THORACIC CAVITY 119 



the left atrium, from the lower border of the left inferior 

 pulmonary vein, downwards and to the right to the coronary 

 sinus, is the oblique vein (Marshall*), which is the remains 

 of the left duct of Cuvier of the foetus. Occasionally it 

 becomes the lower end of a left superior vena cava. 



Dissection. Open the left atrium by three incisions one 

 horizontal and two vertical. The horizontal incision must run 

 from side to side along the lower border of the atrium, immedi- 

 ately above the coronary sulcus ; and the vertical incisions must 

 ascend from the extremities of the horizontal incision to the 



Aorta Left pulmonary veins thrown up- 



Pulmonary artery -> wards and to the ri S ht 



Left auricle 

 (O.T. appendix) 



{ Right pul- 

 /' monary veins 



Position of fossa 

 ovalis in right atrium 



Coronary sinus 



FIG. 60. The Left Atrium opened from behind. The greater part of 

 the posterior wall has been thrown upwards. 



upper border of the posterior surface, each passing to the medial 

 side of the terminations of the corresponding pulmonary veins. 

 When the incisions have been made, the posterior wall of the 

 atrium must be turned upwards whilst the cavity is being 

 examined. From the lower end of the left vertical incision 

 carry the knife forwards, through the lateral wall of the auricle 

 of the left atrium. 



The inner surface of the wall of the left atrium is smooth 

 and generally devoid of muscular bundles, but it will be 

 noted that the inner surface of the wall of its auricle (O.T. 



