chap. xi.] STERNO-CLAVICULAR /OINT. 173 



of the clavicular surface. Extensive muscular exercise 

 appears to increase the dissimilarity between the two 

 joint surfaces by producing enlargement of the sternal 

 end of the collar bone. The facet on the sternum 

 looks upwards, outwards, and a little backwards. 

 This obliquity has important relations to respiration, 

 for " by this backward slope of the facet the sternum 

 is able to advance a little upon the end of the clavicle 

 during its elevation in inspiration " (Henry Morris). 

 The cavity of the joint is V-shaped, since the clavicle 

 only touches the socket at its inferior angle when the 

 arm hangs by the side. When the arm is elevated, 

 however, the two bones are brought in more imme- 

 diate contact, and the joint cavity becomes a mere slit. 

 Thus, in disease of this articulation it will be found 

 that of all movements of the joint the movement of the 

 limb upwards is the most constant in producing pain. 

 The movements permitted at this joint are 

 limited, owing to the anterior and posterior sterno- 

 clavicular ligaments being moderately tense in all 

 positions of the clavicle. Movement forwards of the 

 clavicle on the sternum is checked by the posterior 

 ligament, and resisted by the anterior ligament. 

 This latter ligament is more lax and less sub- 

 stantial than is the posterior band, and would be 

 comparatively weak were it not strengthened by the 

 tendon of the sterno-mastoicl. Its weakness serves in 

 part to explain the frequency of the dislocation 

 forwards. Movement of the clavicle backwards on the 

 sternum is checked by the anterior ligament, while 

 the passage of the head of the bone is resisted by the 

 powerful posterior band. The movement is also 

 opposed by the rhomboid ligament. To produce, 

 therefore, a dislocation backwards considerable force 

 must be used. Movement of the clavicle upwards on 

 the sternum is checked by the rhomboid ligament, the 

 interclavicular ligament, the interarticular cartilage, 



