290 EMBRYOLOGY 



centration of estrone may have the same effect as a low concentration of 

 progesterone. Perhaps the ratio 



estrone 



progesterone 



increases before birth so that muscular contraction occurs. On the other hand, 

 the growth of the fetus may produce mechanical effects such as the stretching 

 of uterine muscle, and stretching is an adequate stimulus for the contraction 

 of some muscle fibers. No simple explanation of the onset of labor is as yet 

 accepted generally. 



If the pituitary is not absolutely necessary for birth, it is nevertheless 

 helpful, and repeated injections of pituitrin from the posterior pituitary are 

 sometimes used to induce labor. 



Labor is a process of rhythmic and extensive contractions of the uterine 

 muscles resulting in the expulsion of the fetus. The abdominal muscles also 

 play an important role. The contractions of these muscles exert a force on 

 the amniotic fluid and push the head of the fetus through the opening of 

 the uterus and through the vagina. Possibly the pelvic ligaments have been 

 previously relaxed by the action of relaxin secreted by the corpus luteum. 



After the fetus is completely expelled, it is still attached to the placenta 

 inside the uterus by means of an umbilical cord about two feet long. The 

 cord is clamped and cut. If not clamped, the umbilical artery constricts and 

 shuts off the blood flow from the newborn child to the placenta. Part of the 

 blood from the placenta then drains back through the umbilical vein into the 

 child. 



Next, breathing begins, no doubt stimulated by the temporary lack of 

 oxygen and excess carbon dioxide in the blood acting upon the respiratory 

 center. As noted previously, experiments on the cat fetus show the respiratory 

 center to be active before birth. For the carbon dioxide to stimulate the 

 respiratory center, the blood must reach the brain, and it may be that the 

 increase in blood pressure with the clamping of the cord sends more blood 

 to the brain. Another factor may be an increased tonus of the respiratory 

 muscles, caused by the change in temperature and exposure to air at birth. 



With the onset of breathing the fetal circulation undergoes changes. Even 

 before birth some blood flows through the lungs, but most of the blood com- 

 ing from the right ventricle is shunted into the systemic artery by the ductus 

 arteriosus. After birth this vessel gradually becomes occluded by a mechanism 



