THORACIC CAVITY 89 



nerve, the superior cardiac branch of the left sympathetic, and the inferior 

 cervical cardiac branch of the left vagus, immediately above the upper 

 border of the aortic arch. Next divide the aortic arch. Enter the knife at 

 the upper border of the arch, between the left common carotid and left 

 subclavian arteries and anterior to the left vagus and the left recurrent 

 nerve, and cut from above downwards, completing the division of the 

 arch at the lower border, immediately to the left of the upper end of 

 the ligamentum arteriosum. The left superior intercostal vein will be 

 divided at the same time, but care must be taken not to injure the left 

 recurrent nerve, which is curving round the arch from the front to the 

 back. When the incisions are completed, pull the anterior part of the 

 aortic arch, with the superior vena cava and the lower parts of the innomi- 

 nate veins, anteriorly, and separate them from the lower part of the 

 trachea and from the bronchi. As the separation proceeds, keep the edge 

 of the knife turned towards the aortic arch, to avoid injury to the deep part 

 of the cardiac plexus, which lies anterior to the bifurcation of the trachea. 

 When the lower border of the arch is reached, the twigs which connect the 

 superficial with the right half of the deep part of the cardiac plexus will 

 be exposed, and must be divided. When this has been done detach the 

 posterior surface of the pericardium from the front of the oesophagus and 

 the descending aorta, taking care to avoid injury to the plexus formed by 

 the vagi nerves on the anterior aspect of the oesophagus. As soon as the 

 separation is completed, the heart, with the remains of the pericardium and 

 the lower parts of the phrenic nerves, can be removed from the thorax, and 

 the investigation of the left atrium and the structure of the heart can 

 be proceeded with ; but, before this is done, the dissector should note that 

 the posterior wall of the pericardium intervenes between the posterior wall 

 of the left atrium and the anterior surfaces of the oesophagus and the 

 descending part of the aorta, as the latter structures lie anterior to the 

 middle four thoracic vertebrae (Fig. 21). 



After the heart and the roots of the great vessels have been removed from 

 the thorax, fasten the left vagus and the recurrent nerve to the part of the 

 arch left in sitti by one or two points of suture ; then cut away the remains 

 of the pericardium from the heart, leaving only those portions of it which 

 mark the lines of reflection of the parietal to the visceral portions of the 

 serous sac. Note, as the posterior wall of the pericardium is removed, that 

 it forms the posterior boundary of the oblique sinus (p. 21). 



The Left Atrium. The left atrium, like the right, is 

 separable into two parts a larger main portion, the atrium 

 proper or body ; and a long narrow prolongation, the auricle 

 (O.T. auricular appendage), which runs from the left margin 

 of the body anteriorly and to the right. The four pulmonary 

 veins, two on each side, open into the left atrium. They enter 

 close to the upper ends of the lateral borders of the posterior 

 surface, and not uncommonly the right or the left pair may 

 fuse into a common trunk at the point of entrance. 



It has been noted previously that the left atrium forms 

 the greater part of the base of the heart, a small part of the 

 anterior or sterno-costal surface, and a still smaller part of 

 the left border. The only part which can be seen from the 

 front, when the heart is in situ^ is the apical portion of the 



