Different methods have been employed to measure the intensity of uterine action. Schatz made use of an apparatus called the Tocody namometer. Poppel, Matthews Duncan, and Ribemont, tried to estimate the force necessary to rupture the membranes, believing that this would approximately represent the force of uterine contraction. Schatz holds that two forces are here at work. One, which he calls the force of general pressure, is the regular pressure which the contraction of a hollow muscle exerts upon its contents. The other, called the restitution of shape, is the force in virtue of which the uterus tends to resume its original form by means of contraction. This force, as a result of the diminution of the transverse diameter of the uterus, forces the foetus into a position of ex tension, (Ahlfeld), and is supposed to act with special force upon the poles of the fetus. Before the rupture of the membranes, the action of this force is always incomplete, but, after their rupture, the two forces simultaneously act, with their full power. Schatz introduces into the uterus, underneath the head, a rubber bag connected by a mercurial manometer, to Ludwig's Kymographe. The pressure to which the bag, moderately full of liquid, is subjected in the uterus, is indicated by the manometer, and recorded on the drum of the Kymographe in the form of a curve. Schatz thus found that the pressure exerted by the uterine tonicity and that of the abdominal muscles, the uterus being quiescent, is about the same and equal to 11 inches of mercury. The pressure at the end of labor of combined uterine and abdominal con tractions, was from 3.1 inches to 9.7 inches. Consequently, the force necessary for the expulsion of the fetus would vary from 7 to 55 pounds. Schatz's method is faulty in that it not only gives the measure of the uterine pressure, but that of all the coordinated forces of labor. Poullet, of Lyons, modified the experiments of Schatz, using two bags, one in the uterus, the other in the rectum, below the head, measuring, thus, both the total expulsive force, and at the same time the force of abdominal con tractions. He obtained the power of the uterine contractions by subtract ink the latter from the former. He called his apparatus the Tocographe.
Poppel found that an average force of five pounds is necessary to rup ture membranes, the surface of which has a diameter of 1.9 inches. The average force required to expel the fetus he found to vary from four to nineteen pounds.
Ribemont took up these experiments and concluded that, when the diameter of the membrane surface amounts to 3.9 inches, the average pressure necessary to produce rupture is 23 pounds.
Going still farther, investigators have studied the modifications in the individual uterine muscular fibres, during labor. When the uterus con tracts down, after parturition, it is seen that its walls are notably thick ened, and this thickening can only have been produced by a displacement of the muscular bundles such that, while some remain at the periphery, others are arranged beneath the first. This displacement is proportional to the shortening and thickening of the separate individual bundles. The thickening is not exclusively due to the cessation of the muscular elastic tension, but also to a state of active contraction, which persists after the stage of expulsion. Schatz denies that the separate fasciculi become shorter and thicker. He believes in a simple change of position. The tension of the uterine muscles, in labor, does not increase until the uterine walls grow thicker, and this tension does not increase more than one half during the course of parturition. Schroeder, therefore, admits the occurrence of shortening and thickening of the individual fasciculi in labor, but Schatz denies it.
4. Dilatation of the Cervix.
At the inception of labor, the cervix presents a projection, hardly per ceptible in primiparaa, but a little more pronounced in multiparx, as a result of the eversion of the os externum. The cervix is open in multi pare, and generally closed in primiparre. Under the influence of labor the cervix gradually opens, its orifice grows larger and larger, until it attains its maximum dimensions—that is, until it is wide enough to allow the passage of the foetus. This is called the dilatation of the cervix. This phenomenon is differently explained by various authors. Some consider it essentially and exclusively due to uterine contraction—that is, they re gard it as an active phenomenon. Others hold that the cervix passively yields to the pressure exerted on it by uterine contractions, at first trans mitted through the amniotic fluid and the bag of waters, and later through the foetus. We believe that the dilatation of the cervix is both active and passive. It is indisputable, indeed, that the chief agent in effecting cervical dilatation is uterine contraction, but we must not disregard the action of the amniotic sac and of the foetus. For in cases where this action is wanting (premature rupture of the membranes, high position of fetus), although the cervix dilates, it does so much less regularly, much more slowly, thus causing the woman more fatigue, and entailing notable prolongation of labor.
Let us study these different conditions in regular order. The fundus and body of the uterus are mainly composed of longitudinal fasciculi of fibres, beneath which is found a layer of transverse fibres. But these transverse fibres pass, as do the longitudinal ones, from one side of the womb to the other, thus making of this organ a muscular sac, in which longitudinal fibres predominate, the action of which will be exerted from above downward, and transversely, tending to particularly diminish the capacity of the contractile sac in a transverse and a longitudinal direc tion. The cervix, on the contrary, is chiefly composed of circular fibres, and the longitudinal fibres of the body are attached to the former. The first effect of the uterine contraction is, thus, to shorten these longitudinal fibres, and consequently to exert traction upon the cervical circular fibres— to act, in brief, upon the whole circumference of the orifice. A contest thus takes place between the longitudinal fibres and the circular cervical ones, and, as the former overcome the latter, the cervix must yield and open. As the intensity of uterine contraction is constantly increased, the cervix opens more and more. At the same time, the cervix tends to rise above the ovum, or above the fcetus, if the membranes have ruptured. The cervix thus tends simultaneously to dilate and to retract above the con tents of the uterus. When the ovum is intact and the fcetus well engaged, owing to regularity of the presentation and favorable pelvic conforma tion, a new force is added to the uterine contraction, which, although not indispensable, since it is wanting in certain cases where dilatation still occurs, yet largely contributes to dilatation of the cervix. The ovum may, in fact, be considered as composed of two parts—one solid, compressible part, the fcetus, and one incompressible, the amniotic fluid. The uterus contracts simultaneously upon both. The solid part, being compressed, reduces its size as much as possible, so as to occupy the smallest .possible space, but the incompressible liquid conveys the uterine pressure to the point where the uterus is least resisting, to the orifice of the cervix. Pushing the membranes before it, the liquid collects within these mem branes, in the cervix, and the membranes, acting as a wedge, tend to en large the opening of the cervix with results proportionate to the force of uterine action, the resistance of the membranes, and the abundance of the contained fluid. After a time, the membranes rupture and the foetal part then becomes engaged in the cervix, and, in its turn, acts as a wedge to finish the dilatation of the cervix. The cervix thus simultaneously dilates, both actively and passively. On the other hand, the presence of those membranes, and of a foetal part in the cervix, irritates it, and con sequently induces, by reflex action, increasingly intense uterine contrac tions. Thus, the role of the bag of waters is not entirely passive. The authors, therefore, who consider cervical dilatation as active, and not passive, are nearer the truth than those who regard this phenomenon as entirely passive. Since the dilatation occurs thus slowly and progressively, the cervix presents variable degrees of patulousness, at the different stages of labor, and pieces of money have been selected to illustrate these degrees comparatively. Thus, one says that the cervix is dilated to the size of a ten cent piece, a twenty-five cent piece, a fifty-cent piece and a silver dollar. When the maximum cervical dilatation has been reached, the borders of the cervix are continuous with the vaginal walls, the vagina and uterus form a single uninterrupted canal, and dilatation is said to be complete. Generally, the membranes do not rupture until dilatation is almost coma pleted, and as the 'fetal part, usually the head, impinges against the cervix, the latter forms a circle around it and the head is said to be " crowned." The head escapes from the cervix, and the latter, retracting more and more, becomes less and less accessible, the foetus filling up the vagina. It would, therefore, seem as if, at a given moment, the cervix must com pletely disappear for two reasons: 1st, because it has become inaccessible; and, 2d, because its borders have become confounded with the vaginal walls. This, however, occurs only exceptionally, for the cervix never recedes sufficiently to be perfectly inaccessible, and the fusion of the margins of the cervical canal with the vaginal walls is never complete. In primi parte, especially, the anterior lip is always more or less appreciable, at the level of the symphysis, between the latter and the head. The posterior lip usually escapes observation on account of the descent of the head. Tarnier found, in the frozen body of a woman who died at the Maternite, that, at the instant when the head was appearing at the vulva, the borders of the uterine orifice were below the symphysis, anteriorly, while they were, posteriorly, l k inches from the perineal margin.