PULMONARY ARTERY. 325 



Where the nnmbrane passes from an a'iricle to a ventricle, or from a ven- 

 tricle to an artery, it forms duplicatures or valves, in which fibrous tissue 

 is inclosed ; and in the ventricles it covers the tendinous cords of the 

 valves, and the projecting muscular bundles. 



The thickness of the membrane is greater in the auricles than in the 

 ventricles, and in the left than in the right half of the heart. In its struc- 

 ture it resembles a serous membrane. 



GREAT VESSELS OF THE HEART. The arteries which take origin from 

 the heart are the aorta and the pulmonary. The large veins entering the 

 heart, besides the coronary, are the superior and inferior cava, and the 

 pulmonary. 



The PULMONARY ARTERY is a short thick trunk, which conveys the 

 dark or impure blood from the right side of the heart to the lungs. From 

 its commencement in the right ventricle the vessel is directed upwards on 

 the left of the aorta ; and at a distance of an inch and a half or two inches, 

 it divides into two branches of nearly equal size for the lungs. The trunk 

 of the pulmonary artery is contained in the pericardium ; and beneath it 

 is the beginning of the aorta. On each side are the coronary artery and 

 the auricular appendix. 



Fig. 104. 



THE FORMATION OF THE SEPTUM VEXTRICCLORUM BY THE FIBRES OF BOTH VEXTRICLES 

 IS REPRESENTED IN THIS CUT. 



Near the bifurcation of the artery is a ligamentous cord about as large 

 as a crow-quill, the remnant of the arterial duct, which passes from the 

 left branch of the vessel to the arch of the aorta, and is named ligamenttim 

 ductus arteriosi. 



The right branch is longer than the left. In its course to the lung it 

 lies beneath the aorta and the vena cava superior, and rests on the bron- 

 chus or piece of the air tube : and as it passes outwards it lies above the 

 level of the right auricle of the heart. At the lung the artery divides 

 into three primary branches, one tor each lobe. 



The left branch is rather smaller than the right ; it is directed in front 

 of the descending aorta and the left bronchus to the fissure of the root of 

 the lung, where it ends in two branches for the two pulmonic lobes. 



