THE SCAPULO-CLAVICULAR 221 



ligaments, moves upon the sternum; so that the entire scapula moves in the arc of 

 a circle whose centre is at the sterno-clavicular joint, and Avhose radius is the 

 clavicle. The scai)ula, in moving upon the clavicle, also moves upon the thorax 

 forwards and backwards, upwards and downwards, and also in a rotatory direction 

 upon an axis drawn at right angles to the centre of the bone. Throughout these 

 luovemonts the lower angle and base of the scapula are kept in contact with the 

 ribs l)y the latisslinus dorsl which straps down the former, and the rhomboids and 

 serratus viarjnus which brace down the latter. The glenoid cavity could not have 

 preserved its obliijucly forward direction had there been no acromio-clavicular 

 joint, ])ut would have shifted round a vertical axis, and thus the shoulder Avould 

 have pointed inwards when the scapula w^as advanced, and outwards when it was 

 drawn backwards. By means of the acromio-clavicular joint, the scapula can be 

 forcibly advanced upon the thorax, the glenoid cavity all the time keeping its face 

 duly forwards. Thus the muscles of the shoulder and forearm can be with advan- 

 tage combined, as, for example, in giving a direct blow. The acromio-clavicular 

 joint also permits the lower angle of the scapula to be retained in contact with the 

 chest wall during the rising and falling of the shoulder, the scapula turning in a 

 hinge-like manner round the horizontal axis of the joint. 



There are no actions in which the scapula moves on a fixed clavicle, or the 

 clavicle on a fixed scapula; the two bones, bound together by their connecting 

 ligament, must move in unison. 



(c) The Proper Scapular Ligaments 



There are three proper ligaments of the scapula, which pass between different 

 portions of the bone, viz. — 



Coraco-acromial. Transverse. 



Inferior transverse. 



The coraco-acromial ligament (figs. 222 and 226) is a flat, triangular band 

 with a l)road base, attached to the outer border of the coracoid process, and a lilunt 

 a])ex which is fixed to the tip of the acromion. It is made up of two broad marginal 

 l)ands, and a smaller and thinner intervening portion. The anterior band, which 

 arises from the anterior portion of the coracoid process, is the stronger, and some of 

 its marginal fibres can often be traced into the short head of the l:)iceps, which can 

 then make tense this edge of the ligament. The posterior band, coming from tlie 

 posterior part of the coracoid process, is also strong. The intei-mediate part, of 

 variable extent, is thin and membranous, containing but few ligamentous fibres; it 

 is often incomplete near the coracoid process, leaving a small gap (fig. 222). The 

 su]ierior surface of the ligament looks upwards and a little forwards, and is covered 

 l»y the deltoid muscle; the inferior looks downwards and a little backwards, and is 

 separated from the capsule of the shoulder-joint by a bursa. At the coracoid 

 l)rocess it overlies the coraco-humeral ligament. It is barely one-third of an inch 

 (8 mm. ) above the capsule of the shoulder, and in the undissected state there is 

 scarcely a quarter of an inch (6 mm.) interv^al. The anterior band projects over 

 the centre of the head of the humerus, and is continued into a tough fascia under 

 the deltoid; the posterior band is continuous with the fascia over the supra-spinatus 

 muscle. It binds the two processes firmly together, and so strengthens each; it 

 holds the deltoid off the capsule of the shoulder, and protects the joint from slight 

 injuries directed downwards and backwards against it. 



The transverse, coracoid, or suprascapular ligament (figs. 222, 223 and 

 224) is a small triangular band of fibrous tissue, the surfaces of which look forwards 

 and l)ackwards; and its edges, which are thin and sharp, are turned upwards and 

 downwards. It continues the superior border of the scapula, l)ridging over the 

 suprascapular notch. It is broader internally, Avhere it springs from the upper 

 border of the scapula on its dorsal surface; and narrow externally, where it is 

 attached to the base of the coracoid process; some of its fibres are inserted under 

 the edge of the trapezoid ligament, and others pass upwards with the conr)id to 



