What are the basic components of the eye?
bony orbit, contents, eyeball, eyelids, lacrimal apparatus, muscles
What is the ‘tear-drop sign’ indicative of?
An orbital floor fracture which has herniated into the maxillary sinus
What is the role of orbital fat?
Cushioning the globe. Key for support
What components make up the outer fibrous coat of the eye?
The sclera and cornea
What components make up to middle vascular coat of the eye?
The iris, ciliary body, choroid
What makes up the outer sensory coat of the eye?
Retina
What liquid like substance resides in the anterior portion of the eye?
Aqueous humour
What gel like substance is contained in the posterior portion of the eye?
Vitreous humour
Describe the cornea
The anterior 1/6 of the fibrous outer coat, transparent window that will allow light rays to enter the eyeball
Describe the sclera
The posterior 5/6 of the fibrous outer coat, gives attachment to the muscles moving the eyeball
Describe the histology of the cornea
Stratified squamous non-keritanized epithelium
What is Bowman’s membrane?
The basement membrane of the corneal epithelium
What are the stroma of the corneal epithelium?
Regularly arranged collagen deposits with no blood vessels
What are the three factors which maintain the transparency of the cornea?
Histologically - regular arrangement of collagen in the stroma
No blood vessels
Endothelium cell layer has a pump that actively keeps the aqueous humour out
What are the two categories of corneal pathology?
Inflammatory - e.g. corneal ulcers
Non-inflammatory - e.g. dystrophies
What is a common result of corneal pathology?
Opacification. May need to be treated by corneal transplant - Keratoplasty
Why is avascularity important to surgeons performing a corneal graft?
The lack of blood supply results in a lesser chance of foreign antigens from a donor graft being detected by the host immune system and therefore less chance of subsequent rejection. Cornea is “immune-privileged’
Describe the function of the iris
Controls the diameter of the pupils and therefore the amount of light rays entering the eyeball
Describe the function of the ciliary body
Suspends the lens and produces Aqueous Humour
Describe the function of the choroid
Supplies the blood to the outer layers of the retina
What are the key histological layers of the retina?
9, the nerve fibre layer
2, the layer of rods and cones
1, the pigment epithelial layer
What are the four quadrants of the retina?
Clockwise from top-left of the left eye; superonasal, superotemporal, inferotemporal, inferonasal
What is the optic disc?
The start of the optic nerve, a physiological blind spot
What is the fovea centralis?
A small, central pit composed of closely packed cones in the eye. It is located in the center of the retina. The high number of cones gives high visual acuity
Describe the lens
A transparent, crystalline biconvex structure which is suspended by zones from the ciliary body. It can change shape
What feature allows the lens to be transparent?
Avascularity
What is another name for lens opacification?
Cataract
What is a function of aqueous humour?
To maintain intraocular pressure
What is a function of vitreous humour?
To cushion the retina
Where is aqueous humour produced?
By the ciliary body, in the posterior chamber - the space behind the iris and in front of the lens
What can an obstruction along the pathway of aqueous humour cause?
Damming up of AH and therefore increased intraocular pressure i.e. Glaucoma
Describe the conjunctiva
Thin vascular membranes that cover the the inner surface of eyelids and loop back over the sclera. It does not cover the cornea
What are the two conjunctiva?
Bulbar - over the sclera
Palpebral - covering the innner surface of the eyelid
Where is the lacrimal gland situated?
Laterally in the orbit
How do tears reach the surface of the eye?
Through punctae on the medial side if each eyelid
Where do tears drain to from the eyelid?
The lacrimal sac which sits over the lacrimal bone, then draining through the nasolacrimal duct into the inferior meatus of the nasal cavity
What are the four functions of the tear film?
Keeps the cornea moist
Washes away particulate foreign bodies
Contains antibodies and lysosomes to kill microbes
Smooths outer surface the cornea providing smooth surface for refraction
What dye can be used to visualise the tear film?
Fluorescein
Describe the first (deepest) layer of the tear film
Mucinous layer which makes up around 30% of the tear film. Overlies the corneal epithelium
Describe the second (middle) layer of the tear film
The aqueous layer which makes up around 69% of the film. Overlies the mucinous layer
Describe the third (superficial) layer of the tear film
The oily layer (~1%)
How is aqueous humour filtered out of the anterior portion of the eye?
When it reaches the angle of the AC it is filtered out through a meshwork called the trabecular meshwork