The Region of the Clavicle . 13 



relax the sternal part of the sterno-mastoid, which passes 

 across the inner side of the articulation. When the arm 

 hangs by the side, the base of the V is widened, and hence 

 the patient depresses the shoulder so as to enlarge the A" 

 when the joint is inflamed. A knife, inserted immediately 

 external to the sternal portion of the sterno-rnasitoid mus- 

 cle, will enter the joint. 



Functions. The clavicle acts as an outrigger or 

 radius, with its centre at the sterno-clavicular articulation. 

 It keeps the scapula, and, therefore, the upper limb, at a 

 certain definite distance from 'the trunk, thus permitting 

 the lateral or rotary movements of the shoulder. In addi- 

 tion, it supports the limb and affords, through its connec- 

 tion with the scapula, a permanent base for the movements 

 of the humerus, for, if the clavicle were absent, the scap- 

 ula would be unsupported and would be thrust, here and 

 there, between the muscular planes, when the arm is in 

 use. The acromio-clavicular joint permits those move- 

 ments between the scapula and the clavicle that are neces- 

 sary to secure the best effects of the former in relation to 

 the head of the humerus, for, if there were no joint here, 

 i.e., if the two bones were united, then, although the hu- 

 merus could rest firmly against the glenoid fossa in one 

 position, it could not in another, since the absence of the 

 joint would prevent the scapula being placed directly be- 

 hind the head of the humerus in the different positions of 

 the arm. Further, this joint allows the lower angle of 

 the scapula to be approximated to the chest wall by the 

 serratus magnus thus increasing its power when it is 

 brought into play in such actions as supporting weights 

 on the shoulder, etc. The sterno-clavicular articulation 

 permits elevation, depression, adduction and abduction of 

 the shoulder, as well as their combination, circumduction. 

 In elevation and depression of the shoulder, the clavicle 



