THE UPPER EXTREMITY. 123 



If this fracture takes place at its centre, the displace- 

 ment of the outer fragment downwards, forwards, and 

 inwards, is due to the weight of the arm, and if put in 

 action, to the deltoid, pectoralis major, and subclavius. 

 The inner fragment is occasionally tilted upwards, but 

 only when the clavicular fibres of the sterno-cleido- 

 mastoid are in action, as in rotation of the neck, the 

 strong ligamentous attachment to the rib (costo-clavic- 

 ular), and perhaps the clavicular fibres of the pectoralis 

 major, retaining it in place. The slightness of the dis- 

 placement, on fracture of the acromial and sternal ex- 

 tremities, is also due to their strong ligamentous attach- 

 ment. The clavicle is separated from the upper part of 

 the axilla and its contents by the subclavius muscle 

 and its aponeurosis ; behind its sternal extremity is the 

 junction of the internal jugular vein and subclavian vein 

 running closely along it ; behind its upper border are 

 the supra-scapular vessels, which are liable to be injured 

 in fracture of the bone or operations upon it. 



External Aspect of the Shoulder. The deltoid muscle 

 forms the external boundary of the region of the shoulder. 

 The subcutaneous tissue contains the terminal twigs of 

 the acromial branches of the descending clavicular 

 nerves, and of the cutaneous branches of the circumflex 

 nerve and artery. The aponeurosis of the deltoid com- 

 pletely covers the muscle, and is continuous with that of 

 the upper arm and axilla, sending down septa between 

 the bundles of the muscular fibres, and passing beneath 

 it, is continuous with the deep fascia. The deltoid itself, 

 arising from the outer half of the clavicle and the lower 

 border of the spine of the scapula, after the convergence 

 and interlacement of its fibres, is inserted into the rough- 

 ened surface on the outer aspect of the humerus. 



