592 DISSECTION OF THE NECK. 



behind and lastly by the anterior scalenus. Behind and below the artery are 



below. 4.1. I J i 



the pleura and lung. 



Position of Veins. Below the level of the artery, and separated from it by 



vein ; t j ie an t e riof scalenus muscle, lies the arch of the subclavian vein. 



of nerves to Nerves. In front of the scalenus descends the phrenic nerve. 



the artery. ^bove the vessel, in the interval between the scaleni, are placed the 

 large cervical nerves ; and the trunk formed by the last cervical and 

 first dorsal nerves is behind the artery. 



Third part THIRD PART. Beyond the scalenus the artery traverses the 

 clavicular part of the posterior triangular space (fig. 210), and is 



is super- nearer the surface than in the rest of its course : this part of the 

 vessel is enclosed in a sheath of the deep cervical fascia, which it 

 receives as it passes from between the scaleni. It is comparatively 



Parts cover- superficial in the greater part of its extent, for it is covered 



ing it ; only by the integuments, the platysma, and deep fascia ; but near 

 its termination, the vessel gets under cover of the clavicle and sub- 

 clavius muscle, and the suprascapular vessels cross in front of it. 



and beneath. In this part of its course the artery rests on the surface of the first 

 rib, which is interposed between it and the pleura ; and the insertion 

 of the scalenus medius is behind it. 



Position of Veins. The subclavian vein approaches the artery, not being 



veins; separated by muscle, but lies commonly at a lower level. The 



external jugular vein crosses it near the scalenus muscle ; and the 

 suprascapular and transverse cervical tributaries, which enter the 

 jugular, sometimes form a plexus over this part of the artery. 



of nerves to Nerves. The large nerves of the brachial plexus are mostly above 

 the artery, but the lowest trunk is still behind and close to it, 

 and the small nerve to the subclavius crosses it about the 

 middle. Superficial to the cervical fascia lie the descending 

 cutaneous branches of the cervical plexus. 



Peculiarities Peculiarities. The artery may spring as a separate trunk from 



of origin, ^ Q ^^ o f ^ 9^^ j n w hich case it takes a deeper course than 

 usual to reach the interval between the scaleni muscles. 



level and The level of the arch formed by the subclavian artery in the 



neck varies in different subjects, and occasion ally the vessel pierces, 

 or even passes in front of the scalenus anticus muscle. 



Branches of ORIGIN OF BRANCHES. The chief branches of the subclavian 

 an ' artery are four in number. Three of these arise from the first part 



from Hrst, of the arterial trunk : one (vertebral) ascends to the head ; another 

 (internal mammary) descends to the chest ; and the remaining one 

 (thyroid axis) is a short thick trunk, which furnishes branches 

 inwards and outwards to the thyroid body and the shoulder. These 

 arise commonly near the inner border of the scalenus anticus muscle, 

 so as to leave an interval at the beginning of the trunk free from 

 offsets. This interval varies in length from half an inch to an inch 

 in the greater number of cases ; and its extremes range from less 

 than a quarter of an inch to an inch and three quarters. In some 

 instances the branches are scattered over the first part of the artery. 



second, On the right side the fourth branch (superior intercostal) arises 



beneath the anterior scalenus from the second part of the artery, 



