ARTICULATIO HUMERI 



89 



of the glenoid cavity the socket in which it moves ; (2) by the great laxity 

 of the ligamentous 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 power of its ligaments. It lies (i) in the intimate 

 manner in which the scapular muscles are arranged around it ; (2) in the 

 overhanging coraco-acromial arch which forms, as it were, a secondary 

 socket for the head of the humerus, and effectually prevents any displace- 

 ment in an upward direction ; and (3) in atmospheric pressure, which 



Supraspinatus 



Scapuh 



Subscapularis 



Serratus anterior 



Capsule of joint 



Humerus 

 Deltoid 



Fold of capsule 

 of joint 



Posterior circumflex artery 

 of humerus and axillary 

 (O.T. circumflex) nerve 



Teres major 



Radial (O.T. musculo-spiral) 

 nerve 



Latissimus dorsi 



FIG. 38. -Coronal or vertical transverse section through the Left 

 Shoulder-joint. (Viewed from behind. ) 



exercises a powerful influence in keeping the opposed surfaces in contact 

 with each other. 



From all points of view, 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 here it is 

 prolonged downwards, in the form of a fold, in the ordinary dependent 

 position of the limb (Fig. 38). When, however, the arm is abducted, this 

 fold is obliterated, and the head of the bone rests upon the inferior part 

 of the capsule, which now receives partial support from two muscles which 



