ACROMIOCLAVICULAR ARTICULATION 315 



articular surface of the sternum, revolving, with a sliding movement, around an axis drawn nearly 

 vertically through the sternum; in the circumduction of the shoulder, which is compounded of 

 these two movements, the clavicle revolves upon the articular disk and the latter, with the clavicle, 

 rolls upon the sternum. 1 Elevation of the shoulder is limited principally by the costoclavicular 

 ligament; depression, by the interclavicular ligament and articular disk. The muscles which 

 raise the shoulder are the upper fibers of the Trapezius, the Levator scapula?, and the clavicular 

 head of the Sternocleidomastoideus, assisted to a certain extent by the Rhomboidei, which pull 

 the vertebral border of the scapula backward and upward and so raise the shoulder. The depres- 

 sion of the shoulder is principally effected by gravity assisted by the Subclavius, Pectoralis minor 

 and lower fibers of the Trapezius. The shoulder is drawn backward by the Rhomboidei and the 

 middle and lower fibers of the Trapezius, and forward by the Serratus anterior and Pectoralis 

 minor. 



n. Acromioclavicular Articulation (Articulatio Acromioclavicularis ; Scapulo- 

 clavicular Articulation) (Fig. 326). 



The acromioclavicular articulation is an arthrodial joint between the acromial 

 end of the clavicle and the medial margin of the acromion of the scapula. Its 

 ligaments are: 



The Articular Capsule. The Articular Disk. 



The Superior Acromioclavicular. m, ^ , . , (Trapezoid and 



m. -r i A i i I he Coracoclavicular i i 



The Inferior Acromioclavicular. I Conoid. 



The Articular Capsule (capsula articularis; capsular ligament) .The articular 

 capsule completely surrounds the articular margins, and is strengthened above 

 and below by the superior and inferior acromioclavicular ligaments. 



The Superior Acromioclavicular Ligament (ligamentum acromioclaviculare), 

 This ligament is a quadrilateral band, covering the- superior part of the articula- 

 tion, and extending between the upper part of the acromial end of the clavicle 

 and the adjoining part of the upper surface of the acromion. It is composed 

 of parallel fibers, which interlace with the aponeuroses of the Trapezius and 

 Deltoideus; below, it is in contact with the articular disk when this is present. 



The Inferior Acromioclavicular Ligament. This ligament is somewhat thinner 

 than the preceding; it covers the under part of the articulation, and is attached to 

 the adjoining surfaces of the two bones. It is in relation, above, in rare cases with 

 the articular disk ; below, with the tendon of the Supraspinatus. 



The Articular Disk (discus articularis) .The articular disk is frequently absent 

 in this articulation. When present, it generally only partially separates the artic- 

 ular surfaces, and occupies the upper part of the articulation. More rarely, it 

 completely divides the joint into two cavities. 



The Synovial Membrane. There is usually only one synovial membrane in this articulation, 

 but when a complete articular disk is present, there are two. 



The Coracoclavicular Ligament (ligamentum coracoclaviculare) (Fig. 326). This 

 ligament serves to connect the clavicle with the coracoid process of the scapula. 

 It does not properly belong to this articulation, but is usually described with it, 

 since it forms a most efficient means of retaining the clavicle in contact with the 

 acromion. It consists of two fasciculi, called the trapezoid and conoid ligaments. 



The Trapezoid Ligament (ligamentum trapezoideum) , the anterior and lateral fas- 

 ciculus, is broad, thin, and quadrilateral : it is placed obliquely between the cora- 

 coid process and the clavicle. It is attached, below, to the upper surface of the 

 coracoid process; above, to the oblique ridge 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, an angle projecting backward. 



The Conoid Ligament (ligamentum conoideum), the posterior and medial fasciculus, 

 is a dense band of fibers, conical in form, with its base directed upward. It is 



1 Humphry, On the Human Skeleton, page 402. 



