178 THE ARTICULATIONS OF THE HORSE 



The cavity for the liead of the first rib is formed by concave facets on the bodies of the last 

 cervical and first thoracic vertebra*. The conjuo;al ligament is absent, but the ligament of the 

 neck is short and strong. The radiate ligament is very strong, and consists of two bands. In 

 the case of the last two or three ribs the costo-central and costo-transverse joints are confluent, 

 and the various structures are correspondingly modified. 



Movements. — The chief movement is rotation around an axis which connects 

 the centers of the head and tubercle of the rib. The movement is very hmited in 

 the anterior part of the series of joints, but very considerable in the posterior part. 



In the case of the first rib, the movement is evidently extremely limited. The facet for the 

 tubercle of the rib is deeply concave, and the axis of rotation is almost transverse. Further back 

 the facets on the transverse processes become flat, and the axis of rotation gradually approaches a 

 longitudinal direction. This, in connection with the mobility of the ventral ends of the asternal 

 ribs and their elasticity, allows a great increase here in the range of movement, the effect of which 

 is to enlarge (chiefly) the transverse diameter of the thorax. 



COSTO-CHONDRAL ARTICULATIONS 



The costo-chondral junctions are synarthroses. The rib has a concave surface 

 which receives the convex end of the cartilage. They are united by the continuity 

 of the strong periosteum and perichondrium. 



CHONDRO-STERNAL ARTICULATIONS 



These joints (Articulationes sternocostales) are diarthroses formed by the 

 junction of the cartilages of the sternal ribs with the sternum. The articular ends 

 of the cartilages (except the first) are somewhat enlarged, and present surfaces of 

 cylindrical curvature. The articular surfaces on the sternum for the first pair of 

 cartilages are placed close together on the dorsal border of the cariniform cartilage 

 (Manubrium sterni) ; the other seven are placed laterally at the jimction of the 

 segments. The capsules are strong and tight; the first pair of joints has a common 

 capsule, and the cartilages articulate with each other medially. The lower ends 

 of the first pair of ribs are firmly attached to each other by dense fibrous tissue, 

 which is prolonged forward along the upjjer margin of the cariniform cartilage and 

 is continuous behind with the sternal ligament. Each of the other capsules is 

 reinforced dorsally by the superior costo-sternal ligament (Ligamentum sterno- 

 costale radiatum), composed of radiating fibers which blend with the sternal liga- 

 ment. The movement is rotation around a nearly vertical axis, except in the case 

 of the first pair of joints. 



INTERCHONDRAL LIGAMENTS 

 The eighth and ninth costal cartilages are firmly united by fibrous tissue. The 

 chondro-xiphoid ligament attaches the ninth costal cartilage to the xiphoid carti- 

 lage. The remaining cartilages are rather loosely attached to each other by elastic 

 tissue. 



STERNAL ARTICULATIONS 

 In the new-born foal the sternum has seven bony segments (Sternel^rae), 

 united by persisting cartilage. The last two segments coalesce within a few weeks 

 after birth. In old subjects there is more or less ossification of the intersegmental 

 cartilage, which may load to partial fusion of adjacent segments, especially pos- 

 teriorly. Tli(> sternal ligament (Ligamentum sterni ])roprium internum) lies on the 

 thoracic surface of the sternum. It arises on the first segment, and divides oppo- 

 site the second chondro-sternal joint into three bands. The median band passes 

 backward and spreads out on the last segment and the xiphoid cartilage. The 



