214 THE ARTICULATIONS 



that tlu" superior ligament of the seventli is blended with the inferior of the 

 sixth rib. 



Dee])er than the fibres of these ligaments are short fibres passing from the mar- 

 gins of the sternal facets to the edges of the facets on the cartilages; they are most 

 distinct in the front and lower part 6f 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. 217) 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 l)undle of fibres passing from the ridge on the facet 

 on the cartilage to the symphysial substance between the manuljrium and body; 

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

 by short, tine, ligamentous fibres. 



The chondro-xiphoid ligament (fig. 217) is a strong fiat band of fibres passing 

 obliqu(4y upwards and outwards 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 of the first cartilage with the sternum being 

 synarthrodial, it has no synovial membrane; the second has usually two, separated 

 l)y the interarticular ligament. The rest usually have one synovial membrane, 

 which may occasionally be subdivided into tAvo (fig. 217). 



The arterial supply is derived from perforating branches of the internal mam- 

 mary; and the nerves come from the anterior branches of the intercostals. 



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

 l)ut the motion of which they are capable is very limited. It consists of a hinge- 

 like action in two directions: first, there is a slight amount of elevation and depres- 

 sion Avhich takes place round a transverse axis, and, secondly, there is some forward 

 and backward movement round an obhquely vertical axis. In inspiration the 

 cartilage is elevated, and the lowest part of its articular facet is pressed into the 

 sternal socket, and the sternum is thrust forwards so that the upper and front edges 

 of the articular surfaces separate a little; in expiration the reverse movement takes 

 ]ilace. Thus the two extremities of the costal arches move in their respective 

 sockets in opposite directions. 



This difference results necessarily from the fact that the costal arch moves upon 

 the vertebral column, and, having been elevated, it in its turn raises the sternum 

 by pushing it upwards and forwards. 



The chondro-xiphoid ligament tends to prevent the xiphoid cartilage from 

 V)eing drawn backwards l»y tlie action of the diaphragm. 



{(l) The Intekchondrai. Aktktlations 

 Class. — 1)1(1 rf/irosis. Subdivision. — Artlirudia. 



A little in front of the point where the costal cartilages bend ui)\var(ls towards 

 the median line, the sixth is united with the seventh, the seventh with the eighth, 

 the eighth with the ninth, and the ninth with the tenth. 



At this point, each of the cartilages from the sixth to the ninth inclusive is 

 deeper than elsewhere, owing to the projection downwards from its lower edge of a 

 broad blunt process, which comes into contact with the cartilage next In-low. Each 

 of the ai)i)Osed surfaces is smooth, and they arc connected at tlieir margins bv liga- 

 mentous tissue, which forms a comjilete capsule for the articulation, and is lined 

 by a synovial membrane (fig. 217). The largest of these cavities is between the 

 seventh and eighth; tliose between the eighth and ninth, and ninth and tenth, are 

 smaller, and are not free to ].lay upon each other in the whole of their extent, 

 being held together by ligamentous tissue at their anterior margins. Sometimes 

 this fi])rous tissue completely obliterates the synovial cavity. 



The arteries are derived* from the muscuio-jihrenic, and the nerves from the 

 intercostals. 



Movements.— Ky means of the costal cartilages and intcnhondral joints. 



