MOVEMENTS OF THE CLAVICLE AND SCAPULA. 263 



The obvious advantage of a joint between the clavicle and the acromion, apart 

 from breaking shocks and making the shoulder-girdle much more elastic, is that it 

 allows the angle between the bones to change with the position of the arm, and thus 

 the direction of the glenoid cavity may be modified so as to give the best support to 

 the arm in different positions. The motion at the outer end of the clavicle is con- 

 siderable, but indefinite. The overlapping clavicle can advance a little laterally 

 onto the acromion, except in the cases in which the plane of the joint is vertical. 

 There is also motion on an approximately vertical axis when the shoulder is thrown 

 forward and the outer end of the clavicle advances, the angle between the back of 

 the clavicle and the spine of the scapula being diminished. When the clavicle can 

 advance no farther, the tension of the trapezoid ligament checks the progress of the 

 coracoid. In the withdrawal of the shoulder the reverse occurs, the movement 

 being finally checked by the conoid. In up-and-down movements of the shoulder 

 the motion is on an approximately antero-posterior axis. When it rises the base of 

 the coracoid comes into direct contact with the clavicle and the rhomboid ligament 

 is strained ; when it falls the clavicle rests on the first rib and the conoid is put on 

 the stretch, as are also the interclavicular ligament and the top of the capsule of the 

 sternal end. Probably the freest movement is when the arm is raised vertically, in 

 which case the lower angle of the scapula swings strongly forward so as to direct the 

 glenoid cavity more nearly upward. The clavicle rises from the sternal end, and 

 perhaps slightly rotates. Possibly the lower end of the scapula is withdl^awn slightly 

 'from the chest. Apart from the movements of the arm the scapula may change its 

 position considerably. It may rotate on either the end of the acromion (as in rais- 

 ing the arm) or on the superior angle, the lower angle being the most movable 

 point. When it is carried far forward a larger portion of the posterior surface of 

 the lungs can be examined. The scapulae may also be raised or brought nearer 

 together. 



Surface Anatomy of the Shoulder-Girdle. — The general shape of the 

 clavicle is easily made out by pressing on its front and superior surfaces with the 

 muscles relaxed. The degree of backward projection of the inner end can be deter- 

 mined. It is placed horizontally in woman ; in man the outer end is slightly raised. 

 The joint with the acromion is easily felt from above, the clavicle being the higher. 

 The outline of the acromion, which slopes somewhat downward, is easily felt. It 

 forms the point of the shoulder-girdle, but not of the shoulder, as the humerus 

 always projects beyond it externally. A plane vertical surface placed against the 

 outside of the shoulder cannot touch the acromion if the head of the humerus is in 



• 



place. The possibility that the outer epiphysis of the acromion may not unite by 

 bone is to be remembered. The finger can be carried from the acromion along the 

 spine to its triangular origin. The tip of the coracoid is to be felt by manipulation 

 in the infraclavicular fossa at the inner side of the humerus. The posterior border 

 of the scaprula is always to be felt ; in thin persons its outline can be traced and the 

 shape of the inferior angle approximately recognized. 



PRACTICAL CONSIDERATIONS. 



The Sterno-Clavicular Articulation. — The interposition of an elastic buffer 

 in the shape of the interarticular fibro-cartilage, united to both the bones by very 

 strong ligamentous fibres, and completely bisecting the joint (Fig. 280), in fact, 

 converting it into two separate joints, prevents the clavicle from transmitting to the 

 sternum the full force of blows and falls received upon the hand or shoulder, and 

 allows of the varied, though limited, movements of the articulation. 



Dislocatioji is rare. The ligaments are stronger than the clavicle, which is 

 therefore usually broken by any force sufficient to threaten the integrity of the joint. 

 The curves of the clavicle, the mobility of the scapula, and the play of the acromio- 

 clavicular joint all tend to diffuse forces that might otherwise have been expended 

 on this articulation, which is furthermore strengthened by the tendinous origins 

 of the sterno-cleido-mastoid and of the pectoralis major. 



The most common form of dislocation is the forward one, the anterior sterno- 

 clavicular ligament being the weaker and thinner. Backward luxation is re- 



