Sometimes there are at first two centers for the body which soon fuse. The process of ossification 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 usually fused at birth, 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 (Costæ) are elongated curved bones which form the skeleton of the lateral thoracic walls. They are arranged serially in pairs which usually correspond in number to the thoracic vertebræ. Each articulates dorsally with the spine and is continued ventrally by a =costal cartilage=. Those which articulate with the sternum by means of their cartilages are termed =sternal= or =“true” ribs= (Costæ sternales s. veræ); the remainder are =asternal= or =“false” ribs= (Costæ asternales s. spuriæ). Ribs at the end of the series which have their ventral ends free in the abdominal wall are named =floating ribs= (Costæ fluctuantes). The intervals between the ribs are termed =intercostal spaces= (Spatia intercostalia). A typical rib[4] consists of a =shaft= and two =extremities=. The =shaft= (Corpus costæ) 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 costæ); 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.