262 HUMAN ANATOMY. 



The capsule (Fig. 280) surrounds the joint, being attached to the borders of 

 the articular surfaces and also to the borders of the interarticular disk. It is 

 strengthened before and behind by bands running upward and outward from the ster- 

 num, of which the posterior are the stronger, and sends some deep fibres to the disk. 

 These bands strengthening the capsule are sometimes described as the a^iterior and 

 posterior sterno- clavicular ligaments. There are two distinct sy7iovial cavities. 



The interclavicular ligament (Fig. 279) is" a fairly well-defined band run- 

 ning from the top of one clavicle across to the other. It is closely connected with the 

 top of the joint and loosely with the top of the sternum, towards which it sinks with 

 a slight curve. This does much towards filling up the deep interclavicular notch. 



The costo-clavicular ^ or rhomboid ligament (Fig. 280) arises from the 

 costal cartilage just outside of the joint, with which it is loosely connected, and runs 

 upward and outward to the rough rhomboid impression on the under side of the 

 clavicle. It is a layer of strong, short fibres. 



THE SCAPULO-CLAVICULAR ARTICULATION. 



The Acromio-Clavicular Articulation. — This joint includes a capsular 

 ligament (Fig. 274) and occasionally an intra-articular fibro-cartilage. The elongated 

 facet on each bone is covered with articular cartilage, that of the clavicle usually 

 overlapping the other. 



The capsule is weak, except above and behind, where there are strong bands 

 extending outward from the clavicle. Of these the posterior are the longer. 



The fibro-cartilage, when present, is wedge-shaped, attached by the base to 

 the superior part of the capsule, the thin ecige reaching, perhaps, half-way through 

 the cavity of the joint. Sometimes it divides the joint into two. There may be 

 merely a thick pad of fibrous tissue attached to the outer end of the clavicle with 

 only a very rudimentary joint. 



The coraco-clavicular ligament is an important ligamentous apparatus 

 divided into an outer part, the trapezoid, and an inner, the conoid (Fig. 289). These 

 are continuous behind, but diverge in front. The trapezoid ligament '"' is a four- 

 sided layer of parallel fibres, springing from the trapezoid ridge and the top of the 

 first part of the coracoid, to run outward to the trapezoid ridge on the under side of 

 the clavicle. The line of attachment to the clavicle is usually the longer, and, as this 

 runs forward and outward, the anterior fibres are almost horizontal. The conoid 

 ligament,^ or inner part, is less strong. It arises from the posterior border of the 

 conoid tubercle at the root of the acromion, and runs to the tubercle of the same 

 name at the back of the under side of the clavicle. Both these tubercles being 

 prominences of some size, this ligament is not a cord, as might be inferred, but 

 another layer continuous with the trapezoid behind. The inner fibres incline inward 

 as they ascend. The general direction is upward and perhaps a little backward, but 

 this changes with the position of the bones. There may be a synovial bursa in the 

 open angle seen from the front between these two parts of the ligament. 



Movements of the Clavicle and Scapula. — The compound joint at the 

 inner end of the clavicle is practically a universal one. The clavicle can be raised, 

 depressed, carried forward or backward, circumducted, and slighdy rotated. The 

 outer and lower end of the disk being attached to the corresponding border of the 

 facet, it follows that the clavicle lies upon it. When the shoulder is raised or 

 depressed the motion is almost wholly between the clavicle and the disk, though the 

 latter slides a litde, and in marked falling of the shoulder the top of the disk starts 

 to come out of the socket, but is restrained by the top of the capsule. Forward and 

 backward mot'ons occur chiefly between the disk and the sternum, but there is some 

 displacement of the former. Circumduction, therefore, involves both parts of the 

 joint ; rotation is chiefly in the inner one. 



It is remarkable that a joint at which there is so much strain, owing to leverage, 

 should be so strong with such apparently imperfect bony arrangements for retention. 

 Part of the safety is due to the subdivision of the joint and a great deal to the assist- 

 ance of muscles. At both ends of the clavicle, as Morris has pointed out, the great 

 muscles are so placed that by their contraction they draw the bones together. 



' Lig. costoclavicularc. " Lig. trapezoidcum. '^ Lig. conoideura. 



