Anatomy of the Orbit
The two orbits are two pyramidal-shaped bony cavities situated on either side of the nose or midline of face. The apex is represented by the optic foramen and the base by the orbital margins of the frontal and maxillary bones. The average volume of orbit is approximately 30cc.
Innervation of muscles:
Orbital contents:
Orbital Septum: Fibrous membrane attaches to the upper margin and rim of the orbit, forming a diaphragm which is closed by the eye-lid ➔ important in preventing infections and can be
1. Orbital Walls
The surfaces of each orbit (roof, floor, medial and lateral wall) are composed of seven bones: ethmoid, frontal, lacrimal, maxillary, palatine, sphenoid and zygomatic bones. The thinnest of these bones is the lamina papyracea over the ethmoid sinuses along the medial wall.
2. Orbital Apertures
Superior orbital fissure: It is located between the greater and lesser wings of the sphenoid. Structures passing through the fissure include:
- 3rd, 4th, and 6th cranial nerves
- 5th cranial nerve—ophthalmic division.
- Sympathetic nerve fibres
- Orbital veins: No valves therefore bi-direction flow➔ infections can easily pass into cavernous sinus and then infect the other nerve
- Recurrent branch of ophthalmic artery.
I. Optic foramen and canal:
Optic canal (5-10 mm long): It is in the lesser wing of sphenoid. It transmits the Optic nerve, Ophthalmic artery, and the Sympathetic nerves.
Optic foramen: It is the orbital end of optic canal. It measures 6 mm in diameter.
Inferior orbital fissure: It is situated in the lower part of the orbital apex. It transmits following structures:
- Fifth cranical nerve—maxillary division
- Zygomatic nerve
- Inferior ophthalmic vein.
3. Surgical Spaces in the Orbit
There are four surgical spaces in the orbit. These are of clinical importance as the inflammatory process remains localized in any one of them and each space may be opened separately.
- Subperiosteal space: It lies between the periosteum and the bone.
- Peripheral orbital space: It lies between the periosteum and the extraocular muscles. It is a continuous circular space.
- Central space: It is a cone-shaped space enclosed by the extraocular muscles. It is known as the ‘muscle cone’.
- Tenon’s space: It is situated around the globe underneath the Tenon’s capsule.
Normal Position of the Eye in the Orbit
When a straight ruler is applied vertically to the middle of upper and lower margins of the orbit, it just touches the closed lids over the apex of the cornea.