The Region of the Clavicle. 15 



the fourth cervical sends a branch to the fifth, and the 

 fifth supplies the joint through its branch, the subclavian 

 nerve. Again, the serratus magnus, a powerful factor in 

 the movement of the joint, is supplied by the external 

 thoracic nerve, and this derives one of its roots from the 

 fifth cervical nerve. Pain, in synovitis of the sterno-cla- 

 vicular articulation, may be local, or may be referred to 

 the shoulder or to the region of the clavicle through the 

 descending branches of the superficial cervical plexus, 

 which arise from the fourth cervical nerve and are, there- 

 fore, related to the joint by the branch from the fourtih to 

 the fifth nerve. 



Dislocation at the sterno-clavicular articulation is 

 very rare, since the tough unyielding ligaments bind the 

 bones, firmly, together, and when it does occur, the sternal 

 end of the clavicle is displaced, generally, forwards, as 

 shown by Hamilton, who reported thirteen cases of sterno- 

 clavicular displacement, of which eleven were forwards, 

 and two upwards. Acromio- clavicular joint. When dis- 

 location is present at this joint, the acromial end of the 

 clavicle is, as a rule, displaced on to the upper surface of 

 the acromion process, since the oblique direction of the 

 line of joint favors over-riding of the clavicle. The in- 

 tegrity of the coraco-clavicular ligament, and especially 

 the trapezoid portion of it, prevents much lateral displace- 

 ment, so that not more than half an inch of the clavicle, 

 as a rule, rests on the acromion. 



Fracture of the Clavicle, when complete, and 

 when due to indirect violence, occurs, as a rule, at the 

 junction of the outer and middle thirds of the bone. Vi- 

 brations, set up by a blow applied to the point of the shoul- 

 der, will alter their amplitude at the inner end of the outer 

 third of the bone for the following reasons: (1) Since 

 both the coraco-clavicular ligament and the trapezius mus- 



