348 THE ARTICULATIONS. 



coracoid process, and the coraco-acromial ligament constitutes a sort of supple- 

 mental articular cavity for the head of the bone. 



The looseness of the capsule is so great that the arm will fall about an inch 

 from the scapula when the muscles are dissected from the capsular ligament and 

 an opening made in it to remove the atmospheric pressure. The movements of 

 the joint, therefore, are not regulated by the capsule so much as by the surrounding 

 muscles and by the pressure of the atmosphere an arrangement which " renders 

 the movements of the joint much more easy than they would otherwise have been, 

 and permits a swinging, pendulum-like vibration of the limb when the muscles 

 are at rest" (Humphry). The fact, also, that in all ordinary positions of the joint 

 the capsule is not put on the stretch enables the arm to move freely in all direc- 

 tions. Extreme movements are checked by the tension of appropriate portions of 

 the capsule, as well as by the interlocking of the bones. Thus it is said that 

 " abduction is checked by the contact of the great tuberosity with the upper edge of 

 the glenoid cavity, adduction by the tension of the coraco-humeral ligament " 

 (Beaunis et Bouchard). 



The intimate union of the tendons of the four short muscles with the capsule 

 converts these muscles into elastic and spontaneously acting ligaments of the joint, 

 and it is regarded as being also intended to prevent the folds into which all portions 

 of the capsule would alternately fall in the varying positions of the joint from being 

 driven between the bones by the pressure of the atmosphere. 



The peculiar relations of the Biceps tendon to the shoulder-joint appear to sub- 

 serve various purposes. In the first place, by its connection with both the shoulder 

 and elbow the muscle harmonizes the action of the two joints, and acts as an 

 elastic ligament in all positions, in the manner previously adverted to. 1 Next, it 

 strengthens the upper part of the articular cavity, and prevents the head of the 

 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. Occasionally, in consequence probably of a greater amount of force being brought to 

 bear on 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), 



1 See p. 318. 



