What is the orbit?
A pyramidal shaped bony cavity within the facial skeleton
What does the orbit contain and protect?
- The eyeball
- Its muscles
- Most of the lacrimal apparatus
What nerve does the orbit contain?
The optic nerve
How does the optic nerve enter the eye?
It enters the back of the eye via the optic canal at the apex of the pyramid
What are the layers of the optic nerve?
- Pia mata
- Arachnoid mata
- Dura mata
Why does brain inflammation lead to photophobia?
Because the layers of the optic nerve also become inflamed
Which walls of the orbit are thin and vulnerable to injuries to the face or eye?
Inferior and medial walls
What is the apex of the orbit?
The optic canal
What makes up the superior boundary of the orbit?
The frontal and sphenoid bone
What makes up the lateral boundary of the orbit?
Zygomatic and sphenoid
What makes up the medial boundary of the orbit?
Ethmoid, maxillary, lacrimal. sphenoid
What makes up the inferior boundary of the orbit?
What holes does the orbit have?
- Optic canal
- Superior orbital fissure
- Inferior orbital fissure
What does the optic canal transmit?
- Optic nerve
- Opthalmic artery
What does the superior orbital fissure transmit?
- Trochlear nerve
- Frontal branch of opthalmic nerve
- Lacrimal branch of opthalmic nerve
- Abducent nerve
- Superior branch of oculomotor nerve
- Nasociliary branch of opthalmic nerve
- Inferior branch of oculomotor nerve
What does the inferior orbital fissure transmit?
Inferior opthalmic vein
What are the different parts of the eyeball?
What holds the lens in place?
What does the lens act to do?
Focus light onto the back of the retina
What is the area of the most accurate vision?
Why is the macula/fovea the site of the most accurate vision?
Because it has the most amount of cells, so convert most light energy to nerve signals
What is the optic disc?
The location at which the optic nerve enters the retina, and thus the blind spot
What are the layers of the eyeball?
- Outer fibrous layer
- Middle vascular layer
- Inner layer
What does the outer fibrous layer of the eyeball comprise of?
Tough, fibrous sclera
How is the outer fibrous layer of the eyeball continuous anteriorly?
As the transparent cornea
What is the function of the outer fibrous layer of the eyeball?
- Provides attachment for the extra-ocular muscle
- Gives shape to the eyeball
What is the outer fibrous layer of the eyeball continuous with posteriorly?
The dural sheath covering the optic nerve at the back of the eye
What covers the sclera of the eyeball?
A thin, transparent layer of cells called the conjunctivae
Where does the conjunctivae extend?
Up to the edge of the cornea (the limbus), and is reflected onto the inner surface of the upper and lower eyelids
What runs through the conjunctivae?
What does the middle vascular layer of the eyeball consist of?
- Ciliary body
What is the function of the ciliary body?
Connects the choroid with the iris
What are the properties of the ciliary body?
Where does the iris lie?
Just anteriorly to the lens
What is the iris?
A thin contractile diaphragm with a central aperture, the lens
What is the purpose of the central aperture of the iris?
Allows for transmission of light
What forms the iris?
- Sphincter muscle
- Dilator pupillae muscle
What are the muscles forming the iris under the control of?
The autonomic nervous system
Why does the middle vascular layer of the eyeball need a rich network of blood vessels?
To help supply nutrients to avascular area of eyeball
What does the inner layer of the eyeball consist of?
What does the retina consist of?
- Optic parts
- Non-visual parts
What are the chambers of the eyeball?
What is the anterior chamber of the eyeball filled with?
Transparent liquid aqueous humour
Where is the anterior chamber of the eyeball found?
Between the cornea and the iris
How does the anterior chamber of the eyeball communicate with the posterior chamber?
Through the pupil
What is the posterior chamber of the eyeball filled with?
Transparent liquid aqueous humour
Where is the posterior chamber of the eye found?
Between the iris and the lens
What does the posterior chamber of the eye contain?
The ciliary body and processes
What is the function of the ciliary body and processes?
Secretes aqueous humour
What is the aqueous humour of the eye important for?
- Supporting the shape of the eyeball (along with vitreous humour)
Providing nourishment to the lens and cornea
Providing nourishment to the lens and cornea
How does the aqueous humour support the shape of the eyeball?
Because of the pressure it exerts
Why is the aqueous humour required to provide nourishment to the lens and cornea?
Because they do not have their own blood supply
Where does the aqueous humour of the eye drain through?
The irido-corneal angle, into the canal of Schlemm
What is the irido-corneal angle?
The space between the anterior surface of the iris and the posterior extremity of the cornea
Where does the aqueous humour drain from the canal of Schlemm?
Via a trabecular meshwork, back into the venous circulation
What is the vitreous chamber filled with?
Transparent, jelly-like vitreous humour
What is the lens?
Transparent biconvex structure
Where does the lens lie?
Posterior to the iris
What is the lens enclosed in?
What is the lens attached to?
How is the lens attached to the ciliary body?
By the circular suspensory ligament
What is the nerve and blood supply to the lens?
It does not have any
How does the lens recieve its nutrients?
Entirely from the aqueous humour that surrounds and bathes it
What is the effect of contraction of the ciliary msucle fibres?
It alters the tension of the suspensory ligaments, and so allows for a change in the shape of the lens
What does a change in the shape of the lens do?
Changes its refractive power
What is the front of the eye exposed to?
The eyelids and the lacrimal glands
What kind of role to the eyelids and the lacrimal glands have?
Have an important protective role
How to the eyelids and the lacrimal glands protect the eye?
They prevent the surface of the eye from drying out, or being injured by small particles
What happens when the cornea becomes dry?
The eye blinks reflexively, and the eyelids carry a film of fluid over the cornea while also sweeping any dust and foreign material across the medial angle of the eye to be removed
How are the eyelids strengthened and given their shape?
By tarsal playes
What are tarsal plates?
Dense bands of connective tissue
What do tarsal plates contain?
Tarsal glands (Meibomian glands)
What is the purpose of the Meibomian glands?
They secrete an oily substance
What is the purpose of the oily secretion of the Meibomian glands?
- Lubricate the edge of eyelids
- Mix with the tear film over the surface of the eye
Why do the oily secretions from the Meibomian glands mix with the tear film over the surface of the eye?
To prevent tears from evaporating too quickly
What does the lacrimal apparatus consist of?
- Lacrimal glands
- Lactimal ducts
- Lacrimal canaliculi
What do the lacrimal glands do?
Secrete the lacrimal fluid (tears)
Where do the lacrimal glands lie?
In a fossa on the superolateral part of the orbit
What happens to the lacrimal fluid after it has been secreted by the lacrimal glands?
It enters the conjuctival sac through the lacrimal ducts, and from the ducts pass into the lacrimal lake at the medial angle of the eye
Where does the lacrimal fluid go from the lacrimal lake?
Fluid drains into the lacrimal sac before passing into inferior meatus of the nasal cavity via the nasolacrimal duct
What is the passage of the lacrimal fluid into the inferior meatus of the nasal cavity the reason for?
You getting a runny nose when you cry
What innervates the structures of the orbit?
The optic nerve and several branches of the opthalmic division of the trigeminal nerve
What supplies blood to the muscles and tissues of the orbit?
Branches of the opthalmic artery
What supplies blood to the retina?
The central retinal artery, a branch of the opthalmic artery
How is the orbit and related structures drained of blood?
Venous drainage is via tributaries into the opthlamic veins