ACKOMIO-CLAVICULAR JOINT. 319 



The ligaments which surround this small joint form a complete articular 

 capsule, of which the superior and inferior parts are specially strong, and are there- 

 fore named the superior and inferior acromio -clavicular ligaments (Fig. 303). These 

 consist of short fibres passing between the adjacent rough margins of the two bones 

 in the positions indicated by their names. 



An articular disc, which is nearly always incomplete, and may occasionally be 

 wanting, is usually found within the joint cavity, where it lies obliquely, with its 

 superior margin farther from the median plane than its inferior margin, and having 

 its borders attached to the surrounding capsule. Frequently the disc is wedge- 

 shaped, with its base directed upwards and its apex free. 



A synovial stratum is found forming either a single or a double sac, according to 

 the condition of the disc. Complete division of the joint cavity, however, is rare. 



Ligamentum Coracoclaviculare. Accessory to this articulation there is the 

 strong coraco-clavicular ligament which binds the acromial end of the clavicle to 

 the coracoid process of the scapula. It is readily divisible into two parts, viz., lig. 

 conoideuni and the lig. trapezoideum. 



The conoid ligament (Fig. 303) is situated medial to and slightly posterior to the 

 trapezoid. It is narrow and pointed at its inferior end, by which it is attached 

 to the superior aspect of the coracoid process, in close proximity to the scapular 

 notch. Its superior end widens out in the manner expressed by its name, and is 

 attached to the coracoid tuberosity of the clavicle. 



The trapezoid ligament (Fig. 303) is attached inferiorly to the superior surface of 

 the posterior half of the coracoid process, lateral and anterior to the attachment of 

 the conoid ligament. Superiorly it is attached to the ridge on the inferior surface 

 of the acromial end of the clavicle. Its lateral and medial borders are free. Its 

 anterior surface is principally directed upwards, and its posterior surface, to a 

 similar extent, looks downwards. 



A mucous or synovial bursa usually occupies the re-entrant angle between these 

 two ligaments. 



Movements at the Clavicular Joints. The movements of the medial end of the clavicle at 

 the sterno- clavicular joint are limited in their range, owing to the tension of the ligaments. 

 When the shoulder is raised or depressed the acromial end of the clavicle moves upwards and 

 downwards, whilst its sternal end glides upon the surface of the articular disc ; when, on the 

 other hand, the shoulder is carried forwards or backwards, the sternal end of the clavicle along 

 with the articular disc moves upon the sternal facet. In addition to these movements of elevation, 

 depression, forward movement and backward movement of the clavicle, there is also allowed 

 at the sterno-clavicular joint a certain amount of circumduction of the clavicle. 



The part which is played by certain of the ligaments in restraining movement requires 

 careful consideration. The costo-clavicular ligament checks excessive elevation of the clavicle, 

 and restrains within certain limits both backward and forward movement of the clavicle. When 

 the clavicle is depressed, as in cases where a heavy weight, such as a bucket of water, is carried in 

 the hand, it receives support by resting upon the first rib, and the tendency for the medial end 

 of the bone to start up out of its sternal socket is obviated by the tension of the articular 

 disc, the interclavicular ligament, and the anterior and posterior sterno-clavicular ligaments. 



The articular disc not only acts as a cushion which lessens the shock of blows received 

 upon the shoulder, but it also acts as a most important bond of union, and prevents the medial 

 end of the clavicle from being driven upwards upon the top of the sternum when force is applied 

 to its lateral end. 



The movements at the acromio -clavicular joint are of such a kind as to allow the inferior 

 angle, and to some extent the vertebral border of the scapula, to remain more or less closely 

 applied to the chest-wall during the various movements of the shoulder. The strong connexion 

 between the coracoid process and the acromial end of the clavicle, by means of the conoid and 

 trapezoid ligaments, renders it necessary that the scapula should follow the clavicle in its various 

 excursions. The presence of the acromio-clavicular joint, however, enables the scapula to change 

 its position somewhat with reference to the clavicle as the shoulder is moved. Thus, when the 

 shoulder is raised and depressed, a marked difference takes place in the angle between the two 

 bones ; again, when the shoulder is thrown forwards or backwards, these movements can be 

 performed without altering in a material degree the direction of the glenoid cavity of the scapula, 

 or in other words, the socket of the shoulder-joint. 



The conoid and trapezoid ligaments set a limit upon the movements of the scapula at the 

 acromio-clavicular joint. They both, but more particularly the trapezoid ligament, prevent the 

 acromion from being carried medially below the lateral end of the clavicle when blows fall upon 

 the lateral aspect of the shoulder. 



