344 THE ARTICULATIONS. 



process, it is usually described with it. It consists of two fasciculi, called the 

 trapezoid and conoid ligaments. 



The Trapezoid Ligament, the anterior and external fasciculus, is broad, thin, 

 and quadrilateral ; it is placed obliquely between the coracoid process and the 

 clavicle. It is attached, below, to the upper surface of the coracoid process ; 

 above, to the oblique line on the under surface of the clavicle. Its anterior border 

 is free; its posterior border is joined with the conoid ligament, the two forming 

 by their junction a projecting angle. 



The Conoid Ligament, the posterior and internal fasciculus, is a dense band of 

 fibres, conical in form, the base being turned upward, the summit downward. It 

 is attached by its apex to a rough impression at the base of the coracoid process, 

 internal to the preceding; above, by its expanded base, to the conoid tubercle on 

 the under surface of the clavicle, and to a line proceeding internally from it for 

 half an inch. These ligaments are 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. 



Actions. The movements of this articulation 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 backward upon the clavicle, the extent of this rotation being 

 limited by the two portions of the coraco-clavicular ligament. 



The acromio-clavicular 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 shoulders), 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 acromio-clavicular 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 ver- 

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



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

 dislocation generally occurs downward ; that is to say, the acromion process of the scapula is 

 dislocated Bunder the outer end of the clavicle ; but dislocations in the opposite direction have 

 been described. The displacement is often incomplete; on account of the strong coraco-claviculai 

 ligaments, which remain untorn. The same difficulty exists, as in the sterno-clavicular disloca- 

 tion, in maintaining the ends of the bone in position after reduction. 



III. Proper Ligaments of the Scapula (Fig. 243). 

 The proper ligaments of the scapula are the 



Coraco-acromial. Transverse. 



The Coraco-acromial Ligament is a broad, thin, flat band, of a triangular shape, 

 extended transversely above the upper part of the shoulder-joint, 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 obliquely backward and inward, its anterior fibres transversely 

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



