The =vertebral extremity= (Extremitas vertebralis) consists of the =head=, =neck=, and =tubercle=. The =head= (Capitulum costæ) is the actual end of the rib, and is rounded and somewhat enlarged. It presents two facets (Facies articularis capituli costæ) for articulation with the bodies of two adjacent thoracic vertebræ; these surfaces are separated by a groove in which the conjugal ligament is attached. The =neck= (Collum costæ) joins the head to the shaft. It varies in length and diameter. Its outer surface is rough, its inner smooth. The =tubercle= (Tuberculum costæ) projects backward at the junction of the neck and shaft. It has a facet (Facies articularis tuberculi costæ) 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 (Sternebræ) 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 sterni= 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 compressed 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 =mesosternum= (Corpus sterni) presents laterally, at the junction of the segments, concave facets (Incisuræ 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.