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.
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.