304 THE ARTICULATIONS, OR JOINTS 



the acromio- and sternoclavicular 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 con- 

 siderable, 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 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 supplemental 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 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 coraco- 

 humeral ligament" (Beaunis et Bouchard). Cleland 2 maintains that the limitations of move- 

 ment 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 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 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 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 adverted to. 4 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 Infra- 

 spinatus 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 coracoacromial 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. 



Applied 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 fre- 

 quently dislocated than any other joint in the body. Dislocations of the shoulder contribute 

 about forty per cent, of the cases in tables of dislocations. Dislocation occurs when the arm 

 is thrown into extreme abduction, and when, therefore, the head of the humerus presses against 

 the lower and front part of the capsule, which is the thinnest and least supported part of the liga- 



i gee p 299 J Journal of Anatomy and Physiology, 1884, vol. xviii. 



3 Ibid. .vol. xviii. 4 See p. 267. 



