702 TEXT-BOOK OF PHYSIOLOGY. 



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- 

 monary artery passes through the pulmonary capillaries in large volume and 

 is returned by the pulmonary 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- 

 ulates on boiling, and certain inorganic salts which have a laxative effect 

 on the new-born child. Normal lactation and the phenomena which 

 accompany it are fully established by the end of the second or third day. 



The composition of milk and the mechanism of its production have been 

 stated in the chapter on Secretion. 



Physiologic Activities of the Embryo. During intrauterine life the 

 evolution of structure is accompanied by an evolution of function. The 

 relatively simple and uniform metabolism of the undifferentiated blastoder- 

 mic membranes gradually increases in complexity and variety, as the individ- 

 ual tissues and organs make their appearance and assume even a slight 

 degree of functional activity. As to the periods at which different organs 

 begin to functionate, but little is positively known. 



