THE THORAX 999 



ductus venosus, and partly indirectly by means of the hepatic veins 

 after having circulated through the liver. 



A small quantity of the mixed blood which enters the right 

 auricle by the inferior vena cava mingles with the venous blood 

 which enters that cavity by the superior vena cava ; but by far the 

 greater part of it is directed by the Eustachian valve to the foramen 

 ovale, through which it passes into the left ventricle. Here it 

 receives the blood which has been returned from the lungs by the 

 pulmonary veins, and then it passes through the left auriculo- 

 ventricular orifice into the left ventricle. From the left ventricle 

 it is driven into the aorta, and the greater part of it passes into 

 the innominate, left common carotid, and left subclavian arteries 

 to be distributed to the brain, head and neck, and upper limbs, as 

 well as to the substance of the heart by the coronary arteries. A 

 small quantity, however, of this mixed blood passes into the de- 

 scending thoracic aorta, and mixes with the venous blood entering 

 by the ductus arteriosus. The blood is returned to the right 

 auricle of the heart from the brain, head and neck, and upper 

 limbs, by the superior vena cava, and from the substance of the 

 heart by the coronary sinus. 



Changes at Birth. — At birth respiration is established, and the 

 lungs perform their respiratory function ; the right and left pul- 

 monary arteries undergo rapid enlargement ; and the placental 

 circulation is entirely arrested. The hypogastric arteries become 

 obliterated and transformed into fibrous cords, except at their 

 roots, where they persist as the internal iliac arteries. The um- 

 bilical vein becomes obliterated, and is now known as the round 

 ligament of the liver. The ductus venosus becomes transformed 

 into a fibrous cord, called the ligamentwn ductus venosi, which 

 occupies the venosal fissure of the liver. The foramen ovale is 

 closed by the valve of the foramen ovale becoming adherent to the 

 margin of the annulus ovalis or limbus Vienssenii on its left side. 

 In some cases, however, this union is incomplete, and a ndnute 

 opening persists, through which a small probe may be passed. In 

 yery rare cases a fairly large opening may remain, as in the con- 

 dition known as morbus ccertdeus. As viewed from the interior of 

 the left aiu-icle, the upper crescentic border of the valve of the 

 foramen ovale is visible upon the interauricular septum, and above 

 this border there is a slight depression. The ductus arteriosus 

 becomes obliterated, and persists as a fibrous cord, called the 

 ligamentum ductus arteHosi, which passes from the root of the left 

 pulmonary artery to the arch of the aorta immediately beyond the 

 origin of the left subclavian artery. 



The closure of all the peculiar structures associated with. the 

 fcetal circulation is usually complete from the eighth to the tenth 

 day after'birth. 



Development of the Principal Arteries. — In the embryo the aortic bulb or 

 bulbus arteriosus divides into two vessels, called the primitive ventral aorts, 

 right and left, which pass cephalicwards on the ventral aspect of the fore-gut. 



