Limbs : Upper Limb 67 



checked by the rhomboid ligament, and the amount of play allowed by the ligament 

 depends on its nearness to the axis of rotation combined with the direction of its 

 fibres : they run upwards and somewhat inwards, and thus a certain amount of move- 

 ment outwards on the elevating clavicle is allowed before they become tense. 



The security of the joint depends on these two connections, so that the remaining 

 capsular fibres are comparatively weak and loose, particularly below and in front : the 

 joint is supported in front by the sternal head of Sterno-mastoid passing over it, and 

 the capsule is strengthened behind by the origin of Steraio-hyoid from it, extending 

 out from it on to the back of the clavicle. A certain amount of antero-posterior 

 movement, and even of very slight rotation, is possible in addition to elevation (see 

 under " Scapula "). 



The markings of the acromio-clavicular ligaments are slight because these liga- 

 ments are feeble, though the upper ones are strengthened by fibres derived from the 

 Trapezius. The security of the joint really depends on the coraco-clavicular ligaments, 



FIG. 52. Diagram of sterno-clavicular joint, opened on one side to show 

 the meniscus dividing it into two cavities. 



and especially on the trapezoid fibres, whose direction enables them to withstand the 

 upward and outward movement of the clavicle on the scapula that is necessary to 

 allow dislocation of the bone. The conoid ligament is not placed obliquely, and it I 

 necessary, when endeavouring to articulate the two bones and place them in their 

 proper relative position, to ensure that the conoid tubercle on the clavicle is immediately 

 over that on the base of the coracoid process of the scapula (see Fig. 51).* 



There is a smooth area on the bone (Fig. 53) behind the trapezoid ridge, under 

 which the Supraspinatus plays, particularly if the arm is inverted this is shown in 

 Fig g z and under certain circumstances the subacromial bursa may extend under 

 this surface of the bone. 



The insertion of Subclavius extends (Fig. 53) well in front of the conoid ligament. 

 The muscle is enclosed by the costo-coracoid membrane, indicated in the figure by 

 thick black lines, and this attachment of the membrane to the bone can usually be 

 traced by the finger, extending from the rhomboid to the coraco-clavicular impressions : 

 the membrane is attached at its margins to the corresponding ligaments. 



* For action of the ligaments and movements in this joint, see under " Scapula, p. ; 



5* 



