104 



THORAX 



anterior part of the mediastinal surface of the left pleural 

 sac. As it descends in the substance of the lung it lies 

 along the postero-lateral aspect of the stem bronchus and 

 between its ventral and dorsal branches (p. 62). 



Branches. The branches of the left pulmonary artery 

 are similar to those given off by the right pulmonary artery, 

 except that it has no branch corresponding with that which 

 accompanies the eparterial bronchus on the right side. 



Left common carotid artery 



Left innominate vein-. 



Right auricle^ 

 Pulmonary artery,^ 



Conus arteriosus 

 Upper left pulmonary vei 

 Left auricle 



Right innominate vein 

 --- Innominate artery 



Left subclavian 

 artery 



, Arch of aorta 



Vena azygos 



Cavity of arch of 

 aorta 



Ductus arteriosus 



Left pulmonary 

 artery 



Lower left pulmon- 

 ary vein 



Descending aorta 



Inferior vena cava 



FIG. 52. Dissection of the Heart and Great Vessels of a Foetus, showing 

 the angular junction of the Ductus Arteriosus with the Aorta. 



Ligamentum Arteriosum. The ligamentum arteriosum 

 has already been displayed. It is a fairly strong fibrous band 

 which connects the commencement of the left pulmonary 

 artery with the lower surface of the arch of the aorta. In 

 the adult it has no particular importance, and its interest lies 

 in the fact that it is the remains of the walls of a wide 

 channel, the ductus arteriosus (Fig. 52), which united the left 

 pulmonary artery with the aorta throughout 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 that period, therefore, the blood which had entered the right ventricle, 

 through the superior vena cava and the right atrium (see p. 92), was ejected, 



