thus the second period is that of engage ment or of descent. In the third period the fwtus executes a movement of rotation, which brings the part which is to engage to the front, and arranges its long axis antero-posteriorly, in the conjugate diameter. Hence, the third period is that of internal rotation. In the fourth period,
the foetal part in question frees itself, and emerges from the genital canal by describing the are of a circle below and in front of the symphysis, the lower border of which may be regarded as the central point. This fourth period is that of expulsion. In the fifth period, the foetal part experi ences a movement of external rotation, corresponding to a movement of internal rotation performed by that part of the fietus still remaining in the genital canal. This is followed by expulsion of the rest of the fcetus, by a process identical with that of the fourth period. This is Tar nier's sixth period. So the fifth period is one of external rotation, and the sixth that of expulsion of the rest of the foetus. These periods are not absolutely identical in all presentations and positions of the foetus, but we will encounter them in all labors, whatever the presentation and the position. Let us pass in review the different presentations and posi tions.
We have stated that the foetus may present by its cephalic extremity, flexed or extended (vertex or face), by the pelvic extremity, completed or incomplete (breech or feet), and by the trunk (right or left shoulder), and that each of these presentations has two cardinal positions (right and left), each of these positions presenting the varieties anterior, transverse or posterior. Let us study the mechanism of labor in these different pre sentations and positions. We will then see Pajot's absolute law justified by the facts.