THORACIC CAVITY 91 



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, immediately above the coronary sulcus ; and 

 the vertical incisions must ascend from the extremities of the horizontal to 

 the 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. 



Aorta Left pulmonary veins 



Pulmonary artery 



Left auricle 

 (O.T. appendix) 



;' Right pul- 

 monary veins 



Position of fossa 

 bvalis in right atrium 



Coronary sinus 



FIG. 46. The Left Atrium opened from behind. The greater part of 

 the posterior wall has been thrown upwards. 



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

 and generally devoid of muscular bundles, but the inner 

 surface of the wall of its auricle (O.T. auricular appendage) is 

 covered with musculi pectinati, a fact which can be demon- 

 strated by carrying an incision anteriorly into it. As this 

 incision is made, the dissectors should note that, in a formalin 

 hardened heart, a strong muscular ridge descends along the 

 left border of the cavity anterior to the orifices of the left 

 pulmonary veins, entirely concealing them from view when the 

 cavity is examined from the front. 



On the right or septal wall of the left atrium the position 



