Situated 1 on either side of the root of the nose and gives sockets for rotatory movements of the eyeballs, the orbits are a pair of pyramidal-shaped bony cavities. The medial walls of 2 orbits are parallel to every other but the lateral walls are set at right angle to every other. With apex directed behind at the optic canal, every orbit is a four sided pyramid and signified by the orbital margin, base forward.
The long axis of every orbit (orbital axis) enters backwards and medially.
The borders of the orbit are as follows:
Medial wall (thinnest): Created by 4 bones; from before backwards these are as follows:
- Frontal process of maxilla.
- Lacrimal process of maxilla.
- Orbital plate of ethmoid.
- Body of sphenoid.
Lateral wall (most powerful): Created by 2 bones, viz.
- Orbital surface of the zygomatic bone in front.
- Orbital surface of greater wing of sphenoid behind.
Floor: Created by 3 bones:
- Orbital outermost layer of the body of maxilla.
- Orbital surface of the zygomatic bone, anterolaterally.
- Orbital process of the palatine bone, posteromedially.
Roof: Created by 2 bones, viz.
- Orbital plate of the frontal bone in front.
- Lesser wing of the sphenoid behind.
Presenting Features in 4 Boundary Walls of Orbit
Medial wall: Presents 2 features, viz.
- Lacrimal fossa is bounded in front by the anterior lacrimal crest of maxilla and behind by the posterior lacrimal crest of the lacrimal bone. The lacrimal fossa interacts with all the nasal cavity via nasolacrimal canal. The lacrimal fossa and nasolacrimal canal hostel lacrimal sac and nasolacrimal duct, respectively.
- Anterior and posterior ethmoidal foramina, is located in the junction between medial wall and the roof of the orbit.
Lateral wall: Presents 2 features, viz.
- Zygomatic foramen. Occasionally there are 2 small foramina, for zygomaticofacial and zygomatico-temporal nerves.
- Whitnall’s tubercle, a small bony tubercle just behind the lateral orbital margin and somewhat below the frontozygomatic suture.
Floor: Presents 2 features, viz.
- Infraorbital groove and canal.
- Small rough belief in anteromedial angle for origin of inferior oblique muscle.
Roof: Presents 3 features, viz.
- Fossa for lacrimal gland in the anterolateral part.
- Trochlear notch or spine in the anteromedial angle.
- Optic canal at the extreme posterior part of the roofing.
Apex: Is located at the posterior end of the orbit and is composed by sphenoid. More exactly it’s created by the middle of the bony bridge between optic canal and superior orbital fissure.
Base: Open and quadrangular, its borders create the orbital margins.
The orbital margins are created as follows:
- Supraorbital margin is composed by the frontal bone and presents a notch-supraorbital notch or foramen at the junction of its lateral two-third and medial one-third.
- Infraorbital margin is composed by the zygomatic bone laterally and maxilla medially; it’s constant with anterior lacrimal crest medially.
- Medial orbital margin is ill defined. It’s created by the frontal bone above and lacrimal crest of the frontal process of maxilla below.
- Lateral orbital margin is composed by zygomatic process of the frontal bone above and frontal process of the zygomatic bone below.
The margins of the orbit are easily palpable in vivo.
The orbital margins supply a rational bony protection to the eye with the exception of at the lateral margin. For this particular reason protective eye guards are made to compensate for it, in squash and handball players and still allowing great peripheral vision.
The connections of orbit are as follows:
- Above: Anterior cranial fossa and frontal air sinus (generally).
- Lateral: Temporal fossa in front and middle cranial fossa behind.
- Below: Maxillary air sinus.
- Medial: Ethmoidal air sinuses.
Orbital Fascia or Periorbita
It’s the periosteum of the bony orbit, which lines the bony bounds of the orbit and creates a funnel-shaped fascial sheath that encloses the orbital contents. It’s loosely connected to the bones, for this reason can be easily stripped off especially from roof and medial wall of the orbit.
At the optic canal and superior orbital fissure, it becomes continuous with the periosteum lining the inner part of the skull (endocranium). At the infraorbital fissure and orbital margins, it becomes continuous with the periosteum covering the external outermost layer of the skull (periosteum).
Paths to the orbit
There are 3 primary paths by which structures can enter and leave the orbit:
– The superior orbital fissure transmits the lacrimal, frontal, trochlear (CN IV), oculomotor (CN III), nasociliary and abducens (CN VI) nerves. It also carries the superior ophthalmic vein.
– The inferior orbital fissure transmits the maxillary nerve (a branch of CN V), the inferior ophthalmic vein, and sympathetic nerves.
There are other minor openings into the orbital cavity. The nasolacrimal canal, which drains tears from the eye to the nasal cavity, lies on the medial wall of the orbit. Other little openings consist of the supraorbital foramen and infraorbital canal– they bring little neurovascular structures.