THE PULMONARY ARTERY AND THE AORTA. 465 



Behind it is the right bronchus. At the lung the artery divides 

 into three primary branches, one for each lobe. 



The left branch is directed in front of the descending aorta and Left branch, 

 the left bronchus to the fissure of the lung, where it ends in two 

 branches for the two lobes. 



As the right and left branches of the pulmonary artery pass Space at the 

 outwards, they cross the two bronchi diverging from the end of the bl1 

 trachea, and enclose with them a lozenge-shaped space which 

 contains some bronchial glands (fig. 163). 



Ductus arteriosus (fig. 164). Near the bifurcation of the Ligament of 

 pulmonary artery a fibrous cord, about the size of a crow-quill, 

 passes from the left branch of the vessel to the arch of the aorta. 

 This is the remnant of the ductus arteriosus of the foetus, and is 

 named the ligament of the arterial duct. 



In the fat us the right and left branches of the pulmonary artery Arterial 

 are small, and the trunk is continued by the ductus arteriosus, 

 which opens into the aorta beyond the origin of the last great 

 branch (left subclavian) of the arch. The impure blood from the course of 

 superior venae cavse passes into the right ventricle and thence 

 proceeds by the pulmonary artery, whereby most of it reaches the 

 aorta through the arterial duct, below the attachment of the vessels 

 of the head and neck, in order that it may be transmitted to the 

 placenta to be purified. After birth, when the function of the 

 lungs is established, the great current of blood is directed along the 

 branches of the pulmonary artery to the lungs, instead of through 

 the arterial duct ; and this tube, becoming gradually smaller, is 

 occluded by the eighth or tenth day, and forms finally the ligament 

 of the arterial duct. 



The AORTA (fig. 170, p. 466 ; and fig. 171, p. 467) is the great 

 systemic vessel which conveys the arterial blood from the heart to 

 the different parts of the body. It first ascends for a short distance, 

 and then arches backwards to reach the spinal column, along which through 

 it is continued downwards through the chest and abdomen. In the 

 thorax the vessel is divided into three parts the ascending aorta, 

 the arch of the aorta, and the descending thoracic aorta. 



The ASCENDING AORTA springs from the left ventricle of the Ascending 

 heart behind the left half of the sternum, on a level with the ao 

 lower border of the third costal cartilage. About two inches, or length, 

 a little more, in length, it is directed upwards, w r ith a slight 

 inclination to the right and forwards, and reaches to the inner end extent, 

 of the cartilage of the second rib on the right side. It is contained and 

 nearly altogether in the pericardium, being surrounded by the relatlons - 

 same sheath of the serous membrane as the pulmonary trunk, 

 which is at first superficial to it, but afterwards lies on its left 

 side. Between the ascending aorta and the sternum are the 

 anterior edge of the right lung, with the pleura, and some fatty 

 tissue. Behind it are the left auricle of the heart and the right 

 branch of the pulmonary artery. On the right side is the 

 descending cava. Near the heart the vessel bulges opposite the 

 flaps of the valve (sinuses of Valsalva ; fig. 170). There is 



D.A. H H 



