416 THE SUBCLAVIAN AETERIES. 



artery. It has an average length of about three inches on the right side, an inch 

 more on the left. The subclavian artery is considerably larger than the common 

 carotid ; and as the trunk gives origin to several large branches it diminishes in size 

 towards its termination (from 11 to 9 mm.). The left artery is almost constantly a 

 little smaller than the right. 



Each subclavian artery is conveniently divided into three parts, the first part 

 extending from the origin of the vessel to the inner border of the anterior scalenus 

 muscle ; the second consisting of the portion of the vessel situated behind that 

 muscle ; and the third reaching outwards to the external border of the first rib. In 

 examining each of these portions in detail, it will be necessary to give a separate 

 description of the first part on the right and the left sides, as there is a material 

 difference in the origin, course, and relations of the two vessels. 



THE FIRST PART OF THE RIGHT SUBCLAVIAN ARTERY commences close to the 

 trachea, at the division of the innominate artery, behind the upper part of the 

 sterno-clavicular articulation, and ends at the inner margin of the anterior scalenus 

 muscle. Diverging gradually from the carotid artery, it arches upwards and 

 outwards, and ascends above the level of the clavicle to an extent which varies in 

 different cases. It is deeply placed, being covered by the sterno-mastoid, the sterno- 

 hyoid, and sterno-thyroid muscles, and the deep cervical fascia. It is in contact 

 with the pleura below and behind ; and the apex of the lung separates it from the 

 first dorsal vertebra and the adjoining parts of the first and second ribs. 



Relation to veins. The right innominate vein lies below and somewhat in front of 

 this part of the artery, while the internal jugular and vertebral veins, in their course 

 to join the innominate, descend over its anterior surface. Farther forwards the 

 anterior jugular vein passes transversely outwards, but is separated from the artery 

 by the sterno-hyoid and sterno-thyroid muscles. 



Relation to nerves. The pneumo -gastric nerve crosses the front of the artery to 

 the inner side of the internal jugular vein, and its recurrent laryngeal branch, 

 turning round below the artery, ascends behind. Some cardiac nerves and a loop or 

 two of the sympathetic descend over the artery, while the main trunk of the latter 

 passes behind. 



THE FIRST PART OF THE LEFT SUBCLAVIAN ARTERY arises from the upper aspect 

 of the arch of the aorta near its posterior extremity, and is, therefore, much longer 

 than the first part of the right subclavian. It ascends almost vertically out of the 

 thorax to the root of the neck, where it turns sharply outwards, across the apex of 

 the lung, to reach the interval between the scaleni muscles. Deeply placed in the 

 superior mediastinum, it has the left carotid artery in front of, but not in contact 

 with it, and it is covered on the left side by the pleura. It rests for a short distance 

 on the oesophagus (here deviating to the left side) and the thoracic duct, and after- 

 wards on the longus colli muscle. To the inner or right side of the vessel is at 

 first the trachea, and higher up are the oesophagus and the thoracic duct. The 

 thoracic duct arches from behind forwards and outwards over this part of the artery 

 in its course to the angle of union of the internal jugular and subclavian veins. 



Relation to veins. The internal jugular and vertebral veins are immediately in 

 front of the artery, where it turns outwards from the thorax, close to the scalenus 

 muscle ; and lower down the left innominate vein is anterior to it. 



Relation to nerves. The pneumo -gastric nerve is in front of and parallel to the 

 first part of the left subclavian artery, but in contact with it only at its lower part. 

 The phrenic nerve descends over the artery along the inner margin of the scalenus 

 muscle, immediately outside the thyroid axis. The cardiac nerves of the left side, 

 descending from the neck, are close to the artery, the cord of the sympathetic is 

 behind, and the inferior laryngeal nerve ascends on its inner side. 



THE SECOND PART OF THE SUBCLAVIAN ARTERY, the short portion concealed 



