REPRODUCTION. 



mucous membrane of the uterus, to push against the walls of the 

 large vessels found there and whose structure is similar to that of 

 capillaries. The cells comprising the villi act as an osmotic mem- 

 brane through which osmosis occurs. By this means oxygen and 

 nutritive lymph pass from the mother's blood to that of the foetus. 

 On the other hand, the foetal blood gives off carbon dioxide and 

 probably urea. There is no intermingling of the two blood-currents, 

 since there is always a layer of epithelium to act as a limiting mem- 

 brane. 



Foetal Circulation. The blood is brought to the body of the 

 foetus by the umbilical vein. Some of this oxygenated blood passes 

 through the liver to the hepatic veins, to be emptied into the inferior 

 vena cava. The remainder of the umbilical blood passes into the 

 inferior vena cava through the ductus venosus. 



The blood, mixed with that which is returned from the lower 

 extremities, enters the right auricle. Guided by the Eustachian 

 valve, it passes over into the left auricle through the foramen ovale. 

 The blood now courses through the left ventricle, aorta, the hypo- 

 gastric and umbilical arteries to the placenta. 



The blood is returned from the head and the upper extremities 

 to the right auricle by the superior vena cava. This stream of blood 

 passes through the auricle and auriculo-ventricular opening directly 

 into the right ventricle, guided by the tubercle of Lower. The blood 

 next passes into the pulmonary artery. Some of it (enough to nour- 

 ish the solid lung-substance) passes to the lungs, but the major por- 

 tion passes into the aorta through the ductus arteriosus. When in 

 the aorta it takes the course of the blood from the left ventricle to 

 finally reach the placenta. The blood to the lungs returns to the 

 left auricle through the pulmonary veins. 



After lirth the umbilical arteries are obliterated with the excep- 

 tion of their lower portions, which remain as the superior vesical 

 arteries. The umbilical vein becomes obliterated and remains as the 

 round ligament of the liver. The umbilicals become impervious soon 

 after cessation of the placental circulation. 



The foramen ovale closes, thereby cutting off communication 

 between the right and left hearts. By the second or third day the 

 ductus arteriosus has also become obliterated, to be present in adult 

 life as the ligamentum arteriosum. 



These changes in the circulatory apparatus are dependent upon 

 the establishment of pulmonary respiration at birth. The first in- 

 spiration is said to be due to a sensory reflex from the colder air 



