THE HEART. 189 



the coats of each valve there is an additional fibrous substance, 

 for the purpose of strengthening it. 



The Pulmonary Artery, immediately after its origin, go^s 

 upwards and backwards to the under part of the curvature of 

 the aorta, and there divides into two trunks, one for each lung. 

 These trunks separate widely, and from the middle of their 

 fork proceeds a ligamentous substance, the remains of the Duc- 

 tus Arteriosus of the foetus, to the aorta posteriorly to the origin 

 of the left subclavian artery. The right Pulmonary artery is 

 both longer and larger than the left, and passing transversely 

 behind the aorta and the descending cava, then penetrates the 

 substance of the lung to be distributed as mentioned. The left 

 pulmonary artery passes to the lung in front of the descending 

 aorta. Though the pulmonary artery is quite as large as the 

 aorta, its parietes are thinner. 



The left Auricle, (Auricula Shristra, posterior,) in the na- 

 tural situation of the heart, is concealed by the right auricle 

 and the ventricles. Its figure is more regularly quadrangular, 

 or square, than'that of the right, and into each of its angles is 

 introduced a pulmonary vein, there being two on each side. 

 Sometimes, however, the latter join together previously, so that 

 the two have but a common orifice. Its tip, or ear-like portion, 

 is situated at the left side of the pulmonary artery, and is long- 

 er, narrower, more crooked, and more notched at its margins 

 than the corresponding portion of the right auricle. 



The parietes of this cavity are muscular, and somewhat 

 thicker than those of the right; they are smooth and uniform, 

 both externally and internally, with the exception of its ap- 

 pendix or ear-like portion, in which the musculi pectinati pre- 

 vail. The term sinus venosus or sinus pulmonalis of anato- 

 mists, only means that part of the cavity into which the pul- 

 monary veins empty. The septum between the auricles, when 

 viewed on this side, has the place of the fossa ovalis marked 

 out principally by its diaphanous condition. Occasionally, 

 there is some appearance of the valve which once existed 

 there. 



At the inferior part of the anterior side of this cavity is found 

 the opening between it and the left ventricle, also called Ostium 

 Venosum ; it is circular, and rather more than an inch in dia- 



