cj64 A MANUAL OF ANATOMY 



The left pulmonary artery passes outwards to the left, in front 

 of the left Ijronchus and descending aorta, to the root of the left 

 lung, where it divides into two branches, one for the upper, and 

 the other for the lower, lobe. The ligamentum ductus arteriosi is 

 attached to the upper aspect of its root. 



The right pulmonary artery is larger and longer than the left. 



Development. — The trunk of the pulmonary artery, along with the ascending 

 aorta, is chiefly developed from the aortic bulb, but a small portion of it is 

 formed by the commencement of the sixth left arterial arch, which remains con- 

 nected with that portion of the aortic bulb which becomes partitioned off to 

 form the pulmonary trunk. 



The right and left pulmonary arteries are developed as branches from the 

 sixth left arterial arch near its commencement, the remainder of that arch giving 

 rise to the ductus arteriosus of the foetus. 



Ligamentum Ductus Arteriosi. — ^This is a fibrous cord which is the 

 remains of an important vessel, peculiar to foetal life, called the ductus 

 arteriosus. It extends from the upper aspect of the root of the 

 left pulmonary artery to the under surface of the arch of the aorta, 

 immediately beyond the level of the origin of the Teft subclavian 

 artery. Its direction is upwards, backwards, and slightly to the left. 



During foetal life the right and left pulmonary arteries are of small size, 

 and the ductus arteriosus conveys the greater part of the venous blood from 

 the right vfentricle of the heart into the aorta at a point beyond the origin of 

 the left subclavian artery. None of this blood, therefore, can pass into the 

 great vessels which spring from the upper aspect of the arch of the aorta. 



Development. — The ductus arteriosus is developed from the dorsal part 

 of the sixth left arterial arch. 



Pulmonary Veins. — ^These vessels carry the arterial or oxygenated 

 blood from the lungs to the left auricle of the heart. Though they 

 are called veins, they contain arterial blood, and in this respect 

 resemble the umbilical vein of the foetus. They are four in num- 

 ber, two right and two left, and, at the root of each lung, the upper 

 of the two is on a more anterior plane than the lower. The right 

 veins pass behind the superior vena cava and the right auricle, 

 and the left veins pass in front of the descending aorta. All four 

 vessels open into the left auricle on its posterior aspect. On leaving 

 the roots of the lungs the veins are said to receive small bronchial 

 tributaries from the adjacent bronchial tubes and glands. 



The right pulmonary veins are larger and longer than those of the 

 left side. 



Pneumogastric Nerves in the Thorax. — ^These nerves, right and 

 left, differ so much from each other in their course and relations as 

 to require separate descriptions. 



The right pneumogastric, or vagus, nerve, having descended in 

 front of the first part of the. right subclavian artery," and having 

 given off its inferior or recurrent laryngeal branch at the lower 

 border of that vessel, enters the thoracic cavity. It then descends 

 in the superior mediastinum behind the innominate vein, and, 

 inclining backwards, it reaches the right side of the trachea, alontj 

 which it courses to the posterior aspect of the root of the right lung. 



