_Straining pains._ As the head enters the vagina, not only do the contractions of the uterus become much more powerful, but now another set of forces are called into action, and the half involuntary efforts of the abdominal and other muscles come to aid the uterus in expelling its contents. The sole object of this stage is the expulsion of the child, and even the vagina by its contractions contributes to effect it. The head is therefore subjected to considerable pressure; hence we may now feel the cranial bones overlapping each other at the sutures, and the fontanelles diminished in size; and, from the tightness with which the head is embraced by the vagina, the circulation in the scalp is more or less impeded, and a large oedematous swelling, called _caput succedaneum_, forms on that part of the head which presents. Each pain is attended with a violent and irresistible impulse to bear down, and every muscle which can assist in effecting this object is now brought into play. The tone of the patient's voice, the expression of her face, the hurried breathing and sudden inspiration, stopping short the moment a pain comes on, in order that she may add still greater power to the efforts which she is about to make, all betoken a very different process to that of the first stage, and one which requires a powerful struggle of muscular strength and energy for its completion. Hence it is that the sound of the patient's voice during the pain is frequently of itself sufficient to inform us how far labour is advanced, for "we never see the really powerful straining pains come on (the head may be never so low in the pelvis,) so long as the os uteri is not fully dilated." (Wigand, _op. cit._ vol. ii. p. 310.) This is a wise provision of Nature, for by this means it prevents the danger of laceration to which the os uteri would be otherwise exposed, and shows the importance of not permitting a patient to strain and bear down until the os uteri be fully dilated. In those cases where a patient has been induced to exert herself prematurely, the efforts being voluntary are never so powerful, and soon produce much fatigue.