480 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



As a result of the tonic contraction of the uterine walls, gradually increas- 

 ing with each new peristaltic wave, the uterus becomes gradually narrower in 

 diameter and longer, and the walls press more and more firmly upon the bag 

 of amniotic liquid containing the embryo. Schatz finds that the uterine pres- 

 sure under the uterine contractions rarely reaches and never exceeds 100 milli- 

 meters of mercury. The direction of least resistance to this pressure lies 

 along the cervical canal, the walls of which do not take part in the uterine labor. 

 With each succeeding contraction this canal is forced wider open and the uterine 

 contents are pressed tightly downward and into the eervix. The head of the 

 embryo is preceded by a bulging portion of the membrane, filled with liquid 

 and forming a distinct bladder-like advance-guard. This bag appears at the 

 os uteri, its contents increase under the increasing pressure, and in the majority 

 of cases, when the os is fully expanded, it bursts and allows the amniotic liquid 

 to escape to the exterior. In some cases the rupture is delayed until the sec- 

 ond stage of labor, and rarely the child is born with the membranes intact. 



Second Stage of Labor. — The uterine contractions frequently eease for a 

 period following the rupture of the membrane. They then begin anew with 

 increased force, and are accompanied by a new feature, namely, analogous 

 vigorous rhythmic contractions of the muscles of the abdominal walls. These, 

 following deep inspiration and accompanied by forced attempts at expiration 

 with a closed glottis, diminish the longitudinal and the lateral diameters of the 

 abdominal cavity, compress the abdominal organs, and help to augment greatly 

 the uterine pressure. At the beginning of the second stage the force of the 

 contractions is expended mainly upon the head of the embryo, which lies like 

 a plug in the cervical canal. This is squeezed gradually through the os into 

 the vagina, followed by the more easily passing trunk and limbs. The con- 

 tractions are frequent, vigorous, and painful, the pains reaching a maximum 

 as the sensitive vulva is put upon the stretch and traversed. The vertex is 

 usually presented first to the exterior, the head and body following as the suc- 

 cessive contractions of the maternal muscles develop sufficient power to over- 

 come the resistance offered to their passage by the surrounding walls. In 

 the human female the vaginal muscles do not appear to engage in the expel- 

 ling act, the uterine and the abdominal muscles alone sufficing and finally 

 forcing the child wholly outside the mother's body. In this gradual manner, 

 painful and dangerous alike to mother and child, the maternal organism forces 

 the offspring to forsake its sheltering and nutritive walls and begin its inde- 

 pendent existence. 



Third Stage of Labor. — During the later expulsive contractions of the 

 second stage the placenta, being greatly folded by the diminution in the uterine 

 surface of attachment, is loosened from the uterine wall by a rupture taking 

 place through the Loose tissue in the region of the blood-sinuses. The child, 

 when born, is joined to the loosened placenta by the umbilical cord, until the 

 latter is tied and cut by the obstetrician. The muscular contractions, now 

 almost painless, continue through the third stage, and the placenta is torn 

 from its attachment, everted, and carried gradually outward. The lining 



