The intra-articular ligament of head of rib (interarticular in older texts) is situated in the interior of the articulation of head of rib

Articulation of head of rib

The Articulations of the Heads of the Ribs constitute a series of gliding or arthrodial joints, and are formed by the articulation of the heads of the typical ribs with the facets on the contiguous margins of the bodies of the thoracic vertebræ and with the intervertebral fibrocartilages between...

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It consists of a short band of fibers, flattened from above downward, attached by one extremity to the crest separating the two articular facets on the head of the rib

Rib

In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

, and by the other to the intervertebral fibrocartilage; it divides the joint into two cavities.

In the joints of the first

First rib

The first rib is the most curved and usually the shortest of all the ribs; it is broad and flat, its surfaces looking upward and downward, and its borders inward and outward....

, tenth, eleventh

Eleventh rib

The eleventh rib has a single articular facet on the head, which is of rather large size.It has no necks or tubercles, and is pointed at its anterior ends.The eleventh has a slight angle and a shallow costal groove....

, and twelfth rib

Twelfth rib

The twelfth rib has a single articular facet on the head, which is of rather large size. It has no necks or tubercles, and is pointed at its anterior ends....

s, the interarticular ligament does not exist; consequently, there is but one cavity in each of these articulations.

This ligament is the homologue

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 of the ligamentum conjugale present in some mammals, and uniting the heads of opposite ribs, across the back of the intervertebral fibrocartilage.