7. The Dilatation of the Vagina and of the Perineum.
At the beginning of labor the vagina, which is already shortened by the engagement of the foetus, is farther dilated and enlarged by the de scent of the latter, at the expense of its length. It does not, however, yield so easily in its lower portion and particularly not at its anterior ex tremity. Budin's researches, already described in our remarks on the anatomy of the vagina and hymen, show that, particularly in primiparfe, the vaginal orifice may resist several hours, and that this resistance, attri buted up to the present time to the perineum, must, in the main, be referred to this vaginal orifice, since the vulva offers but little resistance when the ostium vaginae is once overcome.' We hold to Budin's opinion, and have often seen, in primiparte of course, this vaginal orifice pushed through the vulva and projecting as a little livid prominence, .19 inches broad. In this case the head only escapes after little lacerations have occurred in the projecting vaginal fold. In one case, Budin succeeded in bringing a labor, which was protracted without other cause, to a close by dividing the vaginal orifice, the vulva and perineum being left untouched. But the resistance of the perineum also plays an important part. Its di mensions in a state of repose, as compared with those it attains at the end. of labor, when the head is being expelled, prove the above statement.
The perineum, which normally is only from .39 to an inch long, at the outside, often attains a length of seven inches, and this distension is only slowly produced by the incessant pressure of the head. At each pain, in deed, the head, having reached the pelvic floor, presses upon the perineum, which it causes to project more and more, while it pushes out the anus, which opens and reveals a certain area of the rectal mucous membrane. Then, as the contraction ceases, the perineum retracts, in virtue of its elasticity, and the head, which was appearing at the vulva, disappears within the vagina to reappear with the next pain. At the end of a vari able time, this head, which has appeared and disappeared several times, each time showing a greater extent of its surface, becomes fixed under the symphysis. It recedes no more, and the over-distended perineum forms only a kind of thin, bluish membrane which seems ready to rupture at the slightest effort. The head rests thus immovable for some instants, and finally, a last contraction expels it, and the perineum glides over it from before backward, passing successively over the large fontanelle, the forehead and the face of the child. At this moment the perineum is, ordinarily, torn. Hence the indispensable precautions which we will study, in detail, when we come to the management of labor.