THE BRANCHES OF THE ARCH OF THE AORTA 259 



First Portion of the Right Subclavian. Relations. In front, 

 the sternomastoid, hyoid, and thyroid muscles; deep cervical 

 fascia; internal jugular, vertebral, and right innominate veins; 

 and superficially, the anterior jugular vein, some loops of the 

 sympathetic nerve and its cardiac branches, the vagus and 

 phrenic nerves; behind, the transverse process of the seventh 

 cervical or first dorsal vertebra, longus colli, recurrent laryngeal, 

 and sympathetic nerve and pleura; below, the pleura and recur- 

 rent nerve. . 



First Portion of the Left Subclavian. Relations. In front, the 

 left lung and pleura, left carotid artery; internal jugular, verte- 

 bral, and left innominate veins; vagus, phrenic, and cardiac 

 nerves; and superficially, the sternothyroid, hyoid, and mastoid 

 muscles ; behind, the sympathetic nerve, esophagus, and thoracic 

 duct, the longus colli separating it from the spine; externally, 

 the pleura; internally, the trachea, esophagus, and thoracic duct. 



Second Portion of the Subclavian. Relations. In front, the 

 scalenus anticus, phrenic nerve, and the vein; behind and below, 

 the pleura. 



Third portion of the Subclavian. Relations. In front, the 

 clavicle, subclavius, cervical fascia, suprascapular artery, ex- 

 ternal jugular, suprascapular, and transverse cervical veins, 

 supraclavicular nerves from the cervical plexus, and the nerve 

 to the subclavius; behind, the scalenus medius; above, the 

 omohyoid, brachial plexus; below, the first rib. 



The branches of the Subclavian are the vertebral, thyroid axis, 

 internal mammary, and superior intercostal. They are all 

 derived from the first portion on the left side; on the right the 

 superior intercostal arises from the second portion. 



I. The vertebral, the first and largest branch, comes off from 

 the upper and back part of the first portion and enters the trans- 

 verse foramen of the sixth cervical vertebra, ascends through 

 those of the other cervical vertebrae, and, grooving the upper 

 border of the atlas from without, backward, and inward, around 

 the superior articular process, pierces the dura mater. It then 

 ascends to the front of the medulla through the foramen mag- 

 num, uniting at the lower border of the pons with its fellow to 

 form the basilar. The thoracic duct crosses the left artery. 

 It is at first behind the internal jugular and its own vein; then 

 between the scalenus anticus and longus colli. In the foramina 



