THE SUBCLAVIAN ARTERY. 755 



while the third portion extends from the outer border of the scalenus to the con- 

 ventional termination of the artery at the lower border of the first rib. On account 

 of the difference in their origins, the relations of the first portions of the right and 

 left vessels differ somewhat. 



The first portion of the right subclavian artery lies behind the clavicular 

 portion of the sterno-cleido-mastoid, and is crossed in fro7it by the internal jugular 

 and vertebral veins and by the right pneumogastric, phrenic, and superior sympa- 

 thetic cardiac nerves. Behind, it is in relation with the transverse process of the 

 seventh cervical vertebra, with the inferior cervical sympathetic ganglion, and with 

 the right recurrent laryngeal nerve, which winds around its under surface from in 

 front. Below, it is in contact with the dome of the right pleura. 



The first portion of the left subclavian artery, at its origin, is deeply seated 

 in the thoracic cavity and ascends almost vertically through the superior mediastinum. 

 Behind, and somewhat medial to it, are the oesophagus, the thoracic duct, and the 

 longus colli muscle, and at its emergence from the thorax the lower cervical sympa- 

 thetic ganglion. Medial, or internal to it, are the trachea and the left recurrent laryn- 

 geal nerve, and lateral to it, on its left side, are the left pleura and lung, which also 

 overlap it in front. Near its origin it is crossed by the left innominate (brachio- ceph- 

 alic) vein, and, shortly before it passes over into the second portion, it is crossed by the 

 internal jugular, vertebral, and subclavian veins, as well as by the phrenic nerve and 

 the thoracic duct, the latter arching over it to reach its termination in the subclavian 

 vein. The left pneumogastric and cardiac sympathetic nerves descend into the 

 thorax in front of it, the pneumogastric, before passing over the aortic arch, coming 

 into contact with the anterior surface of the vessel. As it emerges from the thorax 

 the subclavian lies behind the clavicular portion of the sterno-cleido-mastoid. In the 

 neck it rests below upon the dome of the left pleura.. 



The second portion of the subclavian artery, the relations of which 

 and of the succeeding portion of the vessel are the same on both sides, in front 

 is covered by the scalenus anticus muscle, anterior to which and on a slightly lower 

 plane is the subclavian vein. Behind and above it are the trunks of the brachial 

 plexus, which separate it from the scalenus medius, and beloiv it is in contact with 

 the pleura. 



The third portion of the subclavian artery lies in the supraclavicular fossa, 

 and is covered only by the skin, the platysma, and that part of the deep cervical 

 fascia which contains the external jugular vein and the supraclavicular branches of 

 the cervical plexus, and encloses a quantity of fatty tissue, in which the suprascapular 

 artery passes outward. Behind, it is in contact with the scalenus medius and the 

 brachial plexus, and above it are the brachial plexus and the posterior belly of the 

 omo-hyoid. Below, it rests upon the first rib, at the lower border of which the vessel 

 becomes the axillary artery. 



Branches. — Considerable variation exists in the arrangement of the branches 

 of the subclavian, but in what is probably the most frequent arrangement the 

 branches are as follows : 



From the first portion arise (i) the vertebral, (2) the internal mammary, (3) 

 the superior intercostal, and (4) the thyroid axis ; from the second portion no 

 branches are given of? ; from the third portion (5) the transverse cervical. 



Variations. — The variations in the origin of the subclavian artery have already been consid- 

 ered in describing the variations of the aortic arch (page 725). Other anomalies occur in its 

 relation to the scalenus anticus, in front of which it sometimes passes, and it may also traverse 

 the substance of the muscle obliquely. More rarely the artery divides at the inner border of the 

 muscle, the two branches so formed continuing onward through the axilla and down the arm 

 to become the radial and ulnar arteries. 



Numerous supernumerary branches may arise from the subclavian. These may be either 

 ( I ) accessory to the branches normally arising from the artery, such as an accessory vertebral, 

 an accessory internal mammary, or an accessory inferior thyroid ; (2) they may be branches, 

 such as the long thoracic, dorsal scapular, subscapular, and the anterior and posterior circum- 

 flexes, which normally arise from the axillary artery, but have secondarily shifted to the sub- 

 clavian as the result of the enlargement of anastomoses which they make with branches of that 

 vessel ; or (3) they may be branches to neighboring organs, such as a bronchial or a pericar- 

 dial branch, or occasionally the thyroidea ima (page 729). 



