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SHOULDER JOINT (NORMAL ANATOMY). 



shoulder joint, and as being intended to com- 

 pensate for the incomplete reception of the 

 head of the humerus bytheglenoid cavity. The 

 centre of this arch is formed by the coraco- 

 acromial or triangular ligament, of which the 

 apex is situated at the acromion, and the base 

 at the outer edge of the coracoid process. 

 The ligament consists of two bundles, sepa- 

 rated by a cellular interval, and placed more 

 anteriorly the one than the other. The acro- 

 mion and coracoid processes constitute re- 

 spectively the extremities and the points of 

 support to the arch, whilst its under surface 

 is accurately adapted to the convexity of the 

 head of the humerus, the tendon of the supra- 

 spinatus muscle intervening. The existence 

 of the large bursa (elsewhere noticed) between 

 this tendon and the coraco-acromial ligament, 

 abundantly proves that considerable motion 

 takes place between them : the upper surface 

 of the ligament is concealed by the deltoid 

 muscle. 



In this arrangement may be recognised a 

 provision for protecting the shoulder-joint 

 against violence from above (Voute protec- 

 tatricc, Blaudin), and the humerus against 

 displacement from below, either directly up- 

 wards or with an inclination backwards or 

 forwards. And for such a provision there is 

 the greater necessity, as the upper extremity 

 is constantly exposed to forces which act upon 

 it from below. 



Mechanical functions. In common with 

 other enarthrodial articulations, the shoulder 

 joint enjoys the following varieties of motion : 

 1. Flexion ; 2. extension; 3. adduction; 4. ab- 

 duction ; 5. circumduction ; and 6. rotation. 



1. Of the opposed motions of flexion and 

 extension, the former possesses the greater 

 latitude. When carried to its utmost extent, 

 the humerus appears to move through the 

 arc of half a circle of which the centre is at 

 the joint ; for the arm from being parallel to 

 the trunk in the direction downwards, may by 

 this motion be raised vertically upwards. 



2. Extension, on the other hand, is much 

 more limited, being restrained by the great 

 strength of the anterior portion of the capsule, 

 by the inter-articular ligaments, and by the 

 contact of the head of the humerus with the 

 coracoid process, which are all calculated to 

 check the advance of the head of the humerus, 

 the necessary result of extension. 



Flexion and extension, although apparently 

 performed in the scapulo-humcral articulation 

 solely, are really distributed over a much 

 more extended sphere, being shared by the 

 scapula, and by the articulations of the clavicle 

 with the acromion and with the sternum. 



When extreme flexion or extension takes 

 place, the scapula undergoes a motion of 

 rotation upon its axis (an imaginary line pass- 

 ing through the centre of the bone) ; and the 

 result is, that when the humerus is flexed the 

 superior angle of the scapula moves back- 

 wards,and its inferior angle for wards ; whereas, 

 in extension of the arm, a change of position 

 the reverse of this is produced in the scapula. 

 This rotation of the scapula is favoured by 



the looseness of the ligamentous connections 

 of the acromio-clavicular articulation, whilst 

 it is restrained within bounds by the coraco- 

 clavicular ligaments (conoid and trapezoid). 

 The trapezoid limits the advance of the upper 

 angle of the scapula ; the conoid checks the 

 rotation which would carry it in the opposite 

 direction. 



The muscles which chiefly effect the rotation 

 of the scapula are, the trapezius, latissimus 

 dorsi, levator anguli scapula?, rhomboidei scapu- 

 las, serratus magnus anticus, and the pectoralis 

 minor. Of these the trapezius and serratus 

 magnus rotate the scapula, so as to elevate its 

 acromial end ; whilst the rhomboidei muscles 

 and the pectoralis minor produce the contrary 

 effect ; the latissimus dorsi can only act on 

 the scapula when it takes an origin from its 

 inferior angle. If it were possible for the 

 levator anguli scapulae to act independently 

 of the other scapular muscles, it would depress 

 the acromion ; but as this rarely, if ever occurs, 

 its ordinary action is to assist those muscles 

 which elevate the entire scapula, and, conse- 

 quently, the shoulder joint. 



3. The motions of adduction and abduction 

 are remarkably contrasted : the former can 

 hardly, with strict propriety, be said to exist, 

 being prevented by the immediate contact of 

 the arm with the side ; adduction, however, 

 in an oblique direction forwards and inwards, 

 is permitted. This motion is limited by the 

 projection of the thorax : when the arm is 

 placed in this position the head of the humerus 

 is strongly pressed against the posterior por- 

 tion of the capsule, and if force were to be 

 applied to the distal extremity of the lever 

 under these circumstances, dislocation back- 

 wards might be produced. 



4. The motion of abduction is the most 

 extensive of those enjoyed by the shoulder- 

 joint; it permits the separation of the arm 

 from the side, until it becomes parallel to the 

 trunk in a direction upwards ; flexion has 

 been stated to be capable of the same range, 

 but the latter owes much of its freedom to 

 the mobility of the scapula, whereas in abduc- 

 tion the scapula moves but little, and nearly 

 all the motion takes place in the scapulo- 

 humeral articulation. 



Abduction is limited by the contact of the 

 neck of the humerus with the acromion, and 

 by the resistance of the capsular ligament. 

 When fully performed, the head of the humerus 

 revolves in the glenoid cavity, and in its de- 

 scent presses strongly against the inferior 

 portion of the capsule ; if force be now ap- 

 plied to the upper extremity from above, the 

 ligament may give way and dislocation be 

 effected. More frequently this accident oc- 

 curs when the arm is moderately abducted, 

 and the mechanism by which, under such cir- 

 cumstances, it is effected, may be briefly ex- 

 plained. When a person falls on the inside 

 of the elbow, while the arm is abducted, the 

 upper extremity represents a lever of the third 

 order, of which the fulcrum is at the point of 

 contact of the elbow with the ground, and the 

 power at the " folds of the axilla;" the at- 



