THE FCETAL CIRCULATION. 841 



right ventricle passes in small quantity to the lungs, the greater part passing 

 through the ductus arteriosus, into the descending portion of the arch of 

 the aorta. This duct is half an inch (12-7 mm.) 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. 



In Fig. 315, which is diagrammatic, the foetal circulation is illustrated. 

 In endeavoring, in this figure, to give a clear idea of the second circulation, 

 no attempt has been made to preserve the exact relations or the relative size 

 of the organs. The Eustachian valve, the foramen ovale and the two auric- 

 ulo-ventricular orifices are represented by dotted lines. The liver and the 

 bladder are also represented by dotted lines. 



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

 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 dis- 

 tended 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, upon 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 foramen ovale has nearly 

 closed. The arrangement of the valve of the foramen ovale is such that at 

 birth a small quantity of blood may pass from the right to the left auricle, 

 but none can pass in the opposite direction. The situation of the Eustachian 

 valve, on the right side of the interauricular septum, is marked by an oval 

 depression, called the fossa ovalis. 



As a congenital malformation, the foramen ovale may remain open, pro- 

 ducing the condition known as cyanosis neonatorum. This may continue into 

 adult life, and it is then attended with more or less disturbance of respiration 

 and difficulty in maintaining the normal heat of the body. Usually the fora- 

 men ovale is completely closed at about the tenth day after birth. The ductus 

 arteriosus begins to contract at birth, and it is occluded, being reduced to 

 the condition of an impervious cord, between the third and the tenth days. 



When the placental circulation is arrested at birth, the hypogastric arter- 

 ies, the umbilical vein and the ductus venosus contract, and they become 



