366 SUBCLAVIAN ARTERIES. 



descends over the artery along the inner margin of the scalenus muscle, im- 

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

 descending from the neck, are close to the artery. 



THE SECOND PAIIT OF THE SUBCLAVIAN ARTERY, the'short portion concealed 

 by the anterior scalenus muscle, forms the highest part of the arch described 

 by the vessel across the neck. Somewhat less deeply placed than the first 

 part, it is covered by the platysma and the stern o-mastoid muscle, with 

 layers of the cervical fascia. Behind, it rests against the middle scalenus 

 muscle ; and below, it lies on the pleura. 



Relation 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 close 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 separated by the muscle from 

 the second part of the artery. 



THE THIRD PART OF THE SUBCLAVIAN ARTERY lies 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 muscle ; the omo-hyoid is in some 

 instances immediately over the artery. The subclaviau artery is nearer to 

 the surface here than elsewhere, being covered only by the platysma and 

 layers of the cervical fascia, but towards its termination it becomes deeper, 

 sinking under the clavicle and the subclavius muscle. 



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 suprascapular 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 brachial 

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

 dorsal nerve behind and in contact with it. The small nerve of the 

 subclamus 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 described as arising from each 

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

 mammary, 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 superior intercostal, is usually found 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 vetebral artery springs from the upper and back part of the sub- 

 clavian, 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 part 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 outwards across the 

 neck to the shoulder ; lastly, the superior intercostal and deep cervical 

 arise by a common stem from the back part of the artery, 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. 



