Definition of Orbit of the Eye

The eye orbit provides support and protection for the eye.

The orbit of the eye, sometimes called the eye socket is the bony part of the skull that houses and protects the eyeball. In addition to the eye itself, the orbit contains related soft tissues that act as shock absorbers, as well as the blood vessels, nerves, and muscles that permit the eye to function.

Purpose

According to the Howard Hughes Medical Institute, 25% of humans' cerebral cortex cells relate to vision.

The orbit of the eye, or eye socket, is the area of the skull that surrounds the eye. It protects and supports the eye and the lacrimal (tear-producing) glands, provides areas of attachment for the muscles that move the eye and eyelids and allows nerves and blood vessels running from inside the skull to reach the eye. Because the eye is a part of the brain that extends outside the skull, and because vision is the primary sense humans use to gather information about the environment, the orbit serves a particularly important purpose.

Bones of the eye orbit

Seven bones of the skull contribute to the eye orbit and encase the eye. Above the eye, the bottom edge of the frontal bone forms the supraorbital torus or brow ridge. The frontal bone also extends downward along the outside of the orbit, where it meets with the zygomatic bone to form the outer rim (lateral margin) of the orbit. The zygomatic bone also extends along the bottom edge of the orbit, where it meets the maxillary bone to form the bottom or inferior margin of the orbit. The maxillary bone meets with the ethmoid, lacrimal, and sphenoid bones to form the medial wall, the part of the orbit closest to the nose. Behind the eye itself the palatine bone also contributes to the floor of the orbit.

Soft tissues of the eye orbit

The orbit provides rigid, bony protection for the eye while also accommodating the lacrimal glands, which produce tears, and the soft tissues that cushion the eye and allow it to move. Of the six muscles that move the eye (also called the extraocular muscles), four originate from a tendon encircling the optic nerve, whereas two attach directly to the bones of the orbit. Two muscles that move the eyelid also attach to the bony orbit. The eye and the muscular sheath that surrounds it are cushioned by layers of fat that keep the eye centered in the orbit and allow it to move freely.

Communication between the orbit and the interior of the skull

Although the orbit primarily functions to protect the eye from injuries, it also must allow access to the nerves and blood vessels that supply the eye and related tissues. Some of the individual bones comprising the orbit have holes called foramina (singular foramen) in them to allow these to pass from inside the skull to the orbital structures. Larger nerves and vessels, including the optic nerve, pass through gaps between the bones of the orbit, which are known as fissures.

Evolutionary history of the orbit

Along with monkeys and apes, humans have the only bony orbit in nature that fully encloses the eye. According to John Fleagle, Professor of Anatomical Sciences at Stonybrook University, the evolutionarily recent development of the fully enclosed orbit reflects the fact that humans gain most of their information about the environment through the visual sense.

References

• Mission for Vision, USA: Anatomy of the Human Eye
• "Primate Adaptation and Evolution (2nd Ed.)"; John G. Fleagle, 1998
• Howard Hughes Medical Institute: Sensing Change in the Environment

Resources

• Eye Info: Diagrams of the Eye