PREGNANCY, PARTURITION, AND LACTATION 447 



irregularly rather than in a complete rhythm, and each is harder than the 

 preceding. In consequence every succeeding contraction gives rise to 

 greater pain than the one before it. The first pains of labor are caused 

 by the violent stimulation of the uterine nerves, whose delicate endings 

 are injured by the strong compression of the hardening uterine walls. 

 As the compressions become stronger the pain is radiated over the abdo- 

 men generally. Each peristaltic wave leaves the uterus narrower and 

 in consequence longer than before, but the pressure in the sac about the 

 fetus does not reach more than that of 10 cm. of mercury. The liquid 

 forms a sac in front of the now somewhat advancing head, and serves 

 until the sac ruptures as the best of dilators for the external mouth of 

 the womb. 



In the second stage of labor the abdominal muscles are put in action 

 to aid the uterine contractions, and rhythmically contract, thus powerfully 

 pushing from above upon the elongated uterus now closely pressing the 

 child on all sides save in front. The vagina is, then, the path of least 

 resistance, and gradually and slowly the advancing head makes its way 

 through it along the curve of the pelvis. At last, after a few minutes 

 of excessive pain (properly always relieved by an anesthetic), caused by 

 the over-stretching of the nerve-filled vulva, the relatively rigid head 

 emerges from the maternal body. The softer shoulders and body so on 

 follow, and the second stage of labor is finished. These two stages 

 differ greatly in length. The first stage averages about thirteen hours 

 in primiparse and nine hours in multipart, while the second stage lasts 

 about one-and-one-half hours in both. Sometimes, however, the first 

 stage lasts two days or more, and seldom less than an hour, depending 

 on the variable relations of the size of the child, of the pelvis, and the 

 muscular powers of the mother. 



The third stage is the expulsion of the placenta by the uterine contrac- 

 tions. It is usually complete within ten or twenty minutes after the child 

 is born. The coming-away of the placenta gives rise to more or less 

 hemorrhage, the average amount being about 400 c.c. After the pla- 

 centa is delivered the uterus normally contracts strongly, thus preventing 

 further and dangerous hemorrhage. 



The muscular phenomena of labor may occur by direction of the spinal 

 cord alone, for the whole process from conception to delivery is possible 

 in dogs whose spinal cords have been severed at the first lumbar vertebra 

 (Goltz). As we have said, the movements of the uterus are largely 

 "automatic," but are regulated and adapted by the inherent nerves. 

 They may be initiated by reflex impulses from any part of the reproduc- 

 tive tract (e. g., the nipples) or even from the brain, as when fright causes 

 miscarriage. 



Lactation. After the muscular and mental strain of labor a few hours 

 of sleep are usually necessary and are very beneficial. The child being 

 then put to the breast, the sucking-process reflexly causes the uterus to 

 contract, thus more surely preventing hemorrhage. It also removes the 

 colostrum from the mammary glands and makes them ready to secrete 



