The Orbit Flashcards
Describe the orbit
Bony, cone shaped socket with its apex pointing posteromedially, consisting of 7 bones. It formed around the optic nerve and orb.
Bones of the orbit
7 bones:
- Frontal (orbital plate)
- Sphenoid (greater & lesser wings)
- Ethmoid bone (labyrinth)
- Lacrimal bone
- Palatine bone (orbital process)
- Maxilla (orbital plate)
- Zygomatic bone
Boundaries of the orbit
- Roof: frontal bone
- Floor: maxilla
- Lateral wall: zygomatic bone & greater wing of the sphenoid
- Medial wall: ethmoid bone
- Lacrimal bone & frontal process of maxilla contribute anteriorly
Discuss the lacrimal fossa
- Depression posterior to the inferomedial angle of the orbital rim
- Houses the lacrimal sac
- Below is a large foramen for nasolacrimal duct
What are the groups of muscles of the eye?
- Intraocular
- Extraocular which move the orb
- Extraocular which elevate the eyelid
Intraocular muscles of the eye
- Ciliary muscle - change shape of the lens
- Pupillary muscles - constrict/dilate pupil
Preganglionic parasympathetic fibres from III
Extraocular muscles which move the eye
- Recti - tendinous ring → just anterior to equator, behind the junction between sclera & cornea
- Obliques - insert just posterior to the equator
NB: any coordinated movements involve at least 3 muscles
Extraocular muscles acting on the eyelid
Levator palpebrae superioris - elevates upper eyelid
Mixed III/sympathetic innervation
Superior rectus
Elevates & adducts, III
Inferior rectus
Depressed & adducts, III
Medial rectus
Adducts, III
Lateral rectus
Abducts, VI
Superior oblique
Depresses & abducts, IV
Inferior oblique
Elevates & abducts, III
Blood supply to the orbit
Via branches of the ophthalmic artery (ICA):
- Central artery of the retina
- Branches to the extraocular muscles
- Lacrimal artery
- Gives recurrent meningeal branch with anastomoses with MMA
Pass through optic canal (ophthalmic & CAotR)
Venous drainage of the orbit
- Superior ophthalmic vein → cavernous sinus
- Via SOF
- Inferior ophthalmic vein: 3 options
- Joins superior
- Passes through SOF → cavernous sinus
- Passes through IOF and joins pterygoid venous plexus
Structures running through the orbital fissures (S→I)
SOF - outside of tendinous ring:
- Superior ophthalmic vein
- Lacrimal n. (Va)
- Frontal n. (Va)
- Trochlear n. (IV)
SOF - inside tendinous ring:
- Superior division of III
- Nasociliary n. (Va)
- Inferior division of III
- Abducent n. (VI)
IOF: inferior ophthalmic vein?
Describe the eyelids
- Movable folds which function to protect the eye
- Canthi = angle between eyelids when open
- Palpebral fissure = space between
- Tarsal plates = condensed connective tissue
- Superior plate is insertion for LPS
Describe the tarsal glands
- Modified sebaceous glands, produce an oily secretion
- Make the lid margins waterproof
- Decrease evaporation of tears from cornea
- Open onto the skin posterior to the eyelashes
Diagram: conjunctival sac
Describe the location of the conjunctiva
- Line inner aspect of the eyelids: palpebral
- At root of eyelid reflect onto anterior aspect of the eyeball: bulbar
Describe the lacrimal gland
- Occupies a depression in the superolateral angle of the orbit
- Its ducts drain into the lateral part of the superior conjunctival fornix
Path of tears
- Swept from blinking → lacrimal lake
- Lake → lacrimal canaliculi → lacrimal sac → nasolacrimal duct → inferior meatus of nose
NB: opening of NL duct guarded by folds of mucous membrane preventing air being blown into the sac
