254 THE ARTICULATIONS. 



humerus from being pressed up against the acromion process, when the Deltoid 

 contracts, instead of forming the centre of motion in the glenoid cavity. By its 

 passage along the bicipital groove it assists in rendering the head of the humerus 

 steady in the various movements of the arm. When the arm is raised from the 

 side it assists the Supra- and Infraspinatus in rotating the head of the humerus in 

 the glenoid cavity. It also holds the head of the bone firmly in contact with the 

 glenoid cavity, and prevents its slipping over its lower edge, or being displaced by 

 the action of the Latissimus dorsi and Pectoralis major, as in climbing and many 

 other movements. 



Surface Form. The direction and position of the shoulder-joint may be indicated by a line 

 drawn from the middle of the coraco-acromial ligament, in a curved direction, with its con- 

 vexity inward, to the innermost part of that portion of the head of the humerus which can be 

 felt in the axilla when the arm is forcibly abducted from the side. When the arm hangs by the 

 side, not more than one-third of the head of the bone is in contact with the glenoid cavity, and 

 three-quarters of its circumference is in front of a vertical line drawn from the anterior border of 

 the acromion process. 



Surgical Anatomy. Owing to the construction of the shoulder-joint and the freedom of 

 movement which it enjoys, as well as in consequence of its exposed situation, it is more frequently 

 dislocated than any other joint in the body. Dislocation occurs when the arm is abducted, and 

 when, therefore, the head of the humerus presses against the lower and front part of the cap- 

 sule, which is the thinnest and least supported part of the ligament. The rent in the capsule 

 almost invariably takes place in this situation, and through it the head of the bone escapes, so 

 that the dislocation in most instances is primarily subglenoid. The head of the bone does not 

 usually remain in this situation, but generally assumes some other position, which varies accord- 

 ing to the direction and amount of force producing the dislocation and the relative strength of 

 the muscles in front and behind the joint. In consequence of the muscles at the back being 

 stronger than those in front, and especially on account of the long head of the Triceps pre- 

 venting the bone passing backward, dislocation forward is much more common than back- 

 ward. The most frequent position which the head of the humerus ultimately assumes is on the 

 front of the neck of the scapula, beneath the coracoid process, and hence named subcora- 

 coid dislocation. Occasionally, in consequence probably of a greater amount of force being 

 brought to beai un the limb, the head is driven farther inward, and rests on the upper part of 

 the front of the chest, beneath the clavicle (subclavicular). Sometimes it remains in the position 

 in which it was primarily displaced, resting on the axillary border of the scapula (subglenoid), 

 and rarely it passes backward and remains in the infraspinatous fossa, beneath the spine (sub- 

 spinous). 



The shoulder-joint is sometimes the seat of all those inflammatory affections, both acute and 

 chronic, which attack joints, though perhaps less frequently than some other joints of equal size 

 and importance. Acute synovitis may result from injury, rheumatism, or pyaemia, or may fol- 

 low secondarily on the so-called acute epiphysitis of infants. It is attended with effusion into 

 the joint, and when this occurs the capsule is evenly distended and the contour of the joint 

 rounded. Special projections may occur at the site of the openings in the capsular ligament. 

 Thus a swelling may appear just in front of the joint, internal to the lesser tuberosity, from effu- 

 sion into the bursa beneath the Subscapularis muscle; or, again, a swelling which is sometimes 

 bilobed may be seen in the interval between the Deltoid and Pectoralis major muscles, from effu- 

 sion into the diverticulum, which runs down the bicipital groove with the tendon of the biceps. 

 The effusion into the synovial membrane can be best ascertained by examination from the axilla, 

 where a soft, elastic, fluctuating swelling can usually be felt. 



Tubercular arthritis not unfrequently attacks the shoulder-joint, and may lead to total de- 

 struction of the articulation, when ankylosis may result or long-protracted suppuration may 

 necessitate excision. This joint is also one of those which is most liable to be the seat of osteo- 

 arthritis, and may also be affected in gout and rheumatism ; or in locomotor ataxy, when it 

 becomes the seat of Charcot's disease. 



p]xcision of the shoulder-joint may be required in cases of arthritis (especially the tuber- 

 cular form) which have gone on to destruction of the articulation ; in compound dislocations and 

 fractures, particularly those arising from gunshot injuries, in which there has been extensive 

 injury to the head of the bone ; in some cases of old unreduced dislocation, where there is much 

 pain ; and possibly in some few cases of growth connected with the upper end of the bone. The 

 operation is best performed by making an incision from the middle of the coraco-acromial liga- 

 ment down the arm for about three inches : this will expose the bicipital groove and the tendon 

 of the Biceps, which may be either divided or hooked out of the way, according as to whether it 

 is implicated in the disease or not. The capsule is then freely opened, and the muscles attached 

 to the greater and lesser tuberosities of the humerus divided. The head of the bone can 

 then be thrust out of the wound and sawn off, or divided with a narrow saw in situ and 

 subsequently removed. The section should be made, if possible, just below the articular surface, 

 so as to leave the bone as long as possible. The glenoid cavity must then be examined, and 

 gouged if carious. 



