Orbit and Eyes Flashcards
How can you get a blowout fracture to the orbit?
Blunt trauma to orbit
How is a broken bone in the orbit a protective measure?
Contents of orbit sink down into broken bone and are thus protected
What forms the supraorbital margin?
Frontal bone
What is the significance of the supraorbital notch?
Neurovascular structures pass through it
What forms the infraorbital margin?
Zgomatic bone laterally
Maxilla medially
Which bones break in a blowout fracture?
Thinnest and weakest bones
- Lacrimal
- More commonly, ethmoid in papyrus region
- Or junction between 2
Which bones make up the roof of the orbit?
Frontal
Lesser wing of sphenoid
Which bones make up the floor of the orbit?
Maxilla - orbital plate of
Zygomatic
Palatine
Which bones make up the lateral wall of the orbit?
Zygomatic
Greater wing of sphenoid
Which bones make up the medial wall of the orbit?
Maxilla
Lacrimal
Ethmoid
Body of sphenoid
What colour is the sclera?
White
How much of the surface of the eyeball is the sclera?
5/6
What are the functions of the sclera?
Maintains shape of globe
Offers resistance to internal and external forces
Provides attachments for extraocular muscles
What is the sclera made of?
Collagen
What is the pattern of collagen in the sclera?
Laid down in whorls
Makes it very tough
How much of the surface of the eyeball is the cornea?
Anterior 1/6
Are the cornea and sclera continuous?
Yes
What is the principal refracting component of the eye?
Cornea
How much light does the cornea bend?
60%
What makes the cornea optimum for vision
Avascular
Transparent
Structure of collagen
Not many cell bodies
What are the five histological layers of the cornea, from outside to inside?
Epithelium Bowman's layer Stroma Descemet's membrane Endothelium
What kind of layer is the epithelium?
Mucous membrane
Multiple cells
How many layers is the endothelium of the cornea?
1
Which part of the cornea is continuous with the sclera?
Stroma
How is collagen organised in the cornea?
Uniform in diameter
Evenly spaced
Run parallel to each other in bundles = lamellae
Adjacent lamellae at angles to each other
What does scleral opacity vary depending on?
Composition of stroma
Hydration
Size and distribution of collagen
How does the collagen of the cornea often heal?
Doesn’t grow back in ordered way
Scar formation
Loss of transparency
What is the anterior chamber?
Area just behind cornea
What is the anterior chamber angle?
Junction between iris and cornea
What are the key structures in the anterior chamber angle?
Cornea
Trabecular meshwork
Canal of Schlemm
Ciliary body
What is the trabecular meshwork?
Holes through which aqueous humour drains
What can happen if the trabecular meshwork is clogged?
Pressure in eye increases
What is the route of aqueous flow through the anterior chamber angle?
Past ciliary body
Through trabecular meshwork
Through canal of Schlemm
What are the three layers of the eye and what are their functions?
- Cornea and sclera > strength
- Uvea > nutrition
- Retina > sight
What are the functions of the ciliary body?
Formation of aqueous humour
Tethers lens
Accommodation
What forms the aqueous humour?
Ciliary epithelium
What tethers the lens?
Ciliary processes
What is responsible for accommodation?
Ciliary muscle
Where are the ciliary processes located?
Anteriorly only
What is aqueous humour important for?
Maintaining health of lens and cornea
Creates intraocular pressure
How many cell layers is the ciliary epithelium?
2
Where is the ciliary epithelium?
On ciliary processes
What does aqueous humour drain into?
Venous supply
What structures does accommodation involve?
Lens
Ciliary muscle
Zonules attached to lens
What are zonules?
Ligaments between ciliary processes and lens
Pull lens to change its shape
Describe the ciliary muscle
Within ciliary body
Innervated by parasympathetic nervous system carried by CN III
Smooth muscle - like sphincter
What does the ciliary muscle do to allow you to see things far away?
Relax > aperture of sphincter open > zonules stretch > lens becomes skinny
What does the ciliary muscle do to allow you to see things close by?
Contract > aperture of sphincter smaller > zonules relax > lens becomes fat
What is presbyopia?
Loss of accommodation with age
Why does presbyopia happen?
Reduction in flexibility of lens and zonules
What are the structures that make up the uvea?
Ciliary body
Iris
What is the function of the iris?
Controls size of pupil > controls light entry
Which muscles make up the iris?
Sphincter pupillae
Dilator pupillae
What does the sphincter pupillae do?
Constrict pupil
What is the sphincter pupillae innervated by?
Parasympathetic fibres of CN III
What does the dilator pupillae do?
Dilate pupil
What is the dilator pupillae innervated by?
Sympathetic nervous system
Is the dilator pupillae a real muscle?
No, it’s specialised myoepithelium
What is the choroid?
3 layers of blood vessels
What is the most important layer of the choroid and where is it?
Choriocapillaris
Just below retina
Where is the choroid?
Underneath retina
What does the choroid supply?
Retina
What are the parts of the retina?
Optic nerve head/optic disc
Fovea/foveola - in middle
Macula - around fovea, defined by pigment
Posterior pole - around macula
Ora serrata - retina joins rest of inside of eyeball
What are the fovea and macula important for?
Central vision
Describe the fovea?
High visual acuity
Avascular - gets nutrients from choroid
High density of cones
No rods
What forms the optic nerve?
Axons of ganglion cells as they exit retina to pass visual information to higher cortical areas
What is the lamina cribrosa?
1/3 of scleral collagen crosses optic nerve
Dense - 3-10 sheets
Sieve-hole transmitting nerve fibres
Provides some structure to optic nerve
What happens to the lamina cribrosa when intraocular pressure increases?
Holes stretched
Press on axons of ganglion cells
Causes damage
What are the four main arteries of the eye?
Central retinal artery Ciliary arteries - Long posterior ciliary artery - Short posterior ciliary artery - Anterior ciliary artery
Where is the central retinal artery from?
Internal carotid > ophthalmic > 1st branch
What is the course of the central retinal artery?
Branches off ophthalmic nerve
Pierces optic nerve
Travels in optic nerve
Fans out on surface of retina
What does the central retinal artery supply?
Inner retina
What artery are the posterior ciliary arteries branches of?
Ophthalmic artery
What is the course of the posterior ciliary arteries?
Branch off ophthalmic artery
Enter back of eye around optic nerve
Depending on how far they travel called short/long
How many branchlets of posterior ciliary arteries are there?
7-11
What do the long posterior ciliary arteries supply?
Travel in choroid all the way forward
- Ciliary body
- Iris
- Some regions of retina via choroid
What do the short posterior ciliary arteries supply?
Choroid around optic nerve
What is the course of the anterior ciliary artery?
Ophthalmic artery > muscular artery > anterior ciliary artery
Travels on outside of globe towards front
What does the anterior ciliary artery supply?
Anterior structures of eye
- Conjunctiva
- Some of sclera
- Forms ring around cornea
What supplies the outer retina - and the photoreceptors within it?
Posterior ciliary artery
What structures make up the eyelid?
Skin - Glands - Eyelashes Muscles - Orbicularis oculi - Levator palpebrae superioris Lacrimal apparatus - Lacrimal gland and ducts - Nasolacrimal sac and duct
What is the function of orbicularis oculi?
Close eyelid
Which nerve innervates orbicularis oculi?
CN VII
What is the function of levator palpebrae superioris?
In upper eyelid only
Opens eyelid
Which nerve innervates levator palpebrae superioris?
CN III
