Orbit

The 7 bones of the orbit:

• Frontal
• Zygoma
• Maxillary
• Sphenoid
• Ethmoid
• Palatine
• Lacrimal

• Volume: 30ml
• There are no lymphatic vessels or nodes in the orbit
• Medial periocular tissues: drain to submandibular lymph nodes
• Lateral periocular tissues: drain to superficial preauricular nodes

Osteology

• Pear-shaped
• Supraorbital margin formed by frontal bone

• Supraorbital ridge only extends over the medial ½ to ⅔ of the superior orbital rim

• Infraorbital margin formed by zygomatic and maxilla
• Lateral margin formed by frontal and zygomatic

• Medial margin formed by maxillary process of frontal and lacrimal crest of the frontal process of maxilla

• Roof: frontal and lesser wing of sphenoid
• Floor: zygomatic, maxilla, palatine
• Lateral wall (thickest): zygomatic, greater wing of sphenoid
• Medial wall: frontal process of maxilla, lacrimal, ethmoid, body of sphenoid
• The lamina papyracea of the ethmoid bone is the weakest part of the orbit

Relations

• Nasal cavity, ethmoid air cells and sphenoid sinus medially
• Frontal air sinus, meninges and frontal lobe superiorly
• Maxillary air sinus inferiorly
• Temporalis muscle, middle cranial fossa and temporal lobe laterally

Openings

Optic canal

• Optic canal within the lesser wing of the sphenoid only
• 4-10mm long
• Connects to middle cranial fossa
• Contains optic nerve and ophthalmic artery (with surrounding sympathetic plexus)

Superior orbital fissure

• Superior orbital fissure lies between the greater and lesser wings of sphenoid
• Also connects to middle cranial fossa

• Common tendinous ring (formed by thickening of periorbita) overlies the medial end of the superior orbital fissure, attached to

• Lesser wing of sphenoid
• Greater wing of sphenoid
• Orbital plate of ethmoid

• SOF outside ring: lacrimal, frontal and trochlear nerves and superior ophthalmic vein. Inferior ophthalmic vein passes below ring (also outside)

• SOF within ring: both branches of oculomotor nerve, nasociliary and abducens nerves

Refresher note: the nasociliary, frontal and lacrimal nerves are the three branches of the first (ophthalmic) division of the trigeminal nerve

Inferior orbital fissure

• Inferior orbital fissure lies between greater wing of sphenoid and maxilla
• Connects to
• Pterygopalatine fossa
• Infratemporal fossa
• Contains
• Maxillary nerve which becomes infraorbital nerve on entering the orbit
• Zygomatic nerve (also a branch of V2) 
• Inferior ophthalmic vein draining to pterygoid plexus
• Infraorbital artery and vein
• Branches of the pterygopalatine ganglion
• Covered by vestigial muscle of Muller (function unknown)

Ethmoidal foramina

• Found where the roof joins the medial wall
• Anterior ethmoid foramen at the junction of the frontal and ethmoid bones 
• Connects to anterior cranial fossa 
• Contains the anterior ethmoidal nerve and artery
• Posterior foramina
• Contains the posterior ethmoidal nerve and artery supplying ethmoidal sinuses

Lateral tubercle of Whitnall

• Insertions of:
• Check ligament of eyeball
• Suspensory ligament of eyeball
• Lateral canthal tendon
• Aponeurosis of levator

Suspensory ligament (of Lockwood)

• Thickening of the inferior part of the fascial sheath of the eyeball (Tenon’s)
• Receives fascial contributions from the inferior rectus and oblique muscles
• Attached medially to the lacrimal bone
• Attached laterally to the zygomatic bone (at the tubercle of Whitnall)