THE SUBCLAVIAN ARTERIES. 419 



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, while 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 arises in common with the superior intercostal artery. 



Another branch, in the majority of instances, arises from the second or third 

 part of the artery. This is most frequently the posterior scapular artery, a branch 

 which otherwise is derived from the transverse cervical, one of the divisions of the 

 thyroid axis ; but it may be the transverse cervical itself, or the suprascapular, or a 

 common trunk for the posterior scapular and suprascapular arteries ; and in rare 

 cases the thyroid axis or internal mammary is found arising in this situation. 



Varieties. The variations in oriyin of the subclavian arteries have been considered 

 along with the peculiarities of the arch of the aorta. 



Course and relations. The height to which these vessels reach in the neck is liable to 

 some variation. Most commonly the second part of the artery is placed from half to three- 

 quarters of an inch above the clavicle (the shoulder being- depressed to the full extent), but it 

 is sometimes, especially on the right side, placed as high as an inch, or even an inch and a 

 half above the level of the bone ; or on the other hand it may be entirely under cover of the 

 clavicle. The third part of the artery is not unfrequently concealed by the posterior belly of 

 the omo-hyoid, when that muscle has an attachment to the clavicle (p. 300) or is bound down 

 to the bone by the fascia. More rarely the artery is covered by an unusually wide clavicular 

 attachment of the tiapezius (p. 205), or crossed by a sterno-scapular muscle (p. 212). 

 Occasionally the subclavian artery perforates the anterior scalenus muscle ; and in rarer cases 

 it has been found altogether in front of the muscle, and close to the subclavian vein. The 

 vein has also been seen to pass with the artery behind the scalenus muscle ; and two or three 

 cases are recorded in which the positions of the artery and vein were reversed. 



Branches. The places of origin of the several branches of the subclavian artery are 

 subject to frequent slight variations, and it occasionally happens that one or more of the first 

 three branches are moved considerably inwards from their usual position, or outwards to 

 another division of the subclavian. Sometimes two, and much more rarely three, branches 

 arise from the third part of the vessel. A small muscular and spinal branch is frequently 

 given off from the second part of the artery. 



Aberrant artery. This is a small vessel arising from the right subclavian artery near the 

 vertebral, and descending to the left between the oesophagus and spine to join the aorta near 

 the termination of the arch : it is a vestige of the right aortic root (p. 386). A branch arising 

 in this situation, and terminating on the trachea and oesophagus, is not uncommon. (J. Wood, 

 Trans. Path. Soc., x. 123 ; F. J. Shepherd, Journ. Anat., xxiv. 71.) 



BRANCHES OF THE SUBCLAVIAN ARTERY. 



1. Vertebral artery (iii). The vertebral artery arises from the upper and back 

 part of the subclavian, on the right side generally about three-quarters of an inch 

 from its commencement, on the left side from the bend formed by the main trunk 

 as it turns outwards at the root of the neck. Passing upwards, and a little back- 

 wards and outwards, the artery enters the foramen in the transverse process of the 

 sixth cervical vertebra. It then ascends in a vertical direction through the series of 

 foramina in the transverse processes, as far as the upper border of the axis, where it 

 is curved outwards to reach the corresponding foramen of the atlas. After passing 

 through that aperture it winds backwards and inwards in the vertebral groove of the 

 atlas, and beneath the arched band of the posterior occipito-atlantal ligament (p. 158). 

 Immediately piercing the dura mater and arachnoid, it turns upwards and forwards in 

 the subarachnoid space, enters the skull through the foramen magnum, and finally, 

 inclining from the side to the front of the medulla oblongata, it unites with the vessel 

 of the opposite side, at the lower border of the pons Varolii, to form the basilar artery. 



VOL. II. E 



