120 



THE SUPERIOR EXTREMITY 



of the glenoid cavity ; (2) by the great laxity of the ligament- 

 ous structures which connect the humerus with the scapula. 

 These provisions for allowing an extensive range of movement 

 at this articulation might, at first sight, lead one to doubt the 

 security of the joint. Its strength certainly does not lie in 

 the adaptation of the bony surfaces to one another, nor in 

 the strength of its ligaments. It lies (i) in the powerful 

 muscles by which it is closely surrounded; (2) in the over- 

 hanging coraco-acromial arch, which forms, as it were, a 



Coraco-acromial 

 ligament 



Coracoid process / 



Acromion 



Coraco-humeral 

 ligament 



Communication 

 between joint- 

 cavity and sub- 

 scapular bursa 



Capsule of joint 



FIG. 51. Shoulder-joint as seen from the front. 



secondary socket for the head of the humerus, and effectually 

 prevents any displacement in an upward direction ; and (3) 

 in atmospheric pressure, which exercises a powerful influence 

 in keeping the opposed surfaces in contact with each other. 



On all aspects, except over a small area below, the loose, 

 ligamentous capsule which envelops the shoulder-joint is 

 supported by muscles, the tendons of which are more or less 

 intimately connected with it. Above, it is covered by the 

 supraspinatus ; behind, the infraspinatus and teres minor are 

 applied to it ; in front is the subscapularis. Below, the 

 capsule is to a certain extent unsupported by muscles, and 



