Incisurae costales

Each outer border, at its superior angle, has a small facet, which with a similar facet on the manubrium, forms a cavity for the cartilage of the second rib; below this are four angular depressions which receive the cartilages of the third, fourth, fifth, and sixth ribs.

The inferior angle has a small facet, which, with a corresponding one on the xiphoid process, forms a notch for the cartilage of the seventh rib.

These articular depressions are separated by a series of curved interarticular intervals, which diminish in length from above downward, and correspond to the intercostal spaces.

Most of the cartilages belonging to the true ribs, articulate with the sternum at the lines of junction of its primitive component segments.

This is well seen in some other vertebrates, where the parts of the bone remain separated for longer.