516 THE THORAX AND VERTEBRAL COLUMN 



forwards, crossing the lateral aspect of the oesophagus to enter 

 the superior vena cava, and, if necessary, it may be ligatured 

 in this part of its course. 



In the upper part of the thorax the oesophagus lies slightly 

 to the left of the median plane, and, as it is situated above the 

 arch of the aorta, it may be approached from the left side. When 

 the pleura is stripped off the vertebral column and retracted, 

 the sympathetic remains intact on the heads of the ribs, but the 

 thoracic duct and the left recurrent nerve are exposed to injury. 

 In this part of its course the thoracic duct is closely applied to 

 the left border of the oesophagus, and it should be identified and 

 separated before the oesophagus is freed. As the vagus crosses 

 the arch of the aorta it gives off the left recurrent nerve, which 

 ascends behind the vessel, to the left of the trachea and in front 

 of the left edge of the oesophagus (Fig. 155). The left subclavian 

 artery passes upwards, backwards, and to the left from the 

 aortic arch, and opposite the second thoracic vertebra it is in 

 close contact with the lateral border of the oesophagus, from 

 which it is separated by the thoracic duct. The position of this 

 large vessel must be kept in mind while the oesophagus is being 

 freed. 



THE VERTEBRAL COLUMN AND THE 

 SPINAL MEDULLA. 



Ossification of the Vertebrae. Each typical vertebra 

 possesses three primary centres of ossification one for the 

 body, and one for each half of the verte- 

 bral arch. The two halves of the 

 vertebral arch unite with each other 

 posteriorly, and this process, which begins 

 in the cervical region at the end of the 

 first year, extends very gradually to the 

 other regions, so that it is not completed 

 in the sacrum till between the seventh 



. and tenth years. The vertebral body is 

 FIG. 156. Ossification of , ' . 



a Vertebra. separated from the vertebral arch on 



each side by a bar of cartilage, which is 



termed the neuro-central synchondrosis, and it disappears about 

 the third or fourth year. 



Centre for 

 body 



