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fice of the vagina to the lower part of the third sacral vertebra. lines cross, at an angle of about 95°, at a point on the floor of the pelvis a little above (in the line of the uterine axis) the tip of the coccyx. The uterus and vagina together form a muscular tube having a blind upper extremity, and bent on itself in such a way that it might be likened to an elbow in a stove pipe. (See Fig. 1.) This bend forms the utero-vaginal angle, and performs, as we shall see, a very important part in the mechanism of labor.

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Before discussing the mechanism of labor let us glance at the anatomy of the floor of the pelvis, which is composed of muscles; fascia, connective tissue, blood vessels, nerves, etc., and is in reality the bottom of the pelvic basin. The most important structure in the pelvic floor, and the only one we will examine, is the levator ani muscle, which "arises in front from the posterior surface of the body and ramus of the pubes

* Dickinson, Am. Jour. Obstet., 1889.

on the outer side of the symphysis, posteriorly from the inner surface of the spine of the ischium, and between these two points, from the angle of division between the obturator and vesico-rectal layers of the pelvic fascia at their lowest part. The fibres pass downward to the middle line of the floor of the pelvis, and are inserted, the most posterior fibres into the sides of the apex of the coccyx, those placed more anteriorly unite with the muscle of the opposite side in a median fibrous raphe which extends between the coccyx and the margin of the anus. The middle fibres, which form the larger portion of the muscle, are inserted into the sides of the rectum, blending with the fibres of the sphincter muscles,

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and the anterior fibres become blended with the longitudinal fibres of the vagina and with the external sphincter and transversus perenei muscles." It is reinforced by the other perineal muscles and by the strong pelvic fascia.

It must readily be seen, from this description of the origin and insertion of the muscle, that, when united with its fellow of the opposite side, a horseshoe-shaped trough or gutter is made (see Fig. 2), and that the *Dickinson, loc cit,

sides of this gutter, being smooth, would naturally tend to carry any object impinging on them to the lowest point of junction, which is in the median line; and as the bottom of this trough slopes anteriorly, the object would be carried on toward the vaginal orifice in exactly the same way that water runs off a roof into the eaves-trough, thence to the ground. (See Fig. 3.)

Bearing in mind that the axis of the uterus crosses the axis of the vagina at an angle of about 95°, let us consider the mechanism of labor, taking first the occipito-anterior positions. First there is flexion, then descent of the head through the pelvis in a straight line (following the

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line of the axis of the uterus) until it reaches the side of the trough at the floor of the pelvis; then it begins to rotate from the oblique diameter in which it started to the antero-posterior diameter. When the head reaches the bottom of the trough it has fully rotated-i. e., the occipito-frontal diameter of the head corresponds with the antero-posterior diameter of the pelvic trough-and, being at the extreme end of the uterine canal, can of course go no further until extension begins. Why? The head, being at the lower extremity of the uterine canal, can go no further in that direction; but on the anterior side of this extremity is the vaginal canal, placed, as has already been stated, at an angle of about 95° with

the uterine canal. In other words, the head is at the utero-vaginal angle, which it can pass only by extension, and that is accomplished in the following manner: The uterine contractions are constantly crowding the child down into the pelvic groove, the pressure being exerted through the spine to its junction with the head. The result of this crowding down must be that the head will either be extended or further flexed. It cannot be further flexed, because the chin already rests on the sternum ; consequently it must be, and is, extended, and in this way: The head is now in the hollow of the sacrum and fully flexed on the body. Its progress is barred in every direction save one, viz., the opening into the vaginal canal. Pressure from above tends to force the occiput in the direction where there is least resistance; the head glides over the smooth surface of the sacrum, and extension begins. (See Fig. 4.) This allows the body to descend a little, which pushes the occiput still further into the vaginal canal, thereby increasing the extension. This process continuing, more and more of the head is forced around the angle, until the occiput emerges from the vaginal orifice under the pubes, when it is immediately followed by the forehead, nose, and chin. Meanwhile, a shoulder being forced down against the side of the groove causes the body to rotate, the shoulders are pushed around the angle, the body is bent laterally, and the shoulders are delivered in precisely the same manner that the head was, i.e., by being bent around the angle and under the pubes. The hips follow the same course, and, being smaller, are easily forced through the now greatly dilated canal.

In occipito-posterior cases, when anterior rotation does not occur, labor is much more difficult-not, as we have been taught, because it takes the occiput so much longer to sweep over the perineum, but because it is extremely difficult for an already flexed head to be further flexed so that it may pass the utero-vaginal angle.

The head in occipito-posterior positions, when it reaches the extremity of the uterine canal, is flexed on the body; and as pressure is exerted through the spine in the same manner as in occipito-anterior positions, the tendency is to do one of two things: either to begin extension by crowding the face into the vaginal opening, thus converting it into a face presentation, or to still further flex the head on the body. In the latter case it takes so long to turn the angle that the head is moulded so that it may be adapted to the canal; and even then, unless the back bends a little, it is doubtful if it can be delivered unassisted. But the back bends a little, the head passes the angle, and finally the parietal protuberances emerge from the vaginal orifice under the pubes, followed by the forehead, nose, and chin, and, as the text books say, "the head is extended over the perineum," while in reality it drops on to the perineum simply of its own weight. At the same time the shoulders come down and are

Yulve.

Fig. 5.

delivered as in occipito-anterior cases. In occipito-posterior cases the perineum is enormously distended, because the head cannot be flexed to a right angle with the body, and an imperfectly bent object is being forced around the angle. (See Fig. 5.)

In face presentation, mento-anterior, there is extension, descent to the pelvic trough, rotation, and, in precisely the same manner as in occipito-anterior cases, the face is pushed into the vaginal canal, and flexion begins. (See Fig. 6.) The face is forced further and further into the vagina, increasing the flexion, until the chin emerges under the pubes, followed by nose, forehead, and occiput. Meanwhile the body descends, rotates, and is delivered as in occiput cases.

In mento-posterior cases, when anterior rotation of the chin does not occur, there is extension, descent, rotation, and further progress is impossible. Why? Because the head is fully extended on the body, and when it reaches the extremity of the uterine canal it cannot be further extended so as to allow it to pass the utero-vaginal angle. The body cannot be bent backward sufficiently to allow the head to pass the angle, therefore labor is arrested-not because the neck is 11⁄2 inches long, and the chord of the sacro-coccygeal curve is 41⁄2 inches long, nor because the sternum impinges on the sacral promontory, but

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