Orbit | Orbital Cavity

Orbit is a pyramid shaped (conical) cavity in the skull, in which the eye and its appendages are placed. The base of the pyramid is in front and its apex behind.

Orbital margin:

Orbital margin denotes the boundaries of the base of the orbit, which opens in the face. It is formed as:

  • Superiorly: By frontal bone
  • Laterally: By the processes of frontal and zygomatic bones
  • Inferiorly: By the zygomatic bone and the maxilla
  • Medially: By the processes of maxilla and the frontal bone

Orbital walls:

As stated above, the orbit has its base in front (towards the face) and its apex behind (towards the back of the head). The orbital cavity from its base to the apex is bounded by four walls:

  • Roof: It is the superior wall of the orbit, formed by the orbital plate of frontal bone. The roof separates the orbital cavity from the anterior cranial fossa, which contains the frontal lobe of the cerebral hemisphere.
  • Lateral wall: It is formed by the zygomatic bone and greater wing of sphenoid bone
  • Floor: It is the inferior wall of orbit, formed by the orbital plate of maxilla. It separates orbit from the maxillary sinus.
  • Medial wall: It is formed by four bones, which in order from anterior to posterior are:
    • Frontal process of maxilla
    • Lacrimal bone
    • Orbital plate of ethmoid bone (which separates the orbital cavity from ethmoid sinuses)
    • Body of sphenoid bone

Openings into the orbital cavity:

Orbital cavity receives a number of openings, as described below:

Orbital opening: It is the large anterior opening, (the base of the pyramid), where about one-sixth of the eye is exposed.

Supraorbital notch or foramen: It is situated on the superior orbital margin. It transmits the supraorbital nerve and the blood vessels.

Infraorbital groove and canal: These are situated on the floor of the orbit. They transmit the infraorbital nerve and blood vessels.

Nasolacrimal canal: It is present in the medial wall, anteriorly. It communicates with the inferior meatus of the nose and transmits the nasolacrimal duct.

Inferior orbital fissure: It is located posteriorly between the maxilla and the greater wing of sphenoid bone, communicating with the pterygopalatine fossa. It transmits the following structures:

  • Maxillary nerve
  • Zygomatic branch of maxillary nerve
  • Inferior ophthalmic vein
  • Sympathetic nerves

Superior orbital fissure: It is located posteriorly between the greater and lesser wings of the sphenoid bone, communicating with the middle cranial fossa. It transmits the following structures:

  • Superior ophthalmic vein
  • Lacrimal nerve
  • Frontal nerve
  • Trochlear nerve
  • Oculomotor nerve
  • Abducent nerve
  • Nasociliary nerve

Optic canal: Also known as optic foramen, it is located posteriorly in the lesser wing of the sphenoid bone, communicating with the middle cranial fossa. It transmits the optic nerve and ophthalmic artery.

Orbital fascia:

Orbital fascia is the periosteum of the bones, which form the walls of the orbit. It is loosely connected to bones and can be readily separated from them. It is continuous through the foramina and the fissures.

Posteriorly, it is continuous with the dura mater, via processes that pass through the optic canal and superior orbital fissure.

Anteriorly, it is connected to the periosteum of the margin of the orbit, and assists in forming the orbital septum.

It also gives off two processes, one to enclose the lacrimal gland and the other to hold the pulley of the superior oblique muscle in position.

Nerves of the orbit:

  • Optic nerve
  • Lacrimal nerve
  • Frontal nerve
  • Trochlear nerve
  • Oculomotor nerve
  • Nasociliary nerve
  • Abducent nerve
  • Ciliary ganglion

Blood vessels of the orbit:

  • Ophthalmic artery
  • Ophthalmic veins