84 THE SUPERIOR EXTREMITY 



deltoid. It springs from the posterior cord of the brachial 

 plexus, turns round the lower border of the subscapularis, 

 and passes backwards, with the posterior humeral circumflex 

 artery, through the quadrilateral space to the back of the limb. 

 There it divides into an anterior and a posterior division. 

 The articular branch takes origin from the trunk of the nerve 

 in the quadrilateral space, and enters the joint from below. 

 The posterior division gives off the branch to the teres minor, 

 and, after furnishing a few twigs to the posterior part of the 

 deltoid, is continued onwards, as the lateral cutaneous nerve 

 of the arm> which has already been dissected in the superficial 

 fascia over the distal part of the deltoid (Figs. 31, 32, 35). 

 The nerve to the teres minor is distinguished by the presence 

 of an oval, gangliform swelling upon it. 



The anterior division proceeds round the humerus with the 

 posterior circumflex artery of the humerus, and ends near the 

 anterior border of the deltoid. It is distributed, by many 

 branches, to the deep surface of the muscle, whilst a few 

 fine filaments pierce the deltoid and reach the skin. 



Dissection. Clean the coraco-acromial ligament, which 

 extends from the coracoid process to the acromion. Note that 

 the coracoid process, the coraco-acromial ligament, and the 

 acromion form an arch the coraco-acromial arch. The arch 

 lies above the shoulder- joint, but is separated from it by the 

 subacromial bursa, and the humeral ends of the supraspinatus, 

 the infraspinatus, and the subscapularis. 



Arcus Coracoacromialis. The coraco-acromial arch should 

 be examined at the present stage, in order that its relation- 

 ship to the subacromial bursa and the supraspinatus may be 

 appreciated. It is the arch which overhangs the shoulder- 

 joint and protects it from above. It is formed by the 

 coracoid process, the acromion, and a ligament the coraco- 

 acromial which stretches between them. 



The coraco-acromial ligament is a strong band of a some 

 what triangular shape. By its base it is attached to the lateral 

 border of the coracoid process, whilst by its apex it is attached 

 to the extremity of the acromion (Figs. 51, 52, 54). 



The coraco-acromial arch plays a very important part in 

 the mechanism of the shoulder ; it might almost be said to 

 form a secondary socket for the humerus. The large sub- 

 acromial bursa, which intervenes between the acromion and 

 the muscles immediately covering the capsule of the shoulder- 



