CHAPTER IV.

MECHANICAL PHENOMENA OF LABOR.

The cavity of the uterus and that of the pelvis form a continuous PASSAGE through which the child must be forced in its exit from the womb at birth. The uterus possesses the character of muscularity and is the main agent in the expulsion of the child. By its own muscular action the cavity of the uterus is diminished and pressure made on the fœtus, forcing it down towards the orifice, distending the cervix, and dilating the passage. During the second stage of labor the power of the uterus is aided by the voluntary muscles of the abdomen and by the depression of the diaphragm.

The character of the passage will be brought to mind by recalling what was heretofore said of the diameter of the pelvis. It will be remembered that the usual antero-posterior diameter of the brim does not exceed 4½ inches while the transverse is 5¼ inches, and that at the lower outlet the antero-posterior diameter is about 5 inches and the transverse about 4 inches.

The FIRST OBSTACLE which the child meets in its progress is the cervex uteri. This being composed partly of muscular fibre which acts somewhat as a sphincter, and partly of elastic celular tissue, holds the sphincter in the tissue with a tenacity which is not easily overcome. But repeated muscular contractions of the womb force down the bag of waters, which forms a sort of wedge, and this is forced down and into the os uteri, compelling it to open.

There are also muscular fibres in the uterus which have a longitudinal as well as some that have a circular course, and the action of the former tend after a time to retract the os, over the fœtal head.

The _second obstacle_ is the bony brim of the pelvis into which the head of the fœtus cannot pass until its long diameter is adapted to certain diameters of the pelvis. The diameter of the bony pelvis is diminished over one-fourth of an inch by the soft parts upon it, but the oblique diameter of the pelvis will admit the long diameter of the head of the child, which does not often exceed 4½ inches. The head usually presents in this way, and passes in a somewhat spiral manner until it arrives at the outlet where the diameters are adjusted to each other. The head is, however, too large to pass, even in this way, were it not that it admits of a degree of compression to facilitate the entrance and progress through; this moulding is effected by the continued pains. The head of the child which presents at the brim with the occiput towards the left acetabulum rotates during the passage, so that the occiput at its exit is directly under the symphasis pubis; the cause of the rotation is found in the form and direction of the passage and in the shape and size of the fœtal head.

This presentation and position is the most common one, though either of the following is liable to occur. By naming the position we indicate just how a presenting part lies, or is turned. We adopt the following classification, which accords with several good authors:

PRESENTATIONS AND POSITIONS.

PRESENTATIONS. │NO. POSITION. NAME OF POSITION.

A—Vertex or head │ 1 Occiput to left Left occipito-iliac │ acetabulum. anterior.

│ 2 Occiput to right Right occipito-iliac │ acetabulum. anterior.

│ 3 Occiput to Symphasis Occipito pubic. │ pubis.

│ 4 Occiput to r. R. occipito-iliac │ sacro-iliac junc. posterior.

│ 5 Occiput to l. L. occipito-iliac │ sacro-iliac junc. posterior.

│ 6 Occiput to promon’y of Occipito sacral. │ sacrum

B—Breach, including │ 1 Sacrum to left Left sacro-iliac inferior │ acetabulum. anterior. extremities. │

│ 2 Sacrum to right Right sacro-iliac │ acetabulum. anterior.

│ 3 Sacrum to symphasis Sacro pubic. │ pubis.

│ 4 Sacrum to r. Left sacro-iliac │ sacro-iliac posterior. │ junction.

│ 5 Sacrum to l. Right sacro-iliac │ sacro-iliac posterior. │ junction.

│ 6 Sacrum to promont’y of Sacro sacral. │ sacrum.

C—Body, including shoulders, elbow and hand.

D—Face, including six varieties.

The right occipito-iliac posterior (A 4) position is not a very uncommon one, but that variety which is described and named as the left occipito-iliac anterior (A 1), in which the occiput is directed in front and to the left, is most frequent. These and other vertex presentations may be recognized even in the commencement of labor through the vaginal walls, the head being known by its rounded spheroidal surface.

Supposing that we have a case of the kind that is most common (A 1), and that labor has begun, we may introduce the finger through the os uteri and we encounter a rounded, smooth and resistant surface, which is the anterior part of the head, and then by directing the finger upwards and backwards it will come in contact with the sagittal suture.

If the direction of the suture is oblique, and if it runs from before backwards and from the left towards the right, the position must be either the left anterior or the right posterior occipito-iliac one. (A 1 or A 4).

To complete the diagnosis we follow with the finger the sagittal suture until it reaches the fontanelle, and this determines the position. If the posterior fontanelle is found to the left and in front, and the anterior one is to the right and behind, the position is A 1, or the left antero-occipito-iliac one. The back of the fœtus is turned forwards and towards the left side, while its face and anterior plane is turned backwards and towards the right, and the occipito-frontal diameter of the child’s head corresponds to the oblique diameter of the pelvic brim.

As the labor progresses and the head is forced down in the pelvis, it is also more strongly flexed on the chest and the occiput is pressed down in the excavation. With the occiput thus presenting, it traverses all the space between the superior and inferior straits until it reaches the floor of the pelvis; there it makes what is sometimes called the pivot turn—it executes a movement of rotation, which carries the occiput behind the symphasis pubis and the forehead towards the hollow of the sacrum; then the head being pressed forwards and stretching the perineum, the forehead and face being disengaged from it, emerge; then after the perfect expulsion of the head it again rotates, the occiput turns somewhat to the left thigh and the face towards the right thigh.

In the beginning of labor the shoulders are turned so as to correspond to the oblique diameter of the pelvic cavity, but they pass through the pelvis in a transverse position. After they reach the inferior strait, the body rotates so that the right shoulder of the child turns towards the left side of the mother and the wide diameter of the shoulders is accommodated to the wide diameter of the strait, and the rotation of the head, which is free externally, is secondary to the rotation of the shoulders.

In the EXPULSION OF THE BODY the right shoulder, or subpubic one, is the first one to appear in the vulvar fissure, but the left or posterior one may be disengaged at the commissure of the perineum before the right one is delivered; the remainder of the trunk is expelled very soon, describing a prolonged spiral course in its passage.

A child originally in the RIGHT POSTERIOR-ILIAC position becomes converted towards the last of the labor into an occipito pubic or anterior one, and the labor terminates as it does in A 1, when the occiput was originally in front. It is the left shoulder, however, which gets behind the arch of the pubis, and the occiput is directed towards the _right_ thigh after the head emerges.

In some instances, though rarely, the child originally in A 4 position remains with the occiput behind to the termination of the labor. In such cases the forehead comes under the pubis and remains there for a time, while the occiput traverses the whole circle of the perineum; then the whole head and face is immediately delivered.

It is not deemed necessary to describe here the mechanism of labor in the more unusual varieties which are so very numerous.

As regards PROGNOSIS, head presentations are the most favorable of all, and those in which the occiput looks anteriorly in the beginning of labor are more favorable than those in which it is turned posteriorly. In occipito-posterior positions the labor is more tedious than when the occiput is in front, and the expulsion becomes particularly difficult when the head maintains its original position and does not rotate or take the pivot turn.

Upon the fœtal head after it is delivered there is almost always a protuberance to be found—a tumefaction, more or less considerable upon some point of the vertex; its greater size indicating a longer continuance of the labor, and its seat indicating in what position the child was born. This tumor is almost always located on one of the posterior superior angles of the parietal bones, and shows that the occiput escaped under the pubic arch. During the labor the whole head is strongly compressed except at one point on the vertex, which therefore becomes the seat of a sero-sanguinolent infiltration. This tumor disappears usually within forty-eight hours; if it does not, it may properly be punctured. It may contain either serum, or serum and blood, or grumous blood.