418 THE SUBCLAVIAN ARTERIES. 



THE THIRD PART OF THE SUBCLAVIAN ARTERY lies for its greater part 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. The arbery is nearer to the surface 

 here than elsewhere, being covered only by the integument, platysma, and layers of 

 cervical fascia ; but towards its termination it becomes deeper, sinking under the 

 clavicle and the subclavius muscle. It rests upon the first rib ; and behind it is 

 the scalenus medius. 



Relation to veins. The subdavian vein is still anterior to, and lower than, the 

 artery. The external jugular vein crosses in front of the artery, and receives on the 

 outer side from the shoulder the two veins which accompany the suprascapular and 

 transverse cervical arteries. These veins in some cases form a sort of plexus over 

 the artery. 



Relation to nerves. Above the vessel are placed the nerves of the Irachial plexus, 

 the lowest trunk, formed by the union of the last cervical and the first dorsal nerves, 





Fig. 350. ORIGIN OF THE BRANCHES OP THE RIGHT SUBCLAVIAN ARTERY (modified from Wyeth). 



(GKD. T.) Natural size. 



v, vertebral artery ; i.t, inferior thyroid artery, giving off a.c, the ascending cervical ; s.c, super- 

 ficial cervical artery; ss, suprascapular artery; the last three arising together by the thyroid axis; 

 i.m, internal mammary artery ; s.i, superior intercostal artery, from which arises d.c, the deep cervical ; 

 p.s, posterior scapular artery. The thick dotted lines indicate the position of the inner and outer 

 borders of the anterior scalenus muscle. 



being behind and in close contact with it. The small nerve to the subclavius passes 

 down over the artery, and the space which lodges the artery is crossed in front by 

 the descending superficial branches from the cervical plexus of nerves. 



BRANCHES. Four branches are commonly described as arising from each sub- 

 clavian artery. Of these, three, namely, the vertebral, the internal mammary, and 

 the thyroid axis, usually spring close together from the first part of the artery, near 

 the inner side of the anterior scalenus muscle ; while the fourth branch, the superior 

 intercostal, is generally found also internal to that muscle on the left 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 part of the subclavian, 

 and ascends in the neck to reach the interior of the skull ; the internal mammary 



