THE BRACHIAL PLEXUS 137 



axilla loses its bulk as the muscles atrophy, and the axillary 

 border of the scapula can then be palpated without difficulty. 



The Axillary (Circumflex) Nerve arises from the posterior 

 cord in the axilla, where it lies behind the third part of the 

 axillary artery. After passing through the quadrilateral space, 

 it winds round the posterior aspect of the surgical neck of the 

 humerus and so reaches the deep surface of the deltoid. It 

 contains both motor and sensory fibres. The former are 

 distributed to the deltoid and the teres minor. 



The Deltoid arises from the anterior border of the lateral 

 thi'rd of the clavicle, the tip and lateral border of the acromion 

 and the lower border of the spine of the scapula. From this 

 wide origin the fibres pass distally and converge to be inserted 

 into the middle of the lateral surface of the shaft of the 

 humerus. The anterior fibres aid in the movements of flexion 

 and medial rotation, while the posterior fibres take part in the 

 opposite movements. Acting as a whole, the deltoid is a 

 powerful abductor of the humerus, and, in this movement, it 

 is aided only by the supra-spinatus (p. 131). 



The teres minor lies along the lateral border of the infra-spinatus, and it 

 performs the same actions as that muscle, i.e. it laterally rotates, adducts 

 and extends the humerus. 



When the axillary nerve is paralysed, abduction of the 

 humerus is the only movement which is markedly affected. 

 A certain degree of this movement, however, is still possible, 

 as the supra-spinatus is not involved, and, further, the latter 

 muscle is assisted by the serratus anterior, which acts through 

 the shoulder-girdle. Atrophy of the deltoid is easy to determine. 

 The deltoid covers the greater tubercle of the humerus and, 

 in this way, it gives the shoulder its normal rounded appear- 

 ance. In atrophy of the muscle, the lateral border of the 

 acromion becomes more distinct and the shoulder loses its 

 normal contour. The tubercles are easy to palpate and the 

 coracoid process, which is normally covered by the anterior 

 fibres of the muscle, may cause a surface elevation below the 

 junction of the intermediate and lateral thirds of the clavicle. 



