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.