

THORACIC CAVITY 105 



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

 through the ductus 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 trunk, 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 rushes through the rapidly enlarged right and left pulmonary arteries 

 into the lungs, as they expand with the first respiratory efforts, the pressure 

 in the pulmonary artery and the ductus arteriosus is reduced below that 

 in the aorta, and blood would flow, from the aorta, through the ductus 

 arteriosus into the pulmonary artery were it not that an alteration of the 

 position of the heart, caused by the expansion of the lungs, produces a 

 twisting of the arterial duct which results in the obliteration of its channel. 

 After blood ceases to flow through it the duct rapidly contracts, and is 

 ultimately reduced to the condition of a fibrous ligament. In a few cases 

 the duct remains open and then peculiar physical signs are produced 

 with which the student will become acquainted during the course of his 

 medical work. 



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. 



Dissection. Cut through the remains of the upper part of 

 the conus 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 

 that has been done, turn the lower end of the pulmonary artery 

 upwards and pin it to the arch of the aorta (see Fig. 53). 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 (Fig. 45). 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 interventricular septum and carry it downwards, cutting 

 through the anterior part of the septum as far as the apex of the 

 heart. From the left extremity of the upper transverse incision 

 carry an incision downwards and forwards 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 wall of the left ventricle for- 

 wards till the base of a large papillary muscle which springs from 



