754 TEXT-BOOK OF PHYSIOLOGY 



exerted from all sides on the body of the child, the head gradually descends 

 still further into the vagina and finally emerges through the vulva to be 

 followed in a short time by expulsion of the trunk and limbs, and a discharge 

 of the remaining amniotic fluid. With the expulsion of the child the uterine 

 contractions cease for a period of ten or fifteen minutes, when they again 

 recur, with the result of detaching the placenta and expelling it into the 

 vagina. It is then removed by the cooperative action of the abdominal 

 and perineal muscles. The hemorrhage which would naturally occur with 

 the detachment of the placenta and the laceration of the maternal vessels 

 is prevented by the firm continuous contraction of the uterine walls, by 

 which the vessels are compressed and permanently closed. 



The Establishment of Inspiration and the Adult Circulation. 

 After the birth of the child and the detachment of the placenta, there speedily 

 occurs a decrease in the quantity of oxygen and an increase in the quantity 

 of carbon dioxid in the blood, a condition which causes a discharge of nerve 

 energy from the inspiratory center, a contraction of the inspiratory muscles, 

 an expansion of the thorax, and an inflow of air into the lungs. 



In addition it is very probable that the stimulation of the inspiratory 

 center is also occasioned by the arrival of nerve impulses from the skin, 

 developed by the cooling of the skin due to the vaporization of the amniotic 

 fluid. 



In the later months of intrauterine life the vascular apparatus undergoes 

 certain anatomic changes which favor the transition from the placental to 

 the adult circulation. Thus the ductus venosus contracts, and sends a 

 a larger volume of blood into and through the liver; the Eustachian valve 

 diminishes in size and at the time of birth has almost disappeared; a mem- 

 branous fold grows upward and backward from the edge of the foramen 

 ovale on the left side; the ductus arteriosus also contracts. With the first in- 

 spiration and the expansion of the lungs, the blood which enters the pul- 

 monic artery passes through the pulmonic capillaries in large volume and 

 is returned by the pulmonic veins to the left auricle. The entrance of 

 the blood into this cavity presses the membranous fold against the margins 

 of the foramen ovale and thus prevents the further flow of blood from the right 

 auricle. The blood entering the right auricle by the inferior vena cava now 

 flows into the right ventricle, which is favored by the small size of the Eu- 

 stachian valve. The foramen ovale is permanently closed at the end of a 

 week or ten days; the ductus arteriosus at the end of four days. The um- 

 bilical vein and ductus venosus, at the end of four or five days, have also 

 become almost impervious from the contraction of their walls. The 

 proximal ends of the hypogastric arteries remain open and carry blood to the 

 walls of the bladder. The distal ends of the arteries are converted into im- 

 pervious cords. 



Lactation. As pregnancy advances the mammary glands increase in 

 size, partly from a deposition of fat and connective tissue and partly from 

 a multiplication of the secreting acini. The lining epithelial cells at the same 

 time increase in size, and toward the end of pregnancy begin to exhibit 

 functional activity. At the time of birth, or within a day or so after birth, 

 the acini are filled with a fluid which in its qualitative composition resem- 

 bles milk and is known as colostrum. It is distinguished from milk more 

 especially in the fact that it contains in large quantity a proteid which coag- 



