THE SHOULDER-JOINT 



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Relations. — The following muscles are in contact with the capsule of the shoulder-joint. 

 In front, the subscapularis; above, the supraspinatus; above and behind, the infraspinatus; 

 behind, the teres minor; below, the long head of the triceps and the teres major. In the interval 

 between the subscapularis and the supraspinatus the subacromial bursa is close to the capsule 

 and occasionally its cavity communicates with the cavity of the joint. 



The axillary (circumflex) nerve and posterior circumflex artery pass beneath the capsule 

 in the interval between the long head of the triceps, the humerus, and the teres major. When 

 the arm is abducted, the long head of the tricepS and the teres major are drawn into closer rela- 

 tion with the capsule and help to prevent dislocation of the humerus. 



The axillary vessels, the great nerves of the axilla, the short head of the biceps, and the 

 coraco-brachialis are separated from the joint by the subscapularis, whilst the deltoid forms a 

 kind of cap, which extends from the front to the back over the more immediate relations. 



The arterial supply is derived from the transverse scapular (suprascapular), anterior and 

 posterior circumflex, subscapular, circumflex scapular (dorsalis scapulae), and a branch from the 

 second portion of the axillary artery. 



The nerve-supply is derived from the suprascapular, by branches in both fossse; and from 

 the axillary (circumflex) and subscapular nerves. 



The movements of the shoulder-joint consist of flexion, extension, adduction, abduction, 

 rotation and circumduction. 



Flexion is the swinging forward, extension the swinging backward, of the humerus; abduc- 

 tion is the raising of the arm from, and adduction depression of the arm to, the side. In flexion 

 and extension the head of the humerus moves upon the centre of the glenoid fossa round an 



Fig. 292. — Biceps Tendon, Bifurcating and Blending on each Side with the Glenoid 



Ligament. 



Tendon of biceps 



Tendon of biceps blended with glenoid 

 ligament 



oblique line corresponding to the axis of the head and neck of the humerus, flexion being more 

 free than extension, and in extreme flexion the scapula follows the head of the humerus, so as 

 to keep the articular surfaces in apposition. In extension the scapula moves much less, if 

 at all. „ , J 



In abduction and adduction the scapula is fixed, and the humerus rolls up and down upon 

 the glenoid fossa; during abduction the head descends until it projects beyond the lower edge 

 of the glenoid cavity, and the greater tuberosity impinges against the arch of the acroimon; 

 during adduction, the head of the humerus ascends in its socket, the arm at length reaches the 

 side, and the capsule is completely relaxed. 



In circumduction, the humerus, by passing quickly through these movements, describes 

 a cone, whose apex is at the shoulder-joint, and the base at the distal extremity of the bone 

 or limb. . . e ,■, ■> 



Rotation takes place round a vertical axis drawn through the extremities of the humerus 

 from the centre of the head to the inner condyle; in rotation forward (that is, medialward) the 

 head of the bone roUs back in the socket as the great tuberosity and shaft are turned forward; 

 in rotation backward (that is, lateral ward) the head of the bone glides forward, and the tuber- 

 osity and shaft of the humerus are turned backward, i. e., lateralward. ^ j u +t. 



Great freedom of movement is permitted at the shoulder, and this is increased by tne 

 mobility of the scapula. Restraint is scarcely exercised at all upon the movements of the 

 shoulder by the ligaments, but chiefly by the muscles of the joint. 



In abduction, the lower part of the capsule is somewhat, and m extreme abduction con- 

 siderably, tightened; and in rotation medialward and lateralward, the upper part ot the capsule 

 is made tense, as is also, in the latter movement, the coraco-humeral hgarnent. . 



The movements of abduction and extension have a most decided and defanite resistance 

 offered to them other than by muscles and ligaments, for the greater tuberosity of the humerus, 

 by striking against the acromion process and coraco-acromial ligament, stops short any turtner 

 advance of the bone in these directions, and thus abduction ceases altogether as soon as the arm 



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