ii40 A MANUAL OF ANATOMY 



nerves also cross it superficially, namely, the pneumogastric and 

 its cervical cardiac branches, the cervical cardiac branches of the 

 sympathetic, and the nerve -loop known as the ansa Vieussenii. 

 Posterior. — ^The recurrent laryngeal nerve, gangliated cord of the 

 sympathetic, fat, longus colli muscle, first thoracic vertebra, dome 

 of the pleura, and apex of the lung. Inferior. — ^The recurrent laryn- 

 geal nerve, part of the ansa Vieussenii, and the pleura. 



The right subclavian and right internal jugular veins unite 

 anterior to this part of the vessel to form the right innominate vein. 



First Part of the Left Subclavian Artery. — ^This part extends from 

 the upper aspect of the arch of the aorta, towards its back part, to 

 the inner border of the scalenus anticus muscle. It is therefore 

 placed at first in the thoracic cavity. Its course is almost vertical 

 until it reaches the root of the neck, where it curves sharply outwards 

 upon the dome of the pleura, and so reaches the scalenus anticus. 

 The relations of the intrathoracic portion have been described in 

 connection with the thorax (see p. 961). 



Cervical Relations — Anterior. — ^Th,e skin, superficial fascia and 

 platysma myoides, deep cervical fascia, clavicular origin of the 

 sterno-cleido-mastoid, sterno-hyoid and sterno-thyroid muscles, and 

 the thoracic duct which arches over it. The left internal jugular 

 and subclavian veins unite in front of it to form the left innominate 

 vein, and the left vertebral vein descends in front of it under cover 

 of the left internal jugular vein. Posterior. — ^The gangliated cord 

 of the left sympathetic with the inferior cervical ganglion, fat, 

 and the left longus colli muscle. Right. — ^The trachea, oesophagus, 

 left recurrent lar5mgeal nerve, and thoracic duct. 



The varieties in origin of the subclavian artery have been described 

 in connection with the arch of the aorta (see p. 963). 



Second Part of the Subclavian Artery. — ^This portion of the vessel 

 is situated behind the scalenus anticus muscle. It lies about J inch 

 above the clavicle, and forms the highest part of the arch described 

 by the vessel. 



Relations — Anterior. — ^The skin, superficial fascia and platysma 

 myoides, deep cervical fascia, clavicular part of the sterno-cleido- 

 mastoid, scalenus anticus, phrenic nerve on the right side, and 

 subclavian vein, the latter vessel being on a lower level than the 

 artery, and separated from it by the scalenus anticus. Posterior. — 

 The pleura. Superior. — ^The nerve-roots of the brachial plexus. 

 Inferior. — ^The pleura. 



In some cases the second part of the vessel passes through the scalenus 

 anticus, and in rare cases it passes in front of the muscle. 



For the third part of the subclavian artery, see p. 1095 . 



Development. — The right subclavian artery, as far as the origin of the 

 internal mammary arterj', is developed from the fourth right arterial arch. 



The left subclavian artery is developed from the seventh left segmental 

 artery. 



Branches of the Subclavian Artery. — These are four in number, 

 namely the vertebral, internal mammar5^ thyroid axis, and 



