Orbit

The bony orbits are structures on both sides of the human skull. They are shaped like pyramids and hold the eyeballs, nerves, blood vessels, lacrimal gland, muscles, tendons, trochlea, fat, and other tissue. The two orbits are located below the anterior cranial fossae and in front of the middle cranial fossae. They surround the nasal cavities in the middle.

Seven cranial bones contribute to each orbit: the Maxilla, Frontal, Zygomatic, Ethmoid, Lacrimal, Sphenoid, and Palatine bones. These bones give the orbit its pyramid-like shape, with a forward-facing base (called the orbital rim) and a backward and inward apex.

Orbital rim: Distinct bones form the margins of the orbital rim:

• The superior margin is made up of the frontal bone.

• The medial margin is formed by the frontal process of the maxilla.

• The inferior margin consists of the zygomatic process of the maxilla and zygomatic bone.

• The lateral margin is formed by the zygomatic bone, frontal process of the zygomatic bone, and zygomatic process of the frontal bone.

Orbital walls: Each pyramid-shaped orbit has a roof, floor, medial wall, and lateral wall, made up of specific bones:

• Roof (Superior wall): Primarily formed by the orbital part of the frontal bone, with a smaller contribution from the lesser wing of the sphenoid bone at the back. There is a bony lacrimal fossa on the anterolateral side, which houses the orbital part of the lacrimal gland. On the anteromedial side, there is a fibrous pulley called the trochlea, to which the superior oblique muscle is attached and passes through, making a medial turn.

• Medial Wall: Formed by the sphenoid bone, orbital plate of the ethmoid bone, lacrimal bone, and frontal process of the maxilla. The ethmoid bone has air-filled spaces (ethmoidal air cells) in the medial wall. At the junction of the medial wall and roof, you can find the anterior and posterior ethmoidal foramina. In the front, there is a lacrimal groove that holds the lacrimal sac. The margins of the groove are known as the anterior and posterior lacrimal crests, formed by the maxilla and lacrimal bones, respectively.

• Floor (Inferior wall): Mainly formed by the maxilla and zygomatic bones.

• Lateral Wall: Composed of the zygomatic bone at the front and the greater wing of the sphenoid bone at the back.

The orbital cavity has important fissures or openings. The superior orbital fissure is located where the roof and lateral walls meet, between the lesser and greater wings of the sphenoid bone. It serves as a passage for various structures, including multiple cranial nerves responsible for eye movement (oculomotor, trochlear, abducent), the ophthalmic division of the trigeminal nerve and its branches, as well as blood vessels that supply the area around the eye. The inferior orbital fissure is situated at the junction of the lateral wall and floor. Additionally, the optic canal is near the apex and allows the optic nerve and ophthalmic artery to pass through.