Eyeball (overview)

The eyeball is the main and most essential part of the eye, containing structures responsible for vision. It receives visual information and transmits it to the brain. The eyeball measures approximately 0.94 inches (24 millimeters) in anteroposterior diameter in adults and occupies the anterior portion of the orbit. Only one-sixth of it is exposed to the external environment.

Even though most people think the eyeball is shaped like a ball, it is actually spherical, with a convex anterior bulge. The eyeball is a two-piece unit consisting of the anterior and posterior segments. The anterior segment is the front part, which accounts for about one-sixth of the eyeball, while the posterior segment is larger and forms five-sixths of the eyeball.

Within the bony orbit, the eyeball is cushioned by adipose tissue called the orbital fat body or the retrobulbar fat. Additionally, its outer surface is enveloped in a thin fascial sheath called Tenon's capsule. Anteriorly, the capsule attaches to the sclera, and posteriorly, it merges with the meninges surrounding the optic nerve (CN II) (Read more!).

Between the sclera and Tenon's capsule is a potential space called the episcleral layer or episcleral space. Together, the adipose tissue and Tenon's capsule protect, position, and stabilize the eyeball within the orbit.

Finally, the eyeball is supported by the suspensory (Lockwood's) ligament, which acts like a hammock, stretching below the eyeball between the medial check ligament and lateral check ligament. This ligament encloses the inferior rectus muscle and inferior oblique muscle, helping maintain the position of the eyeball and stabilize its movements, especially in the vertical plane. By forming a supportive sling, it prevents downward displacement of the eyeball and contributes to the overall integrity of the orbital connective tissue system.