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 (Coste) 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 vertebre. Each articulates dorsally with two vertebre 
and is continued ventrally by a costal cartilage. Those which articulate with the 
sternum by means of their cartilages are termed sternal ribs (Coste sternales); 
the remainder are asternal ribs (Cost asternales). Ribs at the end of the series 
which have their ventral ends free in the abdominal wall are named floating ribs 
(Coste 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 
cost) 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 
certain point, termed the angle of the rib (Angulus cost); 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 (Facies 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 costz), 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 cost) is the actual end of the rib, and is 
rounded and somewhat enlarged. It presents two facets (Facies articularis capituli 
cost) for articulation with the bodies of two adjacent thoracic vertebre; these 
surfaces are separated by a groove in which the conjugal ligament is attached. 
The neck (Collum cost) joins the head to the shaft. It varies in length and 
diameter. Its lateral surface is rough, its medial smooth. The tubercle (Tuber- 
culum cost) projects backward at the junction of the neck and shaft. It has a 
facet (Facies articularis tuberculi cost) 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 
* 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. 
