628 THE VASCULAR SYSTEMS 



The third portion of the subclavian artery passes downward and outward from 

 the outer margin of the Scalenus anticus muscle to the outer border of the first 

 rib, where it becomes the axillary artery. This portion of the vessel is the most 

 superficial, and is contained in the subclavian triangle (see page 608). 



Relations. It is covered, in front, by the skin, the superficial fascia, the Platysma, the 

 descending clavicular branches of the cervical plexus, and the deep cervical fascia; by the 

 clavicle, the Subclavius muscle, the suprascapular artery and vein, and the transverse cervical 

 vein; the nerve to the Subclavius muscle passes vertically downward in front of the artery. 

 The external jugular vein crosses the artery at its inner side, and receives the suprascapular 

 and transverse cervical veins, which frequently form a plexus in front of it. The subclavian 

 vein is below and in front of the artery, lying close behind the clavicle. Behind, it lies on the 

 Middle Scalenus muscle and the lowest cord of the brachial plexus, formed by the union of 

 the last cervical and first thoracic nerves. Above and to its outer side are the upper trunks 

 of the brachial plexus and the Omohyoid muscle. Below, it rests on the upper surface of the 

 first rib, or on the cervical rib if one be present. 



PLAN OF THE RELATIONS OF THIRD PORTION OF SUBCLAVIAN ARTERY. 



In front. 



Skin and superficial fascia. 



Platysma and deep cervical fascia. 



Descending branches of cervical plexus. Nerve to Subclavius muscle. 



Subclavius muscle, suprascapular artery, and vein. 



The external jugular and transverse cervical veins. 



The clavicle. 



Above. / subclavian \ Below. 



Brachial plexus. \ J^ini } First rib. 



Omohyoid. 



Behind. 



Scalenus medius. 



Lower cord of brachial plexus. 



Peculiarities. The subclavian arteries vary in their origin, their course, and the height to 

 which they rise in the neck. 



The origin of the right subclavian from the innominate takes place, in some cases, above the 

 sternoclavicular articulation, and occasionally, but less frequently, .in the cavity of the thorax, 

 below that point. Or the artery may arise as a separate trunk from the arch of the aorta. In 

 such cases it may be either the first, second, third, or even the last branch derived from that 

 vessel; in the majority of cases it is the first or last, rarely the second or third. When it is the 

 first branch, it occupies the ordinary position of the innominate artery; when the second or third, 

 it gains its usual position by passing behind the right carotid; and when the last branch, it arises 

 from the left extremity of the arch, at its upper or back part, and passes obliquely toward the 

 right side, usually behind the trachea, oesophagus, and right carotid, sometimes between the 

 oesophagus and trachea to the upper border of the first rib, whence it follows its ordinary course. 

 Its manner of development is such that the inferior laryngeal nerve is not looped under the ves- 

 sel, but passes in a nearly straight course on its way to the larynx. In very rare instances this 

 vessel arises from the thoracic aorta, as low down as the fourth thoracic vertebra. Occasionally 

 it perforates the Scalenus anticus muscle; more rarely it passes in front of that muscle. Some- 

 times the subclavian vein passes with the artery behind the Scalenus anticus muscle. The 

 artery may ascend as high as an inch and a half above the clavicle or any intermediate point 

 between this and the upper border of the bone, the right subclavian usually ascending higher 

 than the left. 



The left subclavian is occasionally joined at its origin with the left common carotid. 



Surface Marking. The course of the subclavian artery in the neck may be mapped out 

 by describing a curve, with its convexity upward at the base of the posterior triangle. The inner 

 end of this curve corresponds to the sternoclavicular joint, the outer end of the centre of the 



