OF THE STERNUM. 239 



Synovial Membranes. There is no synovial membrane between the first costal 

 cartilage and the sternum, as this cartilage is directly continuous with the sternum. 

 There are two synovial membranes, both in the articulation of the second and third 

 costal cartilages to the sternum. There is generally one synovial membrane in each 

 of the joints between the fourth, fifth, sixth, and seventh costal cartilages to the 

 sternum; but it is sometimes absent in the sixth and seventh chondro-sternal 

 joints. Thus there are eight synovial cavities on each side in the articulations 

 between the costal cartilages of the true ribs and the sternum. After middle life 

 the articular surfaces lose their polish, become roughened, and the synovial 

 membranes appear to be wanting. In old age the articulations do not exist, the 

 cartilages of most of the ribs becoming continuous with the sternum. 



Actions. The movements which are permitted in the chondro-sternal articu- 

 lations are limited to elevation and depression, and these only to a slight extent. 



Articulations of the Cartilages of the Ribs with each other 

 (Interchondral) (Fig. 163). 



The contiguous borders of the sixth, seventh, and eighth, and sometimes the 

 ninth and tenth, costal cartilages articulate with each other by small, smooth, 

 oblong-shaped facets. Each articulation is enclosed in a thin capsular ligament 

 lined by synovial membrane, and strengthened externally and internally by liga- 

 mentous fibres (interchondral ligaments) which pass from one cartilage to the 

 other. Sometimes the fifth costal cartilage, more rarely that of the ninth, articu- 

 lates, by its lower border, with the adjoining cartilage by a small oval facet; more 

 frequently they are connected together by a few ligamentous fibres. Occasionally 

 the articular surfaces above mentioned are wanting. 



Articulations of the Ribs with their Cartilages (Costo-chondral) 



(Fig. 163). 



The outer extremity of each costal cartilage is received into a depression in the 

 sternal end of the ribs, and the two are held together by the periosteum. 



VIII. Articulations of the Sternum. 



The first piece of the sternum is united to the second either by an amphi- 

 arthrodial joint a single piece of true fibro-cartilage uniting the segments or 

 by a diarthrodial joint, in which each bone is clothed with a distinct lamina of 

 cartilage, adherent on one side, free and lined with synovial membrane on the 

 other. In the latter case the cartilage covering the gladiolus is continued without 

 interruption on to the cartilages of the second ribs. Mr. Rivington has found the 

 diarthrodial form of joint in about one-third of the specimens examined by him ; 

 Mr. Maisonneuve more frequently. It appears to be rare in childhood, and is 

 formed, in Mr. Rivington's opinion, from the.amphiarthrodial form by absorption. 

 The diarthrodial joint seems to have no tendency to ossify at any age, while the 

 amphiarthrodial is more liable to do so, and has been found ossified as early as 

 thirty-four years of age. The two segments are further connected by an 



Anterior Intersternal Ligament. 

 Posterior Intersternal Ligament. 



The Anterior Intersternal Ligament consists of a layer of fibres, having a 

 longitudinal direction ; it blends with the fibres of the anterior chondro-sternal 

 ligaments on both sides, and with the tendinous fibres of origin of the Pectoralis 

 major. This ligament is rough, irregular, and much thicker below than above. 



The Posterior Intersternal Ligament is disposed in a somewhat similar 

 manner on the posterior surface of the articulation. 



