[Illustration: Fig. 48.—Extension. _A_, the chin leaves the chest; _B_, extension in progress. (Eden.)]
[Illustration: Fig. 49.—_A_, extension completed; _B_, expulsion. (Eden.)]
Rotation is much retarded or entirely stopped when the head is extended instead of flexed or when it enters the inlet with the occiput posterior instead of anterior.
=Extension.=—After internal, anterior rotation, the head emerges at the vulva, the occiput coming out first, then in succession the vertex, forehead and face and chin. As the chin rolls out over the perineum, it moves away from the chest wall—it becomes extended.
=External Restitution.=—While the head is passing through the outlet, the shoulders are entering the pelvic inlet, and so soon as the head is released from the restraint of the vagina, it naturally falls into its normal relation to the fœtal back; hence in the position now discussed, it turns toward the left.
Therefore, we may summarize the mechanism in a normal left-occipito-anterior position of the head by saying: The head is flexed and forced into the pelvis. It descends to the pelvic floor. The occiput rotates to the front of the pelvis and impinges against the symphysis. Extension ensues in consequence of the necessity for an accommodation between the pelvis and the advancing head, and during this extension, the head delivers over the perineum. External restitution follows.
=The Effect of Labor on the Fœtal Head.=—As the head passes through the canal, it is _moulded_ by contact with the resistances. The degree of moulding is proportionate to the pressure required to drive it through. Thus, in a large head, or a relatively small pelvis, the moulding may be extreme, and changes in the scalp are common.
_Caput Succedaneum_.—Since all parts of the scalp are in contact with a resistant wall, except in the center of the birth canal, an effusion of serum takes place here, which is due to the obstruction of the venous circulation.
[Illustration: Fig. 50.—A cephalhæmatomata. Do not confuse with caput succedaneum. (Bumm.)]
Swelling occurs in the subcutaneous cellular tissue, and a tumor forms—the caput succedaneum—which spontaneously disappears in twenty-four or forty-eight hours. It is useful in confirming the diagnosis of the position.