320 



SYNDESMOLOGY 



It is in consequence of the relative sizes of the two articular surfaces, and the looseness 

 the articular capsule, that the joint enjoys such free movement in all directions. When these> 

 movements of the arm are arrested in the shoulder- joint by the contact of the bony surfaces, 

 and by the tension of the fibers of the capsule, together with that of the muscles acting as accessory 

 ligaments, the arm can be carried conside r ably farther by the movements of the scapula, involv- 

 ing, of course, motion at the acromio- and sternoclavicular joints. These joints are therefore 

 to be regarded as accessory structures to the shoulder-joint (see pages 314 and 316). The extent 

 of the scapular movements is very considerable, especially in extreme elevation of the arm, u 

 movement best accomplished when the arm is thrown somewhat forward and outward, because 

 the margin of the head of the humerus is by no means a true circle; its greatest diameter is from 

 the intertubercular groove, downward, medialward, and backward, and the greatest elevation 

 of the arm can be obtained by rolling its articular surface in the direction of this measurement. 

 The great width of the central portion of the humeral head also allows of very free horizontal 

 movement when the arm is raised to a right angle, in which movement the arch formed by the 

 acromion, the coracoid process and the coracoacromial 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 2.5 cm. from the scapula 

 when the muscles are dissected from the capsule, and an opening made in it to counteract 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 directions. 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 coracohumeral ligament" (Beaunis 

 et Bouchard). Cleland 1 maintains that the limitations of movement at the shoulder-joint are 

 due to the structure of the joint itself, the glenoidal labrum fitting, in different positions of the 

 elevated arm, into the anatomical neck of the humerus. 



The scapula is capable of being moved upward and downward, forward and backward, or, by 

 a combination of these movements, circumducted on the wall of the chest. The muscles which 

 raise the scapula are the upper fibers of the Trapezius, the Levator scapula 1 , and the Rhomboidei; 

 those which depress it are the lower fibers of the Trapezius, the Pectoralis minor, and, through 

 the clavicle, the Subclavius. The scapula is drawn backward by the Rhomboidei and the middle 

 and lower fibers of the Trapezius, and forward by the Serratus anterior and Pectoralis minor, 

 assisted, when the arm is fixed, by the Pectoralis major. The mobility of the scapula is very 

 considerable, and greatly assists the movements of the arm at the shoulder- joint. Thus, in 

 raising the arm from the side, the Deltoideus and Supraspinatus can only lift it to a right angle 

 with the trunk, the further elevation of the limb being effected by the Trapezius and Serratus 

 anterior moving the scapula on the wall of the chest. This mobility is of special importance in 

 ankylosis of the shoulder- joint, the movements of this bone compensating to a very great extent 

 for the immobility of the joint. 



Cathcart 2 has pointed out that in abducting the arm and raising it above the head, the scapula 

 rotates throughout the whole movement with the exception of a short space at the beginning 

 and at the end; that the humerus moves on the scapula not only while passing from the hanging 

 to the horizontal position, but also in travelling upward as it approaches the vertical above; 

 that the clavicle moves not only during the second half of the movement but in the first as well, 

 though to a less extent i. e., the scapula and clavicle are concerned in the first stage as well 

 as in the second; and that the humerus is partly involved in the second as well as chiefly in the 

 first. 



The intimate union of the tendons of the Supraspinatus, Infraspinatus, Teres minor and 

 Subscapularis with the capsule, converts these muscles into elastic and spontaneously acting 

 ligaments of the joint. 



The peculiar relations of the tendon of the long head of the Biceps brachii to the shoulder- 

 joint appear to subserve 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 discussed (see page 287). It strengthens the 

 upper part of the articular cavity, and prevents the head of the humerus from being pressed up 

 against the acromion, when the Deltoideus contracts; it thus fixes the head of the humerus as 

 the center of motion in the glenoid cavity. By its passage along the intertubercular groove it 

 assists in steadying the head of the humerus in the various movements of the arm. When the 

 arm is raised from the side it assists the Supraspinatus 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. 



1 Journal of Anatomy and Physiology, 1867, i, 85. 



'Ibid., 18S4, vol. xviii. 



