=The Course of Labor.=—Labor is divided for convenience into three stages as follows:
The _first stage_, from the beginning of pains until the complete dilatation of the os.
The _second stage_, from the complete dilatation of the os to the delivery of the child.
The _third stage_, from the delivery of the child to the expulsion of the placenta.
The _first stage_ is the stage of dilatation.
Usually at term, the cervix is columnar and unshortened, the canal intact, and closed at both ends, as shown in Fig. 36.
In multiparas the outer opening will usually admit the tip of the finger.
As labor proceeds, the cervix is _effaced_, the os slowly dilates, and the bag of waters forms.
_The Bag of Waters._—When the cervix is effaced and only the os remains, the lower end of the egg with its fluid restrained by the membranes, bulges forward into the canal. The fœtal head, or breech presses into the pelvis, and the fluid in the membranes, compressed between the presenting part above and the cervix below, is called the bag of waters.
When the contraction comes on the longitudinal muscular fibers of the uterus are drawn upward and the bag of waters becomes tense and pushes farther and farther down into the opening; and by its even and universal pressure, mechanically and slowly increases the size of the opening which the muscular traction is pulling apart. At the same time, the fluid around the child prevents, for a time, direct and injurious compression on the body. When no definite cervical projection can be felt, and when the teat-like protrusion of the cervix has disappeared, the cervix is said to be effaced.
[Illustration: Fig. 35.—The bag of waters begins to act on the cervix. (Eden.)]
The os now begins to stretch and widen, the bag of waters becomes more and more evident, vomiting occurs, and at last, when the os has expanded to a diameter of four inches (ten centimeters), the membrane can withstand the pressure no longer. It ruptures, a certain amount of fluid escapes, the presenting part comes down against the opening, and like a valve, prevents the outflow of the waters from above.
[Illustration: Fig. 36.—The effect of the pains. The cervix before labor begins. (Bumm.)]
[Illustration: Fig. 37.—The effect of the pains. The cervix begins to be “effaced.” (Bumm.)]