620 



THE HEART. 



artery, being connected to the latter by their circumference, 

 and having, in the centre of each loose edge, a little white 

 or yellowish body called corpusculum Arantii. These cor- 

 puscles serve as abutments to support each other when the 

 valves are brought together, and thus prevent the blood 

 from regurgitating. Exterior to these valves, and between 

 them and the artery, are three pouches called the sinuses of 

 Valsalva. 



The pulmonary artery ascends obliquely backward to 

 the under part of the arch of the aorta, where it divides 

 into two branches, one for each lung. The right is both 

 longer and larger ; it goes behind the aorta and superior 

 cava to the root of the right lung, where it divides into 

 three branches. The left is shorter, and goes in front of 

 the descending aorta to the root of the left lung where it 

 divides into two branches. The distribution of these 

 branches has been already given in the description of the 

 lungs. Where the pulmonary artery divides into its right 

 and left branches, a ligamentous cord is seen to extend 

 backward and downward to the lower extremity of the 

 arch of the aorta. This, in the foetus, was an open tube 

 called the ductus arteriosus, through which passed to the 

 aorta the balance of the blood which failed to go through 

 the foramen ovale, the pulmonary arteries carrying to the 

 lungs only so much as was just sufficient for their nutrition. 



Left heart, (Fig. 200,) Left auricle. The pulmonary 

 veins, four in number, two from each lung, return the blood, 

 after it has been changed from venous into arterial, into 

 the left auricle. This auricle is situated at the superior 

 and back part of the base of the heart, being hid by the 

 ventricles and right auricle. Its shape is more of a square 

 than the right, and it has a pulmonary vein entering each of 

 its angles. It consists, like the right, of a sinus and appen- 

 dix. Its walls are thicker and stronger than those of the 

 right auricle, though its cavity is smaller. Its appendix 

 contains ihemusculipectinati, and is more indented, crooked 

 and narrower, than that of the right. At the inferior part 

 of this auricle is seen the opening into the left ventricle, 



