THE UPPER LIMB 3i5 



fifth, sixth, and seventh cervical, crosses ovei the first part of the 

 axillary artery from without inwards, and gives a branch to the 

 internal anterior thoracic. Thereafter it pierces the costo-cora- 

 coid membrane, and is distributed to the pectoralis major. 



The musculo-cutaneous or perforating nerve of Casserius, which 

 derives its fibres from the fifth, sixth, and seventh cervical, arises 

 about the level of the pectoralis minor. For a short distance it 

 lies on the outside of the axillary artery, but it soon leaves the 

 vessel, and pierces the coraco-brachialis in a direction dowTiwards 

 and outwards. Before reaching the muscle it parts with a branch 

 to it, which usually enters it in two divisions. The fibres of this 

 branch are derived from the seventh cervical. The subsequent 

 course of the musculo-cutaneous will be described later on. 



The outer root of the median passes slightly on to the axillary 

 artery, where it is joined by the inner root. 



Inner Cord. — ^The branches of this cord are the internal anterior 

 thoracic, the lesser internal cutaneous or nerve of Wrisberg, the 

 internal cutaneous, iimer root of the median, and ulnar. 



The internal anterior thoracic nerve, which derives its fibres 

 from the eighth cervical and first thoracic, passes behind the first 

 part of the axillary artery, and then comes forwards between the 

 artery and the axUlary vein. Having received a branch from the 

 external anterior thoracic nerve, which forms a loop over the first 

 part of the artery, it breaks up into branches which enter the deep 

 surface of the pectoralis minor, a few of them piercing that muscle 

 to enter the deep surface of the pectoralis major. 



The lesser internal cutaneous or nerve of Wrisberg, which derives 

 its fibres from the first thoracic, is at first placed behind the axillary 

 vein, but subsequently descends on its inner side, where it communi- 

 cates with the intercosto-humeral nerve. This nerve is sometimes 

 absent. 



The internal cutaneous, which derives its fibres from the eighth 

 cervical and first thoracic, descends partly on the axillary artery 

 and partly to its inner side. 



The inner root of the median, which is of smaller size than the 

 outer, passes obUquely over the axillary artery, and joins the outer 

 root a little below the lower border of the pectoralis minor. The 

 tnmk of the nerve, which derives its fibres from all the nerves of the 

 plexus, then descends on the outer side of the third part of the 

 artery. 



The ulnar nerve is the largest bremch of the iimer cord, of which it 

 is the continuation, and it derives its fibres from the eighth cervical 

 and first thoracic. It appears at the lower border of the pectoraHs 

 minor, and then descends, lying deeply between the third part of 

 the axillary artery and the vein. 



Posterior Cord. — ^The branches of this cord are the three sub- 

 scapular nerves, circumflex, and musculo-spiral. 



The subscapular nerves are distinguished as upper or short, 

 middle or long, and lower. The upper or short subscapular nerve, 



