The abdominal and respiratory muscles are brought into active use during the second stage, contracting simultaneously with the uterine muscles and increasing their expulsive force. These are apparently controlled by the patient’s will at first, and she is able somewhat to increase their power by taking a deep breath, closing her lips, bracing her feet, pulling against something with her hands, straining with all her might and “bearing down.” Finally, however, the whole bearing down process becomes involuntary, is accompanied by intense pain and the deep grunting sound, which is characteristic of the well-advanced second stage. Under normal conditions, the child descends a little farther into the pelvis with each contraction, and finally the presenting part begins to distend the perineum and to separate the labia advancing at the height of each pain and slipping back a little as it subsides. [Illustration: FIG. 69.—Anterior shoulder being slipped from under symphysis to facilitate birth of posterior shoulder. ] The baby descends into and through the mother’s pelvis by means of a series of twisting and curving motions, accommodating the long axes of its head to the long diameters of the pelvis. The head being somewhat compressible and mouldable, because of imperfect ossification, is capable of a good deal of accommodation to the mother’s pelvis. The mechanism of labor, therefore, is virtually a series of adaptations of the size, shape and mouldability of the baby’s head to the size and shape of the mother’s pelvis. If the head passes through the inlet satisfactorily, the rest of the labor will usually be accomplished with comparative safety. But a marked disproportion between the diameters of the head and pelvis may interfere with the engagement or descent of the head and produce a serious complication. [Illustration: FIG. 70.—Delivery of posterior shoulder. ] The long diameter of the head must first conform to one of the long diameters of the inlet, usually oblique, and then turn so that the length of the head is lying antero-posterior in conformity to the long diameter of the outlet through which it next passes. As the head descends and rotates it also describes an arc because the posterior wall of the pelvis, consisting of the sacrum and coccyx, is about three times as deep as the anterior wall formed by the symphysis. That part of the baby’s head which passes down the posterior wall of the pelvis must therefore travel three times as far in a given time as the part which simply slips under the short symphysis pubis. [Illustration: FIG. 71.—Diagrams showing Duncan and Schultze mechanisms of placental separation. ]