382 THE SUBCLAVIAN ARTERY. 



placed than the first part, it is covered by the phitysma and the sterno- 

 mastoid muscle, with layers of the cervical fascia. Behind, it rests 

 against the middle scalenus muscle ; and belo^^, it lies on the pleura. 



EeJaiioii to Veins and Nerves. — The subclavian vein is lower than the 

 artery, and is separated from it by the anterior scalenus muscle. The 

 phrenic nerve, which descends obliquely inwards over that muscle, 

 usually crosses the first part of the subclavian artery of the left side 

 dose to the muscle, while on the right side, not having quite reached 

 the margin of the muscle at the level of the artery, it is usually sepa- 

 rated by the muscle from the second part of the artery. 



THE THIRD PART OF THE SUBCLAVIAN ARTERY lics in a Small triangular 

 space, the sides of which are formed by the omo-hyoid muscle and 

 clavicle, and the base by the anterior scalenus nniscle ; tlie omo-hyoid 

 is in some instances immediately over the artery. The subclavian artery 

 is nearer to the surface here than elsewhere, being covered only b\' 

 the platysma and layers of the cervical fascia, but towards its termina- 

 tion it becomes deeper, sinking under the clavicle and the subclavius 

 nmscle. 



Relation to Veins. — The subclavian vein continues to be anterior to, 

 and lower than the artery. The external jugular vein lies over the 

 artery, and receives on the outer side from the shoulder the two veins 

 which accompany the supra-scapular and transverse cervical arteries. 

 The veins in some cases form a sort of plexus over the artery. 



Relation to Nerves. — Above the vessel are placed the large Iraehial 

 nerves, the lowest cord ibrmed by the union of the last cervical and the 

 first dorsal nerve being behind and in contact with it. The small 

 nerve of the si/hclavii/s muscle passes down over the artery, and the 

 space which lodges the artery is crossed in front by the superficial 

 descending (clavicular) branches from the cervical plexus of nerves. 



Branches. — Four branches are usually descril)ed as arising from 

 each subclavian artery. Of these, three, namely, the vertebral, the 

 internal mammarfj, and the thyroid axis, generally spring close together 

 from the first part of the artery, near the inner side of the anterior 

 scalenus muscle ; while the fourth branch, the .mperior intercostal, is 

 usually found internal to that muscle on the lelt side, but arising under 

 cover of it, from the second part of the artery on the right. 



The vertebral artery springs from the upper and back ]iart of the 

 subclavian, and ascends in the neck to reach the interior of the skull : 

 the internal mammary proceeds from the lower side of the vessel, and 

 descends into the fore ])art of the chest and abdomen ; the thyroid axis 

 arises from the front of the artery, and divides into three branches, one 

 of which, the inferior thyroid, is distributed in the fore part of the neck, 

 whilst the other two, the suprascapular and the transverse cervical, pass 

 oiitwards across the neck to the sliciulder ; lasth', the superior intercostal 

 and deep cervical arise by a connnou stem from the back part of the 

 arteiy, and pass into the upper part of the thoracic wall and the 

 posterior muscles of the neck. The deep cervical is reckoned by some 

 writers as a fifth branch of the subclavian artery, but it usually rises in 

 common with the superior intercostal artery. 



Another branch, in the great majority of instances, arises from the 

 third part of the artery. This is tlie posterior scapular arteiy, a branch 

 which otherwise is derived from the transverse cervical, one of the divi- 

 sions of the thyroid axis. 



