26 OSTEOLOGY 



is band-like and varies much in length, breadth, and curvature. In the case of 

 some ribs the curvature is not uniform, but is most accentuated at a certain 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 almost vertical, while the remainder slope backward 

 in increasing degree. The external surface is convex, and the internal flattened 

 from edge to edge; on the latter, close to the posterior border, is the costal groove 

 (Sulcus costalis), which fades out ventrally. It contains the intercostal vein. 

 The anterior and posterior borders 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 costse) 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 (CoUum costae) joins the head to the shaft. It varies in length and 

 diameter. Its outer surface is rough, its inner smooth. The tubercle (Tuberculum 

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

 (Facies articularis tuberculi costae) 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 has a rough depression in which the costal cartilage is embedded. 



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

 for the shaft (and sternal end), head, and tubercle; the third center does not occur 

 in the last two ribs. 



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 the remainder 

 overlap and are attached to each other to form the costal arch (Arcus costalis). 



THE STERNUM 



The sternum or breast-bone is a median segmental bone which completes the 

 skeleton of the thorax ventrally, and articulates with the cartilages of the sternal 

 ribs laterally. It consists of six to eight bony segments (Sternebrae) connected by 

 intervening cartilage in the young subject. Its form varies with that of the thorax 

 in general and with the development of the clavicles in animals in which they are 

 present. Its anterior extremity, the manubrium stem! or presternum, is specially 

 affected by the latter factor, being broad and strong when the clavicles are well 

 developed and articulate with it (as in man), relatively small and laterally com- 

 pressed when they are absent (as in the horse) or rudimentary (as in the dog). 

 The cartilages of the first pair of ribs articulate with it. The body or mesostemum 

 (Corpus sterni) presents laterally, at the junction of the segments, concave facets 

 (Incisurae costales) for articulation with the cartilages of the sternal ribs. The 

 posterior extremity or metasternum presents the xiphoid (or ensiform) cartilage 

 (Processus xiphoideus) ; this is thin and plate-like, as in the horse and ox, or narrow 

 and short, as in the pig and dog. 



Development. — The cartilaginous sternum is formed by the fusion medially 

 of two lateral l)ars which unite the ventral ends of the first eight or nine costal 

 cartilages, and is primitively unsegmented. The manubrium ossifies from a single 

 center, but the centers for the other segments appear to be primitively paired. 

 The sternum never becomes completely ossified; details in regard to persisting 



