22 - Contents and the Orbit Flashcards
Orbital margins anteriorally
Supraorbital margin formed by the frontal bone (supraorbital notch is under eyebrows)
Infrorbital margin formed by the zygomatic bone laterally, maxilla medially
Bones of the roof of the orbit
Fontal bone, lesser wing of sphenoid
Bones of the floor of the orbit
Maxilla, zygomatic, palatine
Bones of the lateral wall of the orbit
Zygomatic, greater wing of sphenoid
Bones of the medial wall of the orbit
Maxilla, lacrimal bone, ethmoid, body of sphenoid
Bone through which optic canal and superior orbital fissure run
Greater wing of sphenoid
Bones between which the inferior orbital fissure is
Maxilla and greater wing of sphenoid
Bones often broken in blunt force trauma to the eye
1
2
Part of the medial wall (the weakest part of orbit:
1) Lacrimal bone (very thin).
2) Orbital plate of the ethmoid has a very thin region (papyrus region of ethmoid bone).
Why can double vision arise from a broken orbit?
Extraorbital muscles can become impinged upon by broken bone, resulting in abnormal movement of eye.
Substance used to visualise cornea
Fluoroscein
Amount of eyeball that is sclera
~5/6
Role of sclera
Maintains shape of globe, resistance to internal and external forces.
Provides attachments for the extraocular muscles
How is the collagen of the sclera arranged?
In whorls. Makes it very difficult to pierce the sclera.
Cornea 1 2 3 4 5
1) Anterior 1/6 of eye.
2) Principle refracting component of eye
3) Avascular and transparent
4) Five histological layers (outermost is epithelium, innermost is endothelium)
Layers of cornea
Five layers.
Outermost is epithelium, innermost is endothelium.
Middle is stroma, which is collagenous, and similar to sclera.
Are the cornea and sclera continuous?
Yes
Why is the cornea transparent, if it is continuous with the sclera?
Collagen in cornea is very ordered.
Collagen in sclera is arranged in whorls.
Arrangement of collagen in the cornea 1 2 3 4 5
1) Collagen fibrils in the cornea are uniform in diameter, and evenly-spaced
2) Run parallel to each other inbundles (lamellae)
3) 200-300 lamellae in stroma
4) Adjacent lamellae lie at angles toeach other, but all fibrils 5) within alamella run parallel.
What does sclera opacity vary depending on?
1
2
3
1) Composition of stroma
2) Hydration
3) Size and distribution of collagen
Part of cornea that is vulnerable to abrasion
Epithelium.
Damage to this won’t result in a permanent, opaque scar of the cornea.
Corneal injury that results in an opaque scar
Injury to the very ordered collagen of the stroma. Scar tissue collagen is disordered.
Where does the aqueous humour drain out of the eye?
Anterior chamber angle
Anterior chamber angle
Junction between the iris and cornea.
Key structures involved in the anterior chamber angle 1 2 3 4
1) Cornea
2) Trabecular meshwork (a sieve structure of the cornea)
3) Canal of Schlemm (where aqueous humour drains from the anterior chamber to the venous system)
4) Ciliary body (where fluid of aqueous humour is produced)
Glaucoma
Blockage of the Canal of Schlemm.
Increase in intraocular pressure
Uvea
1
2
3
Middle layer of the eye.
Major role is nutrition of the eyeball.
Consists of conjunctiva, sclera and ciliary body.
Ciliary body
1
2
3
1) Forms aqueous humour (sort of like CSF)
2) Tethers lens of eye via ciliary processes of ciliary epithelium (important in focussing)
3) Accommodation (ciliary muscle)
What is aqueous humour important for?
Maintaining health of lens and cornea
What are involved in accommodation?
Ciliary muscles and zonules (ligaments that attach ciliary muscles to lens)
What innervates the ciliary muscles?
Parasympathetic nervous system of CNIII
Which type of muscle are ciliary muscles?
Smooth
What happens when ciliary muscles contract?
Flatten out, stretch lens. Makes lens thinner.
Relaxation makes lens bulge, become thicker.
‘Accommodation or focussing’
Term for loss of accommodation with age
Presbyopia.
Can’t focus on near objects.
What leads to presbyopia?
Reduction in flexibility of lens capsule and zonules.
How is presbyopia treated?
Using + lenses.
Muscles in iris
1
2
1) Sphincter pupillae
2) Dilator pupillae
Sphincter pupillae
1
2
3
Of iris.
Constricts pupil (runs along circumference of pupil)
Innervated by parasympathetic NS
Dilator pupillae
Of iris
Dilates pupil (‘spokes on a wheel’)
Innervated by the sympathetic NS
Choroid location
Sits behind retina
Choroid structure
1
2
1) Three blood vessels
2) For nutrition to the retina
Retina structures 1 2 3 4 5
1) Optic nerve/optic disc
2) Fovea/foveola
3) Macular
4) Posterior pole
5) Ora serrata (junction between retina and ciliary body)
Fovea 1 2 3 4
1) High-visual acuity (cells, other than cones, are shifted away from the fovea, making a pit)
2) Avascular (gets nutrients from choroid)
3) High density of cones
4) No rods
How does optic nerve pierce sclera at the back of the eye?
~2/3 of sclera bends, forms optic sheath of optic nerve.
~1/3 of sclera continues across the optic nerve to form lamina cribosa.
Lamina cribosa
Where ~1/3 of sclera continues across the optic nerve to form lamina cribosa.
This forms a collagenous mesh which adds strength to optic nerve.
What can happen to lamina cribosa if intraocular pressure increases?
Bows outwards from eye, impinges on visual nerves.
Sheath of optic nerve
Dura and sclera
Blood supply of the orbit and eye
1) All are tributaries of the Opthalmic artery, which is a branch of the internal carotid artery.
2) Central retinal artery branches from the opthalmic artery, pierces optic nerve, travels through optic nerve to retina, fan out across the retina.
3) Ciliary arteries branch from opthalmic artery
Ciliary arteries
Branch from opthalmic artery, travel along exterior of optic nerve.
Pierce eye around the optic nerve.
Short posterior ciliary arteries supply choroid, pierce posterior eyeball.
Long posterior ciliary arteries pierce with short posterior arteries, travel to front of eye.
Anterior ciliary arteries don’t pierce eye with posterior arteries. Supply cornea, anterior structures.
Types of ciliary arteries
1
2
3
1) Short posterior ciliary arteries supply choroid, pierce posterior eyeball.
2) Long posterior ciliary arteries pierce with short posterior arteries, travel to front of eye.
3) Anterior ciliary arteries don’t pierce eye with posterior arteries. Supply cornea, anterior structures.
Blood supply of the retina
1
2
Dual blood supply:
1) Central retinal artery supplies the inner retina
2) Posterior ciliary artery supplies the outer retina (photoreceptors)
Components of eyelid
Skin on outside.
Conjunctiva (mucous membrane) on inside.
Muscles: orbicularis oculae, levator palpebrae superioris
Orbicularis oculae
A sphincter muscle, innervated by CNVII.
Surrounds orbit.
When it contracts, closes eye (depresses upper eyelid).
Levator palpebrae superioris
Lies in upper eyelid.
When it contracts, opens eye.
Innervated by CNIII
