26 THE SUPERIOR EXTREMITY 



Clean the pectoralis minor muscle without injuring the medial 

 anterior thoracic nerve, which pierces it. 



M. Pectoralis Minor. The pectoralis minor muscle is 

 triangular in outline. It arises (i) from the anterior ends 

 of the third, fourth, and fifth ribs, close to their junctions 

 with their cartilages, and (2) from the fascia covering the 

 intercostal muscles in the intervening spaces. Its fibres pass 

 upwards and laterally, and its tendon of insertion is attached 

 to the upper surface and the antero- medial border of the 

 horizontal part of the coracoid process, near its lateral 

 extremity. When the muscle is in action it draws the scapula 

 downwards and forwards, and depresses the shoulder. It 

 is supplied by the medial anterior thoracic nerve. The greater 

 portion of the pectoralis minor is concealed by the pectoralis 

 major, but the lower and medial part of its inferior border 

 appears on the lateral wall of the thorax below the pectoralis 

 major ; its insertion is concealed by the anterior fibres of the 

 deltoid. 



Dissection. Clear away the clavipectoral fascia below the 

 level of the pectoralis minor and open up the lower part of the 

 axilla ; remove also the deep fascia of the arm in the region of 

 the lateral boundary of the axilla. 



Commence at the lateral part of the area below the pectoralis 

 minor and clean the coraco-brachialis and the short head of the 

 biceps, as they descend into the arm from the tip of the coracoid 

 process. Find the distal part of the axillary artery at the medial 

 border of the coraco-brachialis. The trunk of the median nerve 

 lies between the artery and the muscle, and at the lower border 

 of the pectoralis minor the medial head of the nerve crosses the 

 front of the artery. Pull the coraco-brachialis laterally and find 

 the musculo-cutaneous nerve entering its deep surface, just 

 below the pectoralis minor. Above and to the lateral side of the 

 trunk of the musculo-cutaneous nerve find the branch from it 

 which supplies the coraco-brachialis. The axillary vein lies along 

 the medial side of the artery, and in the interval between the 

 artery and vein, anteriorly, is the medial cutaneous nerve of the 

 forearm (O.T. internal cutaneous). Running along the medial 

 side of the vein is the medial cutaneous nerve of the arm (O.T. 

 lesser internal cutaneous nerve) ; secure it and follow it upwards 

 to the communication which it receives from the intercosto- 

 brachial nerve (Fig. 13). At the same time identify and 

 preserve the lateral group of axillary lymph glands, which lie 

 along the medial side of the axillary vein. Secure the intercosto- 

 brachial nerve and follow it medially, to the point where it 

 emerges from the second intercostal space, and laterally to the 

 medial and posterior aspect of the arm, where it is distributed. 



In order to display the distribution of the intercosto-brachial 

 nerve, and to give better access to the medial and posterior walls 

 of the axilla, the axillary fascia must be separated from the fascia 



