318 THE ARTICULATIONS OE JOINTS. 



posterior sides of the triangle are slightly roughened for the attachment of 

 ligaments, while the base or inferior side is smooth and rounded, owing to the 

 prolongation of the articular surface to the inferior aspect of the bone. In the 

 antero-posterior direction the articular surface tends to be concave, while vertically 

 it is slightly convex. 



2. An articular facet, situated on the superior lateral angle of the manubrium 

 sterni, but in a plane slightly behind the supra-sternal notch, articulates with the 

 clavicle. This facet is considerably smaller than the clavicular facet with which it 

 articulates. 



3. The superior surface of the first costal cartilage close to the sternum also 

 participates to a small extent in the articulation. 



It should be noted that the articular surfaces of the clavicle and sternum are 

 covered mainly by fibre-cartilage. 



A capsula articularis is well marked on all sides except inferiorly, where it is 

 very thin. The epiphyseal line of the clavicle is intra-capsnlar. 



Lig 1 . Sternoclaviculare Anterius. The anterior sterno-clavicular ligament 

 (Fig. 301) forms part of the fibrous stratum of the articular capsule, and consists 

 of short fibres which extend obliquely inferiorly and medially from the anterior 

 aspect of the sternal end of the clavicle to the adjoining anterior surface of the 

 sternum and the anterior border of the first costal cartilage. 



Lig. Sternoclaviculare Posterius. The posterior sterno-clavicular ligament 

 also forms part of the fibrous stratum of the capsule, and consists of similarly 

 disposed, but not so strong as the anterior ligament, oblique fibres situated on the 

 posterior aspect of the articulation. 



Discus Articularis. A fibro-cartilaginous articular disc (Fig. 301) divides the 

 joint cavity into two compartments. It is nearly circular in shape, and adapts 

 itself to the articular surfaces between which it lies. It is thickest at the circum- 

 ference and thinnest at the centre, where it occasionally presents a perforation, 

 thereby permitting the two synovial cavities to inter-communicate. By its circum- 

 ference it is in contact with, and adherent to, the surrounding capsule, but its 

 superior margin is attached to the apex of the articular surface of the clavicle, while 

 by its inferior margin it is fixed to the sternal end of the first costal cartilage. 



Two accessory ligaments are associated with this joint, viz., the interclavicular 

 and the costo-clavicular. 



Lig. Interclaviculare. The interclavicular ligament (Fig. 301) is a structure of 

 considerable strength, forming a broad band of fibrous tissue which is attached to the 

 superior rounded angle or apex of the sternal end of the clavicle as well as to the 

 adjacent margins of the articular surface. Its fibres pass across the interclavicular 

 notch to become attached to corresponding parts of the opposite clavicle, but in their 

 course they dip down into the supra-sternal notch, in which many of them are fixed 

 to the sternum. In this way their presence neither bridges nor obliterates the 

 notch between the two clavicles, and the ligament really becomes a superior sterno- 

 clavicular ligament for each joint. 



Lig. Costoclaviculare. The costo-clavicular ligament (Fig. 301) consists of 

 short, strong fibres which are attached inferiorly to the superior surface of the first 

 costal cartilage. They pass obliquely upwards, laterally and posteriorly, to a rough 

 impression situated on the inferior aspect of the sternal end of the clavicle, and are 

 distinct from the articular capsule. Occasionally a bursa is found in the interior 

 of this ligament. 



As a rule there is a synovial stratum lining each of the two joint cavities 

 (Fig. 301), separated from each other by the articular disc. Sometimes, however, 

 the two membranes establish continuity through a perforation in the disc. 



ARTICULATIO ACEOMIOCLAVICULARIS. 



The acromio-clavicular joint is another instance of an arthrodial diarthrosis. 

 It is situated between the acromial end of the clavicle and the medial aspect of 

 the acromion. Each articular surface is an oval, flattened facet, covered with 

 fibro-cartilage. 



