297 



upon the articular disk, and the latter, with the clavicle, rolls upon the sternum." 1 Elevation 

 of the clavicle is principally limited by the costoclavicular ligament; depression by the inter- 

 clavicular. The muscles which raise the clavicle, as in shrugging the shoulder, are the upper 

 fibres of the Trapezius, the Levator anguli scapul A e, the clavicular head of the Sternomastoid, 

 assisted to a certain extent by the two Rhomboids, which pull the vertebral border of the scapula 

 backward and upward, and so raise the clavicle. The depression of the clavicle is principally 

 effected by gravity, assisted by the Subclavius, Pectoralis minor, and lower fibres of the Trape- 

 zius. It is drawn backward by the Rhomboids and the middle and lower fibres of the Trapezius; 

 and forward by the Serratus magnus and Pectoralis minor. 



Surface Form. The position of the sternoclavicular joint may be easily ascertained by feeling 

 the enlarged sternal end of the collar-bone just external to the long, cord-like, sternal origin 

 of the Sternomastoid muscle. If this muscle is relaxed by bending the head forward, a depres- 

 sion just internal to the end of the clavicle, and between it and the sternum, can be felt, indi- 

 cating the exact position of the joint, which is subcutaneous. When the arm hangs by the side, 

 the cavity of the joint is V-shaped. If the arm is raised, the bones become more closely approxi- 

 mated, and the cavity becomes a mere slit. 



Applied Anatomy. The strength of this joint mainly depends upon its ligaments, and it 

 is because of the ligaments and because the force of a blow is generally transmitted along the 

 long axis of the clavicle, that dislocation so rarely occurs, and that the bone is generally broken 

 rather than displaced. When dislocation does occur, the course which the displaced bone takes 

 depends more upon the direction in which the violence was applied than upon the anatomical 

 construction of the joint; it may be either forward, backward, or upward. A complete upward 

 dislocation is also inward. A complete forward or backward dislocation is also inward and 

 downward. The chief point worthy of note, as regards the construction of the joint, in regard to 

 dislocations, is the fact that, owing to the shape of the articular surfaces being so little adapted 

 to each other, and that the strength of the joint mainly depends upon the ligaments, the dis- 

 placement when reduced is very liable to recur, and hence it is extremely difficult to keep the 

 end of the bone in its proper place, and it may be necessary to incise the soft parts and wire the 

 bone in place. 



II. Acromioclavicular or Scapuloclavicular Articulation (Articulatio 

 Acromioclavicularis) (Fig. 244). 



The acromioclavicular is an arthrodial joint formed between the outer ex- 

 tremity of the clavicle and the inner margin of the acromion process of the scapula. 

 The ligaments which surround the joint form a capsule. The ligaments of this 

 articulation are the 



Superior Acromioclavicular. f Trapezoid 



Inferior Acromioclavicular. Coracoclavicular -j and 



Articular Disk. [ Conoid. 



The superior acromioclavicular ligament (ligamentum acromioclaviculare) (Figs 

 244 and 245) is a portion of the joint capsule. It is a quadrilateral band which 

 covers the superior part of the articulation, extending between the upper part of 

 the outer end of the clavicle and the adjoining part of the upper surface of the 

 acromion. It is composed of parallel fibres which interlace with the aponeurosis 

 of the Trapezius and Deltoid muscles; below, it is in contact with the articular 

 disk (when it exists) and the synovial membranes. 



The inferior acromioclavicular ligament, somewhat thinner than the preceding,, 

 and, like it, a portion of the capsule, covers the under part of the articulation 

 and is attached to the adjoining surfaces of the two bones. It is in relation, 

 above, with the synovial membranes, and in rare cases with the articular disk; 

 below, with the tendon of the Supraspinatus. These two ligaments are con- 

 tinuous with each other in front and behind, and form a complete capsule around 

 the joint. 



1 Humphry, On the Human Skeleton, p. 402. 



