THORACIC CAVITY 77 



the upper and left corner of the atrial area is the apex of the 

 auricle of the left atrium (O.T. auricular appendage), and 

 between the two auricles are the roots of the pulmonary 

 artery and the aorta, the former anterior to the latter. The 

 rounded portion of the upper part of the right ventricle, 

 immediately below the pulmonary artery, is called the conus 

 arteriosus. 



If the pulmonary artery and the ascending part of the 

 aorta were removed the upper parts of the anterior surfaces 

 of the right and left atria, which lie behind the arteries, 

 would be exposed, and the upper border of the heart, which 

 is formed by the two atria, could be seen (Figs. 37). The 

 two large vessels must not, however, be interfered with at 

 present. Nevertheless, the dissectors should investigate the 

 cleft which lies between the posterior surfaces of the arteries 

 and the anterior surfaces of the atria. It is called the 

 transverse sinus of the pericardium (Figs. 32, 37). 



To find the transverse sinus pass a ringer across the front 

 of the lower part of the superior vena cava and behind the 

 ascending aorta, then push it to the left until it emerges 

 between the left side of the pulmonary artery and the auricle 

 of the left atrium. The cleft through which the finger has 

 passed is the transverse sinus. As the finger lies in the 

 transverse sinus it has in front of it the ascending aorta 

 and the stem of the pulmonary artery, which are enclosed 

 in a common sheath of the visceral part of the serous 

 pericardium, and form the anterior boundary of the transverse 

 sinus. Behind the finger lie the upper parts of the right and 

 left atria, which form the posterior wall of the sinus. Below the 

 finger, at the lower border of the sinus, the visceral pericardium 

 is reflected forwards from the anterior surface of the atria 

 to the posterior surface of the ascending aorta. And above 

 the finger, at the upper margin of the sinus is the inferior 

 surface of the right pulmonary artery covered by the visceral 

 pericardium as it passes forwards from the upper borders of 

 the atria to the posterior surface of the ascending aorta. 

 In Fig. 37 which represents a sagittal section of a heart, in 

 which the transverse sinus is cut across at right angles to its 

 long axis, the various boundaries of the sinus are in apposition 

 with one another, but their relative positions are quite 

 obvious. 



Whilst a finger is kept in the transverse sinus a pointer 



