216 THE ARTICULATIONS OF THE HORSE 



costse) is a strong band which crosses the joint dorsally. It is attached on the 

 vertebra above the costal facet and on the neck of the rib. 



II. The costo-transverse articulation (Articulatio costo-transversaria) . This 

 is formed by the facet on the tubercle of the rib and on the transverse proc- 

 ess of the vertebra. They are gliding joints. The capsule is reinforced by the 

 dorsal costo-transverse ligament (Lig. costo-transversarium dorsale), a distinct 

 strong band which arises on the transverse process and ends on the non-articular 

 part of the tubercle. It is covered by the levator costse muscle, and begins to be 

 quite distinct at the fifth joint. 



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

 cer^dcal and first thoracic vertebrse. The conjugal ligament is absent, but the hgament of the 

 neck is short and strong. The radiate ligament is very strong, and consists of two parts. 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 limited in 

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



In the case of the fu'st 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, so that the 

 movement is chiefly sagittal in direction. 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 mobiUty of the ventral ends of the asternal ribs and their elasticity, allows a 

 great increase here in the range of movement which is lai-gely transverse, the effect being to enlarge 

 (chiefly) the transverse diameter of tlie 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 perichontlrium. 



CHONDRO-STERNAL ARTICULATIONS 



These joints (Articulationes sternocostaies) 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 ; 

 the other seven are placed laterally at the junctions of the segments. The capsules 

 are strong and tight; the first pair of joints has a common capsule, and the cartil- 

 ages articulate with each other medially. The ventral ends of the first pair of ribs 

 are firml>' attached to each other by dense fibrous tissue, which is prolonged for- 

 ward along the upper margin of the cariniform cartilage and is continuous behind with 

 the sternal ligament. Each of the other capsules is reinforced dorsally by the radiate 

 costo-sternal ligament (Ligamentum sterno-costale radiatum), composed of radi- 

 ating fibers which blend with the sternal ligament. Interarticular bands may be 

 present. The movement is rotation around a nearh' 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. 



