530 THE SUBCLAVIAN AND AZYGOS VEINS. 



scapular, long thoracic, alar -thoracic, acromio-thoracic (opening- in common with the 

 cephalic vein) and the superior thoracic veins ; at the lower border of the sub- 

 scapularis muscle it is joined by one or both of the brachial venae comites, and near 

 its termination by the cephalic vein. 



The subclavian vein is the continuation of the axillary, and extends from the 

 outer margin of the first rib to the inner border of the anterior scalenus muscle, 

 where it terminates by uniting with the internal jugular to form the innominate 

 vein. It crosses over the first rib and behind the clavicle, being placed at a lower 

 level and therefore pursuing a less arched course than the artery, from which it is 

 separated by the anterior scalenus muscle and the phrenic nerve. The subclavian 

 vein is joined, usually close to the outer border of the anterior scalenus, by the 

 external jugular vein, and it has constantly a pair of valves placed immediately out- 

 side the entrance of the latter vessel. 



The wall of the subclavian vein adheres closely to the fascial sheath by which it 

 is invested, and this being intimately connected in front with the costo-coracoid 

 membrane and the back of the clavicle (p. 208), the vessel becomes expanded when 

 the shoulder is carried forwards. Hence care should be taken in operations about 

 the root of the neck or the shoulder in order to avoid the danger of air being drawn 

 into the circulation by movements of the limb. 



Varieties. The subclavian vein is occasionally placed at a higher level than usual as it 

 curves inwards, rising above the clavicle into the neck, and overlapping the subclavian artery. 

 It has also been seen in rare cases passing between the subclavius muscle and the clavicle 

 (Luschka), lying with the artery behind the anterior scalenus, changing places with the 

 artery, or, lastly, dividing into two parts, which were placed, one in front of, the other behind, 

 the anterior scalenus (Luschka). It often receives separately the anterior jugular, the supra- 

 scapular, or the transverse cervical vein ; occasionally the cephalic vein. Other unusual 

 tributaries that have been met with are the brachial venaa comites (W. Krause). and on the 

 left side a bronchial vein (M. J. Weber). 



AZYGOS VEINS. 



The azygos veins are longitudinal vessels resting against the thoracic portion of 

 the spinal column, and formed by the union of the veins corresponding to the 

 arteries of the intercostal spaces. In the lower part of the thorax the two veins of 

 opposite sides are disposed symmetrically, but higher up the blood gathered from 

 most of the veins of the left side is poured into the trunk on the right, which 

 becomes enlarged and unsymmetrical, and has on that account received the name 

 of large or right azygos, while the united vessels from the corresponding parts on 

 the left side constitute the small or left azygos veins. 



The right or large azygos vein (vena azygos major) commences in the abdomen, 

 generally by an anastomotic vessel (ascending lumbar vein} which connects together 

 the several lumbar veins, and establishes a communication below, either directly or 

 indirectly, with the common iliac vein. It is also joined in many cases by a branch 

 which opens distally into the inferior vena cava, or into the renal vein ; and occa- 

 sionally it takes its origin solely in this way. Passing from the abdomen into the 

 thorax through the aortic opening in the diaphragm, or to the outer side of that 

 opening through the fibres of the right crus, the azygos vein ascends on the bodies 

 of the dorsal vertebrae to the level of the fifth rib, where it arches forwards over the 

 root of the right lung, and then opens into the superior vena cava, immediately above 

 the point at which that vessel perforates the pericardium. When passing through 

 the opening in the diaphragm, this vein is accompanied by the thoracic duct, both 

 being situated on the right side of the aorta. In the thorax, maintaining the same 

 position with respect to the duct and the aorta, it passes in front of the intercostal 

 arteries, and is covered by the pleura. It receives the intercostal veins of the right 



