918 AN AMERICAN TEXT-ZOOK OF PHYSIOLOGY. 



and introduced into the uterus. During the earlier part of parturition the 

 contractions gradually increase in intensity up to a maximum which they 

 then maintain. Their rhythm is somewhat irregular ; the duration of each 

 contraction averages about one minute, and a pause, which ensues between suc- 

 cessive contractions, extends from one and one-half to several minutes. The 

 relaxation of the muscle-fibres during the period of rest is incomplete, the 

 result being that the fibres enter gradually into a tonically contracted state. 

 Each contraction is accompanied by a pain, localized in the early part of labor 

 in the uterus alone, but later extending outward, upward into the abdomen, 

 and downward into the thighs. The pains of labor vary greatly in intensity 

 in individuals, but are usually more intense during the first gestation than 

 during later ones. They are due chiefly to direct mechanical stimulation of 

 the sensory uterine and other nerves by compression, tension, and even lacera- 

 tion. 



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 fluid 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 cervix. The head of the 

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

 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 fluid 

 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 cease 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 



