THORACIC CAVITY 



77 



left, across the anterior aspect of the descending aorta and the 

 left bronchus, to the hilus of the left lung. It is covered an- 

 teriorly and on the left by the anterior part of the mediastinal 

 surface of the left pleural sac. As it descends in the sub- 

 stance of the lung it lies along the postero-lateral aspect 

 of the stem bronchus and between its ventral and dorsal 

 branches (p. 98). 



Branches. Except that it has no branch corresponding 

 with that which accompanies the eparterial bronchus on the 



Left common carotid artery 

 Left innominate vein 



Right auricle 

 Stem of pulmonary artery 



Pulmonary vah 



Upper left pulmonary vein 



Left auricle - 



Right innominate vein 

 Innominate artery 



Left subclavian 

 artery 



... Arch of aorta 

 Vena azygos 

 Aorta 



- Ductus arteriosus 



j Left pulmonary 

 i artery 



; ' Lower left pulmon- 

 ary vein 



Descending aorta 



Inferior vena cava 



FIG. 39. Dissection of the Heart and Great Vessels of a Foetus, showing 

 the angular junction of the Ductus Arteriosus with the Aorta. 



right side, the branches of the left pulmonary artery are 

 similar to those given off by the right pulmonary artery. 



Ligamentum Arteriosum. The ligamentum arteriosum is 

 a strong fibrous band which connects the commencement of 

 the left pulmonary artery with the lower surface of the arch of 

 the aorta. It is the remains of the walls of a wide channel, 

 the ductus arteriosus, which united the left pulmonary artery 

 with the aorta throughout the whole period of pre-natal life. 



During foetal life the lungs had no aerating function ; therefore the right 

 pulmonary artery and the part of the left pulmonary artery beyond the 

 origin of the ductus arteriosus were small, for they had merely to convey 

 sufficient blood to maintain the life and growth of the non-functional lungs. 

 At this period, therefore, the blood which had entered the right ventricle, 



