Orbit and Associated Structures: Anatomy and Functions

Orbit and Associated Structures

• The orbit is a quadrangular, truncated cavity with a conical shape.
• The orbit has four boundaries:
  • Superiorly: anterior cranial fossa
  • Medially: nasal cavity
  • Inferiorly: maxillary sinus
  • Laterally: temporal fossa

Boundaries of the Orbit

• The medial wall consists of:
  • Frontal bone
  • Lacrimal bone
  • Ethmoid bone
• The floor consists of:
  • Maxilla
  • Palatine bone
• The lateral wall consists of:
  • Greater wing of sphenoid bone
  • Zygomatic bone
• The roof consists of:
  • Frontal bone (orbital plate)
  • Lesser wing of sphenoid bone

Lacrimal Apparatus

• The lacrimal bone has a grooved area containing the lacrimal sac.
• The lacrimal bone has a shallow fossa for the lacrimal gland.
• The lacrimal apparatus consists of:
  • Lacrimal gland
  • Lacrimal puncta
  • Lacrimal canaliculi
  • Lacrimal sac
• The lacrimal gland is supplied by the oculomotor nerve.

Superior Orbital Fissure

• The superior orbital fissure is located at the apex of the orbit.
• The superior orbital fissure lies between the greater and lesser wings of the sphenoid bone.
• The inferior division of the oculomotor nerve sends a branch to the ciliary ganglion, carrying parasympathetic fibers.

Ciliary Ganglion

• The ciliary ganglion is a parasympathetic ganglion located near the orbital apex.
• The ciliary ganglion is situated between the optic nerve and the lateral rectus muscle.
• Preganglionic fibers travel via the inferior branch of the oculomotor nerve and relay in the ciliary ganglion.
• Postganglionic fibers pass through the short ciliary nerves and supply the sphincter pupillae and the ciliary muscle.

Nerves of the Orbit

• The trochlear nerve (CN IV) enters the orbit through the superior orbital fissure and supplies the superior oblique muscle.
• The abducens nerve (CN VI) enters the orbit through the superior orbital fissure and supplies the lateral rectus muscle.
• The oculomotor nerve (CN III) supplies:
  • The inferior oblique muscle
  • The inferior rectus muscle
  • The medial rectus muscle
  • The superior rectus muscle
  • The levator palpebrae superioris muscle

Blood Supply of the Orbit

• The ophthalmic artery is a branch of the internal carotid artery.
• The ophthalmic artery enters the orbit through the optic canal along with the optic nerve.
• The branches of the ophthalmic artery are:
  • Central artery of the retina
  • Lacrimal artery
  • Long and short ciliary arteries
  • Supraorbital, ethmoidal, muscular, supratrochlear, and dorsal nasal arteries

Veins of the Orbit

• The venous drainage of the orbit is mainly through the superior ophthalmic vein.
• The superior ophthalmic vein is formed by the union of the angular vein and the supratrochlear vein.

Extraocular Muscles

• The extraocular muscles are:
  • Lateral rectus muscle
  • Medial rectus muscle
  • Inferior rectus muscle
  • Superior rectus muscle
  • Inferior oblique muscle
  • Superior oblique muscle
• The actions of the extraocular muscles are:
  • Adduction (medial movement)
  • Abduction (lateral movement)
  • Elevation (superior movement)
  • Depression (inferior movement)
  • Medial rotation (intorsion)
  • Lateral rotation (extorsion)

Eyelid

• The eyelid consists of five layers:
  • Skin
  • Subcutaneous tissue
  • Muscle layer (orbicularis oculi)
  • Tarsal plate
  • Conjunctiva
• The conjunctiva lines the inner surface of the lid and is reflected at the fornices over the sclera.
• The conjunctiva is tightly bound to the underlying sclera.
• The epithelium of the conjunctiva varies from stratified squamous on the sclera to stratified columnar on the eyelid, studded with goblet cells.