THE SHOULDER-JOINT. 253 



joint. 3. The intimate connection of the capsule with the muscles attached to the 

 head of the humerus. 4. The peculiar relation of the biceps tendon to the joint. 



It is in consequence of the relative size of the two articular surfaces that the 

 joint enjoys such free movement in every possible direction. 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 corresponding fibres of the capsule, together with that of 

 the muscles acting as accessory ligaments, they can be carried considerably farther 

 by the movements of the scapula, involving, of course, motion at the acromio- and 

 sterno-clavicular joints. These joints are therefore to be regarded as accessory 

 structures to the shoulder-joint. 1 The extent of these movements of the scapula is 

 very considerable, especially in extreme elevation of the arm, which movement is 

 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 bicipital groove downward, inward, and backward, and the 

 greatest elevation of the arm can be obtained by rolling its articular surface in the 

 direction of the 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 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). Cleland maintains that the limitations of movement at 

 the shoulder-joint are due to the structure of the joint itself, the glenoid ligament 

 fitting, in different positions of the elevated arm, into the anatomical neck of the 

 humerus. 



Cathcart 3 has pointed out that in abducting the arm and i-aising 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 from the hanging to the horizontal position, but also in passing 

 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 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. 4 Next, it 

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

 1 See p. 249. 2 Journal of Anatomy and Physiology, vol. xviii., 1884. 3 Ibid. 4 See p. '2'2'2. 



