264 HUMAN ANATOMY. 



sisted by the more powerful posterior ligament and by the rhomboid. Upward 

 displacement — the least frequent — is resisted by the interarticular cartilage, which is 

 strongly inserted below into the cartilage of the first rib and the sternum, and above 

 into the clavicle itself, by the rhomboid and interclavicular ligaments and by both 

 the anterior and posterior ligaments ; hence the rarity of this luxation. 



In many displacements of the sternal end of the clavicle the shoulder is carried 

 downward or backward until the clavicle is in contact with the strong first rib, which 

 then acts as a fulcrum, the sternal end of the bone continuing its upward or forward 

 motion until the resisting ligaments are torn and the luxation is produced. 



In backward dislocation by indirect violence the force has usually pushed the 

 shoulder forward and inward, as when the patient has been caught between two cars 

 or between a wall and a wagon. 



In this dislocation the sternal end may press upon the trachea, the internal 

 jugular, or the beginning of the innominate vein, and may therefore, if the faulty 

 position has become permanent, require excision. 



Disease of the sterno-clavicular joint is not very common, considering its super- 

 ficial position and its constant motion. This is probably due to the fact that the 

 motion is slight and that strains and injury to the synovial membranes are prevented 

 by the strong and elastic interarticular cartilage and by the strength of the ligaments. 

 Suppuration usually shows itself in front (as the anterior ligament is the thinnest), 

 but may perforate by ulceration the posterior ligament and find its way to the medi-» 

 astinum. With the arm at the side the articulation becomes V-shaped, the clavicle* 

 touching the joint surface only at its lowest angle. With the arm elevated, the two 

 joint surfaces are brought into closer relation, and the shape of the joint viewed from 

 the front becomes linear ; hence raising of the arm is uniformly productive of pain 

 in disease of the joint. 



Ankylosis is rare, probably owing to the separation of the diseased joint surfaces 

 by the thick, resistant fibro-cartilage. 



The Acromio-Clavicular Articulation. — This is one of the shallowest of 

 the articulations, the clavicle being merely superimposed, as it were, upon the upper 

 edge of the acromion. The powerful ligaments which bind the clavicle to the cora- 

 coid (the conoid and trapezoid), although they have no direct relation to the joint, 

 are the most important factors in preserving its integrity when force is applied to the 

 point of the shoulder. 



The movements of the joint are around two axes, an antero- posterior and a 

 vertical one, so that the relations of the glenoid cavity to the humerus may remain 

 relatively unchanged when the arm is elevated or is advanced. The scapula must 

 obviously move backward or forward on the side of the chest in a curVe established 

 by the curve of the ribs. It does this on a radius represented by the clavicle, the 

 centre of the rotation being at the sterno-clavicular joint. The acromio-clavicular 

 joint enables this motion to take place, while at the same time the glenoid cavity 

 continues to point obliquely forward. If it were not for this, the act of pushing 

 or striking with the arm advanced, or of falling upon the hand with the arm in 

 a like position, would bring the head of the humerus against the capsule of the joint 

 instead of against the glenoid cavity, and would thus increase the frequency of 

 luxation. Conversely, "rigidity of this little joint may be a cause of insecurity in 

 the articulation of the shoulder and of weakness in certain movements of the limb" 

 (Treves). 



Dislocation is rare. The dislocation of the acromial end of the clavicle upward 

 (described by some surgical writers, for the sake of uniformity, as dislocation of the 

 scapula downward) is much the more frequent. The capsular ligament is torn or 

 stretched, eveii in the incomplete forms. In the complete variety the coraco- 

 clavicular ligaments must be torn or ruptured, but their great strength, increased in 

 effectiveness by their distance from the joint, renders this accident uncommon. 



Dislocation of the clavicle beneath the acromion — between it and the coracoid 

 process — and dislocation of the clavicle beneath the coracoid are extremely rare 

 accidents. It is not certain that the latter has ever occurred. Both obviously 

 require for their production extensive laceration of all of the ligaments binding 

 together the scapula and the outer portion of the clavicle. 



