310 A MANUAL OF ANATOMY 



of the brachial plexus, behind which is a quantity of fat, separating 

 the vessel from the subscapularis. Externally is the outer cord of 

 the brachial plexus. Internally are the inner cord of the brachial 

 plexus, the internal anterior thoracic nerve, and the axillary vein. 



Third Part. — ^The third part extends from the lower border of the 

 pectoralis minor to the lower border of the teres major, and its 

 length is about 3 inches. The upper half of this part is under cover 

 of the pectoralis major, but the lower half is free from muscular 

 covering, which is due to the posterior wall of the axilla being longer 

 than the anterior. 



Relations. — Anteriorly over its upper half it is covered by the 

 integument and pectoralis major, and over its lower half only by the 

 integument of the arm. The inner root of the median nerve crosses 

 it obliquely from within outwards, and the external vena comes of 

 the brachial artery crosses it from without inwards. Posteriorly 

 it rests, from above downwards, upon the subscapularis, tendon of 

 the latissimus dorsi, and lower part of the teres major, and the cir- 

 cumflex and musculo -spiral nerves descend behind it. Externally 

 is the coraco-brachialis, which sometimes slightly overlaps it. The 

 outer root of the median nerve lies on its outer side, as does also the 

 musculo-cutaneous. Internally is the axillary vein, and, for a short 

 distance below, the internal vena comes of the brachial artery. 

 Between the axillary vein and the artery are the inner root of the 

 median nerve and the ulnar nerve ; internal to the vein is the lesser 

 internal cutaneous nerve ; and the internal cutaneous nerve is 

 partly on the artery and partly to its inner side. 



Branches. — The first part gives off the superior or short thoracic, 

 and the thoracic axis or acromio-thoracic artery. The second 

 part furnishes the inferior or long thoracic, and the alar thoracic, 

 which is very inconstant as a separate branch. The third part 

 gives off the subscapular, the anterior circumflex, and the posterior 

 circumflex. 



Branches of the First Part. — The superior or short thoracic artery 

 is a small branch which arises just below the subclavius. Its course 

 is inwards and downwards behind the axillary vein to the first two 

 intercostal spaces, where it anastomoses with branches of the first 

 and second intercostal arteries. It supplies the adjacent intercostal 

 muscles, upper part of the serratus magnus, and occasionally the 

 pectoral muscles. The thoracic axis or acromio-thoracic artery is 

 a short, but large, trunk which arises immediately above the upper 

 border of the pectoralis minor. Passing straight forwards it pierces 

 the costo-coracoid membrane, and then divides into radiating 

 branches, called thoracic, acromial, humeral, and clavicular. Tlie 

 thoracic branches descend between the two pectoral muscles which 

 they supply. The acromial branches pass outwards over the cora- 

 coid process to the deltoid, in which some of them end, whilst others 

 pierce that muscle, and so reach the upper surface of the acromion 

 process, where they anastomose with branches of the suprascapular 

 and posterior circumflex. The humeral {descending) branch passes 



