THE SCAPULO-CLAVICULAR 219 



Motion takes place in nearly every direction — viz. upwards, downwards, forwards, 

 backwards, and in a circuniductory manner. The upward and downward motions 

 occur between the clavicle and the fibro-cartilage; during elevation of the arm, the 

 upper edge of the clavicle with its attached fibro-cartilage is pressed into the sternal 

 S(jcket, and the lower edge glides away from the cartilage; during depression of the 

 limb, the lower edge of the clavicle presses on to the cartilage, while the rest of the 

 articular surface of the clavicle inclines outwards, Ijringing with it to a slight 

 degree the upper edge of the fibro-cartilage. These movements occur on an antero- 

 posterior axis drawn through the outer compartment of the joint. The forward 

 and backward motions take place Ijetween the cartilage and sternum, the clavicle 

 Avith the cartilage rolling Ijackwards upon the sternum when the shoulder is 

 brought forwards, and forwards when the shoulder is ftn-ced backwards; these 

 iuovements occur round an axis drawn nearly vertically through the sternal socket. 

 The interarticular cartilage serves materially to l)ind the bones together, and to 

 prevent the inward and upward displacements of the clavicle. It also forms an 

 elastic buffer which tends to break shocks. The capsule, by being moderately 

 tight, tends to limit movements in all directions, while the interclavicular ligament 

 is a safeguard against upward displacement during depression of the arm. The 

 rhomboid ligament prevents dislocation upwards during elevation of the arm, and 

 resists displacements backwards. 



(2) THE SCAPULO-CLAVICULAR UXIOX 



The scapula is connected with the clavicle by a synovial joint Avith its liga- 

 ments; and also l;)y a set of ligaments passing betw^een the coracoid process and the 

 clavicle. So that we have to consider — 



(a) The acromio-clavicular articulation. 



(b) The coraco-clavicular ligaments. 



(r) Tlic proper scapular ligaments are also best described in this section — 

 viz. the coraco-acromial and transverse. 



(a) The Ackomio-clavktlar .Ioint 

 Class. — DinrtJtrosi^. Subdivision. — Arthrodia. 



The acromio-clavicular joint is surrounded by a capsular ligament and fre- 

 quently contains an interarticular fibro-cartilage. 



The capsular ligament (figs. 223 and 226) completely surrounds the articular 

 margins, and is C(jmposed of strong, coarse fibres arranged in parallel fasciculi, of 

 fairly uniform thickness, which are attached to the borders as well as the surfaces 

 of the bones. It is somewhat lax in all positions of the joint, so that the clavicle 

 is not tightly braced to the acromion. The fibres extend three-quarters of an inch 

 (2 cm.) along the clavicle posteriorly, but only a c[uarter of an inch (6 mm.) 

 anteriorly. Superiorly, they are attached to an obli(iue line joining these two 

 points, while inferiorly they reach to the ridge for the trapezoid ligament with 

 which they blend. At the acromion they extend half Avay across the upper and 

 lower surfaces, but at the anterior and posterior borders of that process they are 

 attached close to the articular facet. The anterior fibres become blended with 

 the insertion of the coraco-acromial ligament. The fibres are strengthened above 

 by the aponeuroses of the trapezius and rJrJtold muscles; and all run from the 

 acromion to the clavicle inwards and liackwards. 



The interarticular fibro-cartilage is occasionally present, but is i;sually im- 

 perfect, only occui)ying the upper jKirt of the joint; it may completely divide the 

 joint into two cavities, or be perforated in the centre. It is usually thicker at the 

 edge than in the centre, and some of the fibres of the capsular ligament are blendi d 

 with its edges. 



The synovial membrane lining the joint is occai^ionally either partially or 

 entirely divided into two Ijy an interarticular fibro-cartilage. 



