Chap. XL] ACROMIO- CLA VICULAR JOINT. 177 



should be maintained as far as possible at right 

 angles to the long axis of the humerus. When 

 these relations are satisfied, the humerus has the 

 support behind of a stout surface of bone, and it is 

 partly to obtain the value of this support that the 

 boxer strikes out from the side, i.e., with his humerus 

 well backed up by the scapula. If there were no 

 acromio-clavicular joint the glenoid fossa would offer 

 little support to the humerus when the limb 

 was stretched forwards, and a blow given with the 

 limb in that position, or a fall upon the hand under 

 like conditions, would tend to throw the humerus 

 against the capsule of the shoulder joint, and so 

 produce dislocation. Normally, therefore, as the 

 scapula and arm advance, the angle between the 

 posterior border of the acromion and the adjacent 

 portion of the clavicle becomes more and more acute, 

 and the glenoid fossa is maintained with a sufficiently 

 forward direction to give substantial support to the 

 humerus.* It will thus be seen that rigidity of this 

 little joint may be a cause of insecurity in the articu- 

 lation of the shoulder, and of weakness in certain 

 movements in the limb. 



Dislocations of ttie acromio - clavicular 

 joint. The clavicle may be displaced upwards 

 on to the acromion or downwards beneath it. Pol- 

 laillon has collected thirty-eight cases of the former 

 luxation, and six only of the latter. This dis- 

 proportion is, in the main, explained by the direc- 

 tion of the articulating surfaces of the joint. 

 Both luxations are usually due to direct violence. 

 The dislocation upwards is very commonly only par- 

 tial, and is associated only with stretching and some 

 irining rupture of ligaments. In the complete form, 

 where the end of the clavicle rests entirely upon the 



* For an excellent account of the mechanism of these joints 

 see Morris's " Anatomy of the Joints," p. 202, el seq. 1879. 

 M 



