668 TEXT-BOOK OF PHYSIOLOGY. 



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 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 con- 

 tracts, and shunts 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 membranous fold grows upward 

 and backward from the edge of the foramen ovale on the left side; 

 the ductus arteriosus also contracts. With the first inspiration and 

 the expansion of the lungs, the blood which enters the pulmonary 

 artery passes through the pulmonary capillaries in large volume and 

 is returned by the pulmonary veins to the left auricle. The en- 

 trance 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 ven- 

 tricle, which is favored by the small size of the Eustachian 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 hypogastric arteries remain open and carry blood to 

 the walls of the bladder. 



Lactation. As pregnancy advances the mammary glands in- 

 crease 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 resembles 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 coagulates 

 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, page 401. 



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 

 undifTerentiated blastodermic membranes gradually increases in 



