Sclerotic ring

Sclerotic rings are rings of bone found in the eyes of many animals in several groups of vertebrates, except for mammals, amphibians, and crocodilians. The ring is located within the soft tissue of the sclera, commonly known as the "white" of the eye. The structure has also been known as a scleral ring, to prevent confusion with sclerosis, an unrelated medical condition.

They can be made up of cartilaginous material (scleral cartilage) or bony material (scleral ossicles), or often a combination of both, that come together to form a ring. The exact arrangement, size, shape, and number of ossicles is diverse from group to group. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical, or which live underwater. Fossil sclerotic rings are known for a variety of extinct animals, including ichthyosaurs, pterosaurs, and non-avian dinosaurs, but are often not preserved.

The rings help to provide structure to the eye, especially in animals that do not have round eyes. Animals that move rapidly, including both fast flying birds and fast swimming fish have the most robust sclerotic rings, indicating that these thick rings are used to protect the eye during intense changes in pressure in the air and in the water. Additionally, sclerotic rings may help the eye adjust to different viewing differences, also known as visual accommodation. Muscles are used to adjust the shape of the eye for accommodation, and the rings provide attachment sites for these muscles. In aquatic animals, the lens is squeezed in a different way to compensate for differences in light refraction underwater, and so the shape of the ring can be different than those in terrestrial animals.