312 A MANUAL OF ANATOMY 



The posterior circumflex artery, which is of large size, arises 

 from the back of the vessel a little below the subscapular. Its 

 course is backwards in company with the circumflex nerve through 

 the quadrangular space, which is bounded above by the teres minor 

 (subscapularis in front), below by the teres major, internally by the 

 long head of the triceps, and externally by the surgical neck of the 

 humerus. In its course it winds round the inner and posterior 

 aspects of the neck of the bone, and it furnishes many branches, most 

 of which enter the deep surface of the deltoid. A special acromial 

 branch reaches the upper surface of the acromion process, and there 

 anastomoses with the suprascapular and acromial branches of the 

 thoracic axis. On the outer side of the neck of the bone the artery 

 anastomoses with the anterior circumflex, and in this way an arterial 

 circle is formed which closely embraces the surgical neck. Some of 

 the branches supply the muscles bounding the quadrangular space, 

 and one or two pass downwards between the long and outer heads 

 of the triceps, where they anastomose with the superior profunda 

 of the brachial. The posterior circumflex artery is subject to 

 certain variations, (i) It may arise in common with the sub- 

 scapular. (2) Its origin may be transferred to the superior pro- 

 funda of the brachial, in which case it ascends behind the teres 

 major. (3) It may give off the anterior circumflex, superior pro- 

 funda, or dorsal is scapulae. 



Varieties of the Axillary Artery. — (i) The subscapular, circumflex, and 

 profunda; of the brachial may arise by a common trunk. (2) The artery may 

 give origin to a large branch, which may be the radial, ulnar, vas aberrans, or 

 the interosseous trunk of the forearm. 



The part of the axillary artery most accessible for surgical interference is 

 the lower half of the third part, which is covered only by the integument of 

 the arm. 



Axillary Vein. — ^The axillary vein is the continuation of the basilic 

 vein of the arm. It extends from the lower border of the teres 

 major to the outer border of the first rib, where it becomes the sub- 

 clavian vein. It is of large size, and throughout its entire course it 

 lies to the inner side of the artery. Most of its tributaries corre- 

 spond with the arterial branches, but the following two are specially 

 noteworthy : (i) a little above its commencement it receives the 

 trunk formed by the union of the venae comites of the brachial 

 artery, and (2) below the clavicle it is joined by the cephalic vein. 



Axillary Sheatli. — The axillary vessels and the brachial plexus of 

 nerves are enclosed in a loose, infundibuliform sheath, which is 

 called the axillary sheath. It is a downward prolongation of the 

 deep cervical fascia, and it blends with the deep surface of the 

 costo-coracoid membrane. It has been compared to the crural 

 sheath of the femoral vessels, but it is in no sense such a defined 

 structure. 



Brachial Plexus. — ^The brachial plexus is situated in the lower part 

 of the posterior triangle of the neck, behind the clavicle, and in the 

 upper part of the axilla. Its complex formation is rendered simple 



