78 THORAX 



through the superior vena cavaand the right auricle (see p. 68), was ejected, 

 by the ventricle ; into the pulmonary artery and the greater part of it passed 

 through the duclus arteriosus into the aorta, which it entered beyond the 

 origin of the left subclavian artery, and there mingled with the more 

 oxygenated blood from the placenta, the lower part of the body, and the 

 lower limbs, which passed from the inferior vena cava through the right 

 atrium and the foramen ovale to the left atrium, and thence to the left 

 ventricle by which it was pumped into the aorta. 



It is obvious that the passage of blood from the pulmonary artery into 

 the aorta could take place only so long as the pressure in the pulmonary 

 artery was greater than the pressure in the aorta. At birth, when the 

 blood rushed through the rapidly enlarged right and left pulmonary arteries 

 into the lungs, as they expanded with the first respiratory efforts, the pres- 

 sure in the pulmonary artery and the ductus arteriosus was reduced below 

 that in the aorta, and the blood in the aorta would have flowed into the 

 ductus arteriosus had it not been that the angle of union between the 

 ductus arteriosus and the aorta had become more and more acute during 

 the latter part of foetal life, with the result that the upper and right margin 

 of the orifice of communication attained a position overhanging the lower 

 and left margin (Fig. 39) ; and as soon as the blood pressure in the aorta 

 exceeded that in the ductus arteriosus, this margin, acting as a flap valve, 

 was driven against the left and lower margin, closing the orifice effectually. 

 After this occurred the utility of the ductus arteriosus terminated, and it was 

 converted into a fibrous cord the ligamentum arteriosum. 



Note that the left recurrent nerve curves round the 

 lower surface of the aortic arch on the left side of the upper 

 end of the ligamentum arteriosum, and that the superficial 

 cardiac plexus lies below the aortic arch immediately to the 

 right of the ligament. 



In a few cases the ductus arteriosus remains patent for 

 several years of life after birth, and occasionally it is patent 

 throughout the whole of life. 



Dissection. Cut through the remains of the upper part of the conns 

 arteriosus immediately below the bases of the cusps of the pulmonary valve, 

 and carefully dissect the upper part of the conus and the lower part of the 

 pulmonary artery away from the front of the commencement of the ascending 

 aorta. When this has been done, turn the lower end of the pulmonary 

 artery upwards and pin it to the arch of the aorta (see Fig. 40). The 

 upper part of the anterior wall of the left ventricle and the commencement 

 of the aorta are now exposed, and the dissector should note three bulgings 

 at the commencement of the aorta the three aortic sinuses. One of the 

 three sinuses lies anteriorly, and the right coronary artery springs from it. 

 The other two, a right and a left, lie posteriorly, and the left coronary 

 artery springs from the left sinus. 



Make a transverse incision across the upper end of the left ventricle, a 

 short distance below the base of the anterior aortic sinus. On the right 

 side extend the incision into the upper part of the inter-ventricular septum 

 and carry it downwards and anteriorly in the septum to the apex of the 

 heart. From the left extremity of the upper transverse incision carry an 

 incision downwards and anteriorly through the left lateral border of the 

 anterior surface of the left ventricle, parallel with the incision already made 

 in the septum, towards the apex. As this incision is made pull the anterior 



