PROPER LIGAMENTS OF THE SCAPULA 299 



in relation, in front, with the Subclavius and Deltoid; behind, with the Trapezius. 

 They serve to limit rotation of the scapula, the trapezoid limiting rotation forward, 

 and the conoid backward. 



Movements. The movements of this articulatioit are of two kinds: (1 ) A gliding motion of 

 the articular end of the clavicle on the acromion. (2) Rotation of the scapula forward and back- 

 ward upon the clavicle, the extent of this rotation being limited by the two portions of the coraco- 

 clavicular ligament. 



The acromioclavicular joint has important functions in the movements of the upper extremity. 

 It has been well pointed out by Sir George Humphry that if there had been no joint between 

 the clavicle and scapula the circular movement of the scapula on the ribs (as in throwing both 

 shoulders backward or forward) would have been attended with a greater alteration in the 

 direction of the shoulder than is consistent with the free use of the arm in such position, and 

 it would have been impossible to give a blow straight forward with the full force of the arm ; that 

 is to say, with the combined force of the scapula, arm, and forearm. "This joint," as he happily 

 says, "is so adjusted as to enable either bone to turn in a hinge-like manner upon a vertical 

 axis drawn through the other, and it permits the surfaces of the scapula, like the baskets in a 

 roundabout swing, to look the same way in every position, or nearly so." Again, when the whole 

 arch formed by the clavicle and scapula rises and falls (in elevation or depression of the shoul- 

 ders), the joint between these two bones enables the scapula still to maintain its lower part in 

 contact with the ribs. 



Surface Form. The position of the acromioclavicular joint can generally be ascertained by 

 the slightly enlarged extremity of the outer end of the clavicle, which causes it to project above 

 the level of the acromion process of the scapula. Sometimes this enlargement is so considerable 

 as to form a rounded eminence, which is easily to be felt. The joint lies in the plane of a vertical 

 line passing up the middle of the front of the arm. 



Applied Anatomy. Owing to the slanting shape of the articular surfaces of this joint, the 

 commonest dislocation is the passing of the acromion process of the scapula under the outer 

 end of the clavicle; but dislocations in the opposite direction have been described. The first 

 form of dislocation is produced by violent force applied to the scapula so as to drive the shoulder 

 forward. The displacement in acromioclavicular dislocation is often incomplete, on account 

 of the strong coracoclavicular ligaments which remain untorn. The same difficulty exists, as in 

 the sternoclavicular dislocation, in maintaining the ends of the bone in apposition after reduction, 

 and it may become necessary to wire them in place after incision of the soft parts. 



III. Proper Ligaments of the Scapula (Figs. 244, 245). 



The proper ligaments of the scapula pass between portions of that bone, but 

 are not parts of an articulation. They are the 



Coracoacromial. Superior Transverse. 



Inferior Transverse. 



The Coracoacromial ligament (ligamentum coracoacromiale) is a strong triangular 

 band, extending between the coracoid and acromial processes. It is attached, 

 by its apex, to the summit of the acromion just in front of the articular surface 

 for the clavicle, and by its broad base to the whole length of the outer border of 

 the coracoid process. Its posterior fibres are directed inward, its anterior fibres 

 forward and inward. This ligament completes the vault formed by the coracoid 

 and acromion processes for the protection of the head of the humerus. It is in 

 relation, above, with the clavicle and under surface of the Deltoid muscle; below, 

 with the tendon of the Supraspinatus muscle, a bursa being interposed. Its 

 outer border is continuous with a dense lamina that passes beneath the Deltoid 

 upon the tendons of the Supra- and Infraspinatus muscles. This ligament is 

 sometimes described as consisting of two marginal bands and a thinner inter- 

 vening portion, the two bands being attached, respectively, to the apex and base 

 of the coracoid process, and joining together at their attachment into the acromion 

 process. When the Pectoralis minor is inserted, as sometimes is the case, into 



