The engagement, of itself alone, and even the appearance of the fcetus at the vulva, are not sure signs of complete dilatation, for there are, here. several causes of error which we cannot too much insist upon. The first is the engagement of the foetus. We have several times seen students, and even physicians, consider dilatation complete when it had not even begun. The head pushes the inferior uterine segment before it, and the latter is sometimes so thinned that the sutures and fontanelles are almost as plainly felt as if uncovered. If one is not careful to carry his finger backward, he supposes dilatation to be finished before the cervix is fairly effaced. The second cause of error lies in the bulging of the bag of waters. Often-times, particularly with high presentations, the liquor amnii is very abundant in the lower segment, and, provided the mem branes be resisting and yet elastic, they ane largely distended, and one feels so large a bag of waters that he supposes dilatation complete. If, however, one carries the finger up to the base of the hag of waters, he feels that this base is only a pedicle circumscribed by the orifice of the cervix, dilatation being still far from complete. The cervix may be obliterated, and the lips agglutinated, as in cases reported by Depaul, Martin of Lyons and myself; the difficulty in finding the cervix may make one believe that dilatation is complete. The cervix may be displaced and almost inaccessible. Tarnier and Chantreuil mention the fact that often, particularly in primipane, the vagina forms a sort of circular fold, at its upper part. In the centre of this, the cervix, almost effaced, lies hidden, like the end of the water pipe at the bottom of a fountain, and the finger may pass over this, unawares, mistaking the vaginal fold for the cervix itself. All of these errors are committed, of course, only by unpractised fingers.
There is another phenomenon to which we ought to call attention: the so-called dilatability of the cervix. When the foetal presentation is high, after rupture of the membranes, and the escape of the amniotic fluid, the cervix, having been almost completely dilated, recontracts somewhat, preserving, however, such suppleness and flaccidity that one may enlarge the os notably by digital pressure. Then we say that the cervix is dila table, and the difference between dilatation and dilatability is, therefore, as follows: A cervix is completely dilated when its borders have come to be permanently continuous with the vaginal walls, being confounded, as it were, with them. A cervix is dilatable when, no matter what be its actual dilatation, it is sufficiently yielding to be artificially expanded so as to become continuous with the vaginal parietes. It is of great practical importance to fully recognize the dilatability of the cervix. It permits the intervention of the accoucheur if child or mother demand it, while, supposing that one waited for true dilatation to occur, the child might die and the mother be exposed, by the prolongation of labor, to puerperal diseases. which are more likely to occur when labor is protracted, when fatigue is great and traumatism considerable.
Charrier mentions a phenomenon rarely observed, to which he applies the name retrocession of labor. Its nature is the following: It some times happens, before the rupture of the membranes, that the cervix, already somewhat dilated, recontracts owing to interruption of the pains, but still retains a certain degree of dilatation. In the place of forming a simple ring, however, it forms a canal, .3 to .6 of an inch long, which the finger must pass through in order to reach the ovum. In this case some thing analogous to abortion takes place, and we shall return to the subject later.
Since the dilatation of the cervix is accomplished slowly and progres sively, the orifice preserves its circular form throughout the labor. When, on the contrary, the cervix is the seat of cicatrices, and anatomical or pathological degeneration, since it is rare for the cervix to be uni formly diseased, the dilatation takes place at the expense of the healthy part of the cervix. Hence a variety of shapes of the orifice, which may be come elliptical, triangular, rectangular, etc. While the shape of the orifice remains unchanged, the thickness of its borders varies. In primiparte, dila tation is always slower than in multiparm, and the cervix presents, accord ing to the different stages, very different conditions. When dilatation be gins, the cervix is very thin, represented only by the os externum, the lips of which are not thicker than a sheet of parchment, and are only separated from the head by the membranes, which are in almost immediate contact with the head. The head being, generally, very low down, there is very little liquid between the membranes and the head, and, at the moment of contraction, one feels a little sac of fluid shooting, as Madame Lachapelle used to say, through the os. The borders of the latter are so thin that a certain experience is necessary in finding the cervix. The sensation which one now obtains is peculiar: On passing the finger be tween the cervix and the membranes, the margins of the former give the sensation of a thick thread. As the bag of waters becomes more fully developed, and, particularly, when it bursts before complete dilatation of the cervix, and the head comes to press upon the cervix, the latter thickens, becomes tumefied and forms a cushion of greater or less thick ness. If one now passes the finger over this dilated orifice he feels it to be composed of two parts, one external, thick, oedematous, tumefied, and one internal, thin, giving the sensation of a thread, such as we have de scribed. Then, as labor progresses, the cervix grows thin again, without quite resuming its original thinness, presenting still a different appear ance when the head becomes " crowned." At that time the anterior and lateral parts of the cervix remain thicker, while the posterior part is more thin. Then the head, descending still more, pushes before it the anterior cervical lip, which, being thus confined between the symphysis and the head, becomes swollen, and forms a cushion growing constantly thicker and thus offering an obstacle to the complete descent of the head. This obstacle may make itself felt for several hours, and it is only at the end of that time that the head succeeds in passing it, and the cervix becomes permanently inaccessible. It is the rule, when forceps are applied at the upper part of the pelvis, or at the superior strait, the head not having yet completely passed the os, to see this anterior cervical lip projecting at the vulva as a bluish prominence, having the thickness of a finger or even more. In multiparze, on the contrary, the cervix is thicker, but dilata tion is generally more regular, and it is not rare to see a single pain effect both complete dilatation of the cervix, and the passage of the head through this orifice. This oedema of the anterior lip is exceptional, and if this obstacle be absent, and the resistance of the perineum be slight, one often sees labor terminated in a few minutes by two to three pains. In primi parw, the head, even after escaping from the cervical canal, often requires several hours to overcome the obstacle opposed by the anterior lip as well as the resistance of the vagina, perineum and vulva. Budin believes the resistance of the vulva to be insignificant, and thinks that the chief ob stacle is the vaginal orifice.