Each typical rib forms two joints with the vertebral column, one by its
head, and one by its tubercle. They are termed respectively
costo-central and costo-transverse joints.
[Illustration:
FIG. 142.—COSTO-VERTEBRAL ARTICULATION, ANTERIOR VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
I. The =costo-central articulations= (Articulationes capitulorum) are
trochoid or rotatory joints, formed by the junction of the head of the
rib with the bodies of two adjacent vertebræ and the intervertebral
fibro-cartilage. The two facets on the head of the rib are separated by
a non-articular groove, and correspond to the two concave facets (Foveæ
costales) on the vertebral bodies. The capsules are rather tight, and
are covered by the accessory ligaments, which are as follows: 1. The
=radiate ligament= (Ligamentum capituli costæ radiatum) extends
ventrally from the neck of the rib to spread out on the vertebral bodies
and the intervertebral fibro-cartilage. 2. The =conjugal ligament=
(Ligamentum conjugale)—absent from the first joint—is attached to the
groove on the head of the rib, passes transversely into the vertebral
canal, and divides under the superior common ligament into two branches;
one of these is attached to the body of the anterior vertebra; the other
is continued across to the head of the opposite rib, and is attached to
the intervertebral fibro-cartilage. The joint cavity is divided into two
compartments by the conjugal ligament. 3. The =ligament of the neck= of
the rib (Ligamentum colli costæ) is a strong band which crosses the
joint dorsally. It is attached on the vertebra above the costal facet
and on the neck of the rib.
II. The =costo-transverse articulations=. These occur between the
facets on the tubercles of the ribs and those on the transverse
processes of the vertebræ. They are gliding joints. The =capsule= is
reinforced by the =superior costo-transverse ligament= (Ligamentum
costo-transversarium dorsale), a distinct strong band which arises on
the transverse process and ends on the non-articular part of the
tubercle. It is covered by the levator costæ muscle, and begins to be
quite distinct at the fifth joint.
The cavity for the head of the first rib is formed by concave facets
on the bodies of the last cervical and first thoracic vertebræ. The
conjugal ligament is absent, but the ligament of the neck is short and
strong. The radiate ligament is very strong, and consists of two
bands. In the case of the last two or three ribs the costo-central and
costo-transverse joints are confluent, and the various structures are
correspondingly modified.
=Movements.=—The chief movement is rotation around an axis which
connects the centers of the head and tubercle of the rib. The movement
is very limited in the anterior part of the series of joints, but very
considerable in the posterior part.
In the case of the first rib, the movement is evidently extremely
limited. The facet for the tubercle of the rib is deeply concave, and
the axis of rotation is almost transverse. Further back the facets on
the transverse processes become flat, and the axis of rotation
gradually approaches a longitudinal direction. This, in connection
with the mobility of the ventral ends of the asternal ribs and their
elasticity, allows a great increase here in the range of movement, the
effect of which is to enlarge (chiefly) the transverse diameter of the
thorax.