A bone from the middle of the series will show the general characters and various parts of a rib. Such a specimen (Fig. 37) exhibits at its vertebral end an expanded head (A), joined to the remainder of the bone by a somewhat narrower neck which has a rough ridge, the crista colli superior (C), for the upper costo-transverse hgament. The head has two articular facets, which are covered with fibro-cartilage, for the bodies of the two vertebrae that carry it, separated by an inter articular crest (B) for the attachment of the hgament that fastens it to the intervertebral disc. The neck has a smooth pleural surface in front, but is ridged behind by the middle costo-transverse ligament : there is often an inferior crest on the neck.
The rest of the bone, outside the neck, is the shaft or body (D), and where this joins the neck is the tubercle on the back and lower aspect of the bone : the tubercle presents an inner articular part (art.), with a facet for the transverse process of its vertebra, and an outer non-articular part (n. a.) for the external or posterior costo-transverse ligament.
The shaft passes outwards from the tubercle for a little distance, and then turns rather sharply forwards and outwards : this turn makes the angle of the rib. Observe that the angle is properly a sharp curve in the bone, but carries on its surface a secondary marking made by the attachment of the Ilio-costalis and vertebral aponeurosis.
Beyond the angle the curving shaft is continued in a forward, inward and downward direction : it has an upper border, thick and rounded behind and sharper in front, a lower border sharp behind and rather rounded in front, an outer and an inner surface.
The inner surface, covered by pleura in its greater extent, presents below a costal groove (S) for intercostal vessels, best marked behind : the intercostal muscles are attached to the margins of the grove.
The anterior end of the shaft has an oval hollow (CC) which receives the rounded end of the costal cartilage without any intervening synovial cavity.
A slight bend in the shaft may be noticed a little distance from its anterior extremity : this is the anterior angle. The rib is bent in three directions : (i) a general curve forwards and then inwards; (2) a curve upwards of the head and neck and adjoining portion of shaft, connected with a downward turn at the front end in many ribs ; (3) a twist on its longitudinal axis that begins at the angle, so that the outer surface of the bone shows a tendency to look upwards as well as outwards as it is followed forward.