848 EMBRYOLOGY 



of blood ; the one, from the vena cava ascendens, through the foramen 

 ovale, passes into the left auricle and the left ventricle ; and the other, 

 from the vena cava descendens, passes through the right auriculo- 

 ventricular opening, into the right ventricle. It is probable, indeed, 

 that there is very little admixture of these two currents of blood in the 

 natural course of the foetal circulation. 



The blood poured into the left auricle, from the vena cava ascendens, 

 through the foramen ovale, passes from the left auricle into the left 

 ventricle. The left auricle and the left ventricle also receive a small 

 quantity of blood from the lungs, by the pulmonary veins. Thus the 

 left ventricle is filled. At the same time the right ventricle is filled with 

 blood which has passed through the right auricle, in front of the 

 Eustachian valve. The two ventricles, thus distended, then contract 

 simultaneously. The blood from the right ventricle passes in small 

 quantity to the lungs, the greater part passing through the ductus 

 arteriosus, or ductus Botalli, into the descending portion of the arch of 

 the aorta. This duct is half an inch (12.7 millimeters) in length and 

 about the size of a goose-quill. The blood from the left ventricle passes 

 into the aorta and goes to the system. The vessels of the head and 

 superior extremities being given off from the aorta before it receives the 

 blood from the ductus arteriosus, these parts receive almost exclusively 

 the pure blood from the vena cava ascendens, the only mixture with the 

 placental blood being the blood from the lower extremities, the blood 

 from the portal system and the small quantity of blood received from 

 the lungs. After the aorta has received the blood from the ductus 

 arteriosus, however, it is mixed blood ; and it is this which supplies 

 the trunk and lower extremities. 



The Third, or Adult Circulation. When the child is born the pla- 

 cental circulation is suddenly arrested. After a short time the sense of 

 want of air becomes sufficiently intense to give rise to an inspiratory 

 effort, and the first inspiration is made. The pulmonary organs are 

 then for the first time distended with air, the pulmonary arteries carry 

 the greatest part of the blood from the right ventricle to the lungs and a 

 new circulation is established. During the later periods of foetal life, 

 the heart is gradually prepared for the new currents of blood. The 

 foramen ovale, which is largest at the sixth month, after that time is 

 partly occluded by the gradual growth of a valve which extends from 

 below upward and from behind forward on the side of the left auricle. 

 The Eustachian valve, which is also largest at the sixth month, gradually 

 atrophies after this time, and at full term it has nearly disappeared. 

 At birth, then, the Eustachian valve is practically absent ; and after 

 pulmonary respiration becomes established, the foraman ovale has nearly 



