io THE SUPERIOR EXTREMITY 



it will find its way down between the two pleural sacs into the superior 

 mediastinum, following the course of the innominate vein. 



On the right side, the joint may be involved in aneurism of the innominate 

 artery, which lies immediately behind it. 



Acromio- Clavicular Joint. The articular surfaces of 

 the two bones are oblique and the plane of the joint passes 

 downwards and medially. A weak capsule,, thickened above 

 and below, is attached to the articular margins. A wedge- 

 shaped disc protrudes into the joint cavity from the upper 

 part of the capsule ; the disc is covered and the capsule is lined 

 with synovial membrane. The joint is strengthened by the 

 accessory conoid and trapezoid bands, which together form the 

 coraco - clavicular ligament and bind the upper surface of 

 the coracoid process to the under surface of the clavicle near its 

 acrornial end. Part of the weight of the upper limb is conveyed 

 to the clavicle along this ligament. 



The posterior supra-clavicular (supra-acromial) nerves (C. 3 

 and 4) and the acromial branch of the thoraco-acromial artery 

 (p. 30) supply the joint. 



A gliding antero-posterior movement occurs at the acromio- 

 clavicular articulation, so that, when the shoulder is thrust 

 forwards, the angle between the clavicle and the upper border 

 of the spine of the scapula becomes smaller. 



The obliquity of the articular surfaces serves to explain the 

 common injury of upward dislocation of the clavicle. Any force 

 directed medially or downwards on to the acromion will thrust 

 it beneath the clavicle, tearing the superior ligament of the 

 joint. The reverse dislocation is extremely rare. A much 

 exaggerated displacement in either dislocation indicates rupture 

 of the coraco-clavicular ligament. 



The Clavicle fulfils the functions of a prop and keeps the 

 acromion, and, through it, the shoulder-joint at a constant 

 distance from the trunk, to which it transmits the weight of the 

 upper limb. The trapezius supports its acromial end and 

 counteracts the downward pull of the weight of the arm. The 

 clavicle is exposed more often to violence than any other bone 

 in the body and, for that reason, is very commonly injured. 

 These injuries may take the form of fractures, dislocations, or 

 separation of the epiphysis. 



Ossification. The clavicle is the first bone in the body to ossify (fifth 

 week), and the primary centre forms the shaft and the acromial end. A 

 secondary centre appears at the sternal end at about the twentieth year 

 and fuses with the shaft about the twenty-fifth. Congenital absence of both 



