THE COSTAL CARTILAGES 27 



arch. Secondary centers appear later for the summit of the spinous process 

 (except in the cervical region), the extremities of the transverse processes, and the 

 thin epiphyseal plates at the extremities of the body. 



^ Sometimes there are at first two centers for the body, which soon fuse. The process of ossifi- 

 cation extends from 'the lateral centers to form, not only the corresponding part of the arch but 

 also the processes and a part of the body next to the root of the arch (Radix arcus). In the horse 

 and ox the body and arch are fused at birth or unite very soon after, but the epiphyses do not 

 fuse till growth is complete. In the pig, sheep, and dog the body and arch are united at birth 

 by cartilage (neurocentral synchondrosis), but fuse in the first few months. 



THE RIBS 



The ribs (Costae) are elongated curved bones which form the skeleton of the 

 lateral thoracic walls. They are arranged serially in pairs which correspond in 

 number to the thoracic vertebrae. Each articulates dorsally with two vertebrje 

 and is continued ventrally by a costal cartilage. Those which articulate with the 

 sternum by means of their cartilages are termed sternal ribs (Costae sternales); 

 the remainder are asternal ribs (Costae asternales) . Ribs at the end of the series 

 which have their ventral ends free in the abdominal wall are named floating ribs 

 (Costae fluctuantes). The intervals between the ribs are termed intercostal spaces 

 (Spatia intercostalia) . 



A typical rib^ consists of a shaft and two extremities. The shaft (Corpus 

 costae) is band-like and varies much in length, breadth, and curvature in different 

 ribs. In the case of some ribs the curvature is not uniform, but is accentuated at a 

 certttin point, termed the angle of the rib (Angulus costae) ; this occurs at a variable 

 distance from the vertebral end, and is usually marked by a rough ridge. The 

 direction also varies; the first rib is usually almost vertical, while the remainder 

 slope backward in increasing degree. The lateral surface (Fades lateralis) is con- 

 vex, and the medial surface (Facies medialis) flattened from edge to edge; on the 

 latter, close to the posterior border, is the costal groove (Sulcus costae), which fades 

 out ventrally. It contains the intercostal vein. The anterior and posterior 

 borders (Margo cranialis, caudalis) are thin and sharp on some ribs, rounded on 

 others. 



The vertebral extremity (Extremitas vertebralis) consists of the head, neck, 

 and tubercle. The head (Capitulum costae) is the actual end of the rib, and is 

 rounded and somewhat enlarged. It presents two facets (Facies articularis capituli 

 costae) for articulation with the bodies of two adjacent thoracic vertebrae; these 

 surfaces are separated by a groove in which the conjugal ligament is attached. 

 The neck (Collum costae) joins the head to the shaft. It varies in length and 

 diameter. Its lateral surface is rough, its medial smooth. The tubercle (Tuber- 

 culum costae) projects backward at the junction of the neck and shaft. It has a 

 facet (Facies articularis tubercuh costs) for articulation with the transverse process 

 of the posterior vertebra of the two with which the head articulates. The tubercle 

 gradually approaches the head in the posterior ribs, and eventually fuses with it. 



The sternal extremity (Extremitas sternalis) is commonly slightly enlarged, 

 and is roughened at the junction with the costal cartilage. 



Development. — The ribs are ossified in cartilage from three centers — one each 

 for the shaft (and sternal end), head, and tubercle; the third center is absent in 

 some ribs at the terminal part of the series. 



THE COSTAL CARTILAGES 



These (Cartilagines costales) are bars of hyaline cartilage which continue the 

 ribs. Those of the sternal ribs articulate with the sternum, while those of the 



1 The term is employed here, as is usual in descriptive anatomy, to designate only the bony 

 part of the rib (Os costale) ; morphologically it includes the cartilaginous part also. 



