) 



558 THE BLOOD-VASCULAR SYSTEM 



off so many large branches, that it is now seldom or never selected for the applica- 

 tion of a ligature. 



Relations. — In front it is covered by the integuments, the superficial fascia, the platysma, 

 the anterior laj'er of the deep fascia, the clavicular origin of the sterno-mastoid, the sterno-hyoid 

 and sterno-thyreoid muscles, and the deep cervical fascia. It is crossed by the commencement 

 of the innominate, by the internal jugular, and by the vertebral veins; and, in a medio-lateral 

 direction, bj' the vagus and phrenic nerves, and the superior cardiac branches of the sympathetic 

 nerve. A loop of the sympathetic nerve itself also crosses the artery, and forms with the trunk 

 of the sympathetic a ring around the vessel known as the ansa subclavia (annulus of Vieussens). 



Behind, but separated from the artery by a cellular interval, are the longus colli muscle, 

 the transverse process of the seventh cervical or first thoracic vertebra, the main chain of the 

 sympathetic nerve, the inferior cardiac nerves, the recurrent laryngeal nerve, and the apex of the 

 right lung and pleura. 



Below, it is in contact with the pleura and lung and the loop of the recurrent laryngeal 

 nerve, which winds round the artery from the vagus and ascends behind it to the larynx. The 

 subclavian vein is below the artery and on an anterior plane. 



Branches. — The vertebral, internal mammary, superficial cervical, and thyreo- 

 cervical trunk (thyreoid axis) arise from this part of the vessel on the right side. 

 (See p. 559.) Not uncommonly a small aberrant artery also takes origin from this 

 portion of the artery and descends to the left behind the CBsophagus to join a 

 branch of the aorta opposite the third or fourth thoracic vertebra. This vessel is 

 probably the remains of the right dorsal aorta. 



THE SECOND PORTION OF THE SUBCLAVIAN ARTERY 



The second portion of the subclavian artery lies behind the scalenus anterior 

 muscle. It measures about 2 cm. ff in.) in length and here reaches highest in the 

 neck. The subclavian vein is separated from the artery by the scalenus anterior, 

 and lies on a lower and anterior plane (fig. 463). 



Relations. — In front it is covered by the skin, superficial fascia, platysma, anterior layer 

 of deep fascia, the clavicular origin of the sterno-mastoid, posterior layer of deep fascia, and by 

 the scalenus anterior. The phrenic nerve — which, in consequence of its obUque course medially 

 downward, crosses a portion of both the first and second part of the subclavian — is separated 

 from the second portion by the scalenus anterior muscle, as is also the subclavian vein which 

 courses on a somewhat lower plane. 



Behind the artery are the apex of the pleura and lung, and a portion of the scalenus medius; 

 also the scalenus minimus (partially or entirely fibrous, known as Sibson's fascia, see p. 355). 



Above is the brachial plexus. 



Below are the pleura and lung. 



One branch only — the costo-cervical trunk (superior intercostal) — is, as a 

 rule, given off from this portion of the subclavian; occasionally the transverse 

 cervical or the descending branch of the transverse cervical (posterior scapular 

 artery) arises from it. 



THE THIRD PORTION OF THE SUBCLAVIAN ARTERY 



The third portion of the subclavian artery extends from the lateral margin of 

 the scalenus anterior muscle to the lateral border of the first rib. It is more super- 

 ficial than either the first or second portions; it is in relation with less important 

 structures, and as a rule gives off no branch, and for these reasons is the part 

 selected when practicable for the application of a ligature. It is the longest of 

 the three portions of the subclavian artery, and lies in a triangle — the subclavian 

 triangle — bounded by the sterno-mastoid, the omo-hyoid, and the clavicle (fig. 

 445). 



Relations. — In front it is covered by skin, superficial fascia, platysma, supra-clavicular 

 nerves (descending superficial branches) of the cervical plexus; the anterior layer of deep 

 fascia which descends from the omo-hyoid to the clavicle; and the posterior layer of deep fascia 

 which descends from the omo-hyoid to the first rib and passes over the scalenus anterior and 

 phrenic nerve. Between the two layers of fascia is a variable amount of cellular tissue and fat, 

 and running in this is the transverse scapular (supra-scapular) artery. The subclavian is crossed 

 by this artery unless the arm is drawn well downward. Close to the lateral margin of the 

 Btcrno-mastoid, the external jugular vein pierces the fascia, and crosses the subclavian artery 

 to open into the subcl.avian vein. As this vein lies between the two layers of fascia, it receives 

 on its lateral side the transverse scapular (supra-scapular), transverse cervical, and other veins 

 of the neck, which together form a plexus of large veins in front of the artery. The nerve to 

 the subclavius, and, when present, the accessory branch from this nerve to the phrenic, also 



