In gnathostome fish, the first arch develops into the jaws, the second into the hyomandibular complex, with the posterior arches supporting gills.

It emerges between the posterior arch of the atlas and the lamina of the axis, below the Obliquus inferior.

At the C1 level, the vertebral arteries travel across the posterior arch of the atlas through the suboccipital triangle before entering the foramen magnum.

The first cervical or suboccipital nerve lies between the artery and the posterior arch of the atlas.

It is connected above to the posterior margin of the foramen magnum and below to the upper border of the posterior arch of the atlas.

On the under surface of the posterior arch, behind the articular facets, are two shallow grooves, the inferior vertebral notches.

The rectus capitis minor muscle goes from the middle of the posterior arch of the atlas to the occipit.

The floor is formed by the posterior occipito-atlantal membrane, and the posterior arch of the atlas.

In the deep groove on the upper surface of the posterior arch of the atlas are the vertebral artery and the first cervical or suboccipital nerve.

As a result oxygenated blood mostly goes to the anterior gill arches and the deoxygenated blood mostly goes to the posterior arches.