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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
attached by its apex to a rough impression at the base of the coracoid process, medial to the trapezoid ligament; above, by its expanded base, to the coracoid tuberosity on the under surface of the clavicle, and to a line proceeding medialward from it for 1.25 cm. These ligaments are in relation, in front, with the Subclavius and Deltoideus; behind, with the Trapezius.


FIG. 326– The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula. (See enlarged image)


Movements.—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 is limited by the two portions of the coracoclavicular ligament, the trapezoid limiting rotation forward, and the conoid backward.
  The acromioclavicular joint has important functions in the movements of the upper extremity. It has been well pointed out by 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 the 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 positions, 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 shoulder), the joint between these two bones enables the scapula still to maintain its lower part in contact with the ribs.

The Ligaments of the Scapula—The ligaments of the scapula (Fig. 326) are:
        Coracoacromial, Superior and Inferior Transverse.

The Coracoacromial Ligament (ligamentum coracoaromiale).—This ligament is a strong triangular band, extending between the coracoid process and the acromion.

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