STERNO-COSTAL ARTICULATIONS 245 



The junction of the xiphoid with the sternum is on a level somewhat posterior to the junc- 

 tion of the seventh costal cartilage with the sternum. The union is a synchondrosis, each bone 

 being covered by hyaline cartilage which is connected with the intervening fibro-cartilage plate. 



(6) The Costo-chondral Joints 

 Class . — Synarthrosis . 



The extremity of the costal cartilage is received into a cup-shaped depression 

 at the end of the rib, which is somewhat larger than the cartilage. The two are 

 joined together by the continuity of the investing membranes, the periosteum of 

 the rib being continuous with the perichondrium of the cartilage. 



(c) The Sterno-costal Articulations 

 Class. — Diarthrosis. Subdivision — Ginglymus. 



These articulations are between the lateral borders of the sternum and the 

 ends of the costal cartilages. The union of the first rib with the sternum is 

 synchondrodial, and therefore forms an exception to the others. From the second 

 to the seventh inclusive, the articulations have the following ligaments, which 

 together form a complete capsule : — 



Radiate (anterior) sterno-costal. Superior sterno-costal. 



Posterior sterno-costal. Inferior sterno-costal. 



The radiate (anterior) sterno-costal ligament (fig. 282) is a triangular band 

 composed of strong fibres which cover the medial half-inch of the front of the 

 costal cartilage, and radiate upward and downward upon the front of the sternum. 

 Some of the fibres decussate across the middle line with fibres of the opposite 

 ligament. At its upper and lower borders it is in contact with the superior and 

 inferior ligaments respectively. 



The posterior sterno-costal ligament consists of little more than a thickening 

 of the fibrous envelopes of the bone and cartilage, the joint being completed 

 behind by a continuity of perichondrium with periosteum. 



The superior and inferior ligaments are strong, well-marked bands, which 

 pass from the upper and lower borders respectively of the costal cartilage to the 

 lateral edges of the sternum. The sixth and seventh cartilages are so close that 

 the superior ligament of the seventh is blended with the inferior of the sixth rib. 



Deepej- than the fibres of these ligaments are short fibres passing from the margins of the 

 sternal facets £o the edges of the facets on the cartilages; they are most distinct in the front 

 and lower part of the joint, and may encroach so much upon the synovial cavity as to reduce 

 it to a very small size, or almost obliterate it. This occurs mostly in the case of the sixth and 

 seventh joints, especially the latter. 



The interarticular ligament (fig. 282) is by no means constant, but is usually 

 present in the second joint on one, if not on both sides of the same subject. It 

 consists of a strong transverse bundle of fibres passing from the ridge on the facet 

 on the cartilage to the fibrous substance between the manubrium and body; 

 sometimes the upper part of the synovial cavity is partially or entirely obliterated 

 by short, fine, ligamentous fibres. 



The costo-xiphoid ligament (fig. 282) is a strong flat band of fibres passing 

 obliquely upward and laterally from the front surface of the xiphoid cartilage to 

 the anterior surface of the sternal end of the seventh costal cartilage, and most 

 frequently to that of the sixth also. 



Synovial membranes. — The union ol the first cartilage with the sternum being synchondro- 

 dial, it has no synovial membrane; the second has usua,lly two, separated bj^ the interarticular 

 ligament. The rest usually have one synovial membrane, which may occasionally be subdivided 

 into two (fig. 282). 



The arterial supply is derived from perforating branches of the internal mammary; and 

 the nerves come from the anterior branches of the intercostals. 



Movements.— Excepting the first, the chondro-sternal joints are ginglymoid, but the 

 motion of which they are capable is very limited. It consists of a hinge-like action in two direc- 

 tions: first, there is a slight amount of elevation and depression which takes place round a 

 transverse axis, and, secondly, there is some forward and backward movement round an ob- 

 liquely vertical axis. In inspiration the cartilage is elevated, the lowest part of its articular 

 facet is pressed into the sternal socket, and the sternum is thrust forward so that the upper 



