=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. =Cephalhæmatoma.=—Following labor a tumor is sometimes found upon the head, which is often confused with a caput succedaneum. This tumor is caused by an effusion of blood beneath the periosteum or the covering of the bone—usually a parietal bone. It is sometimes single and sometimes double, and it varies in size from a filbert to a peach. The swelling never extends across a suture. The effusion takes place gradually, and may not appear for a day or so after birth. The cause is unknown, for it occurs after normal and easy, as well as after difficult, deliveries, and after breech, as well as vertex, cases. At first it fluctuates, then becomes hard, and in a few weeks or months is gradually absorbed. If symptoms of cerebral pressure develop, it must be remembered that hæmatoma may occur inside as well as outside the cranium. No treatment is necessary. Puncture is inadvisable. In extremely rare instances the tumor may suppurate and require incision. CHAPTER IX THE CARE OF THE PATIENT DURING NORMAL LABOR