Neurology 12 - Structure and Function of the Eye Flashcards
Where does the eye sit?
- In the bony orbit
- Made of the sphenoid bone, frontal bone, zygomatic bone
List the parts of the eye
- The space between the eyelids is called the palpebral fissure
- The outer corner of the eyelid is called the lateral canthus
- Pupil, iris, sclera
- Medial canthus is the medial corner. It contains the caruncle (which forms the third eyelid in some animals)
- The dark outer ring between the sclera and the eyelid is the limbus (also where the corneal stem cells sit in)
List the components of the lacrimal system
- Lacrimal glands produces the largest component of tears
- Absorbed by the tear punctum (medial lid margin), canaliculi, tear sac and tear duct
- Exits into the nose cavity
Describe tear production
- Basal tears
- Reflex tears (in response to irritation)
What is the function of the tear film?
- Maintains smooth cornea air surface
- Provides oxygen supply
- Removal of debris (tear film and blinking)
- Bactericide
List the three layers of the tear film
- Superficial Oily Layer to reduce tear film evaporation (produced by a row of Meibomian Glands along the lid margins) – Aqueous Tear Film (Tear Gland) – Mucinous Layer on the Corneal Surface to maintain surface wetting
What is the conjunctiva of the eye?
- The conjunctiva is the thin, transparent tissue that covers the outer surface of
the eye. - It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids.
- It is nourished by tiny blood vessels that are nearly invisible to the naked eye.
What gives the eye its shape?
Vitreous humour
List the layers of the coat of the eye
- Sclera (hard and opaque)
- Choroid (pigmented and vascular)
- Retina (neurosensory tissue)
What is the diameter of the eye?
24mm
What is the sclera?
- The white of the eye
- Tough opaque tissue that serves as the eyes protective outer coat
- High water content
What is the cornea?
- A continuation of the sclera
- Covers the front of the eye, refracting surface providing 2/3 of the eyes focusing power. It has convex curvature
- Transparent
- Low water content
List the functions of the cornea
- Refraction (2/3)
- Physical barrier
- Infection barrier
Describe the structure of the cornea (5 layers)
- Epithelium
- Bowmans membrane
- Stroma (regularity contributes towards transparence - corneal nerve endings, provides sensation and nutrients for healthy tissue)
- Descements membrane
- Endothelium
What is the function of the endothelium of the cornea?
- 1 layer of cells
- No regeneration power and so decreases with age
- Endothelial cell dysfunction can result in oedema and cloudiness
What happens if the cornea is hydrated?
It will go white
What is the uvea? Describe its structure.
- Vascular coat of the eyeball, lies between the sclera and retina
- Composed of the iris, ciliary body and choroid
- These three parts are connected, so a disease of one part affects the other parts
What is the choroid?
- Between the retina and sclera (posterior part of the uvea)
- Composed of layers of blood vessels that nourish the back of the eye
What is the iris?
- The coloured part of the eye
- Controls light levels similar to the aperture on a camera
- Round opening in the centre is called the pupil
- Embedded with muscles that dilate and constrict pupil size
Describe the structure of the lens of the eye
- Outer acellular capsule
- Regular inner elongated cell fibres (transparent)
- May loose transparency with age (cataract)
List the functions of the lens of the eye
- Transparency (regular structure)
- Refractive power (1/3 power, higher refractive index than aqueous and vitreous fluid)
- Elasticity allows for accommodation
What are lens zonules?
- Suspends the lens
- A fibrous ring
- Passive connective tissue
- Connects the lens to the ciliary muscle
What is the result of contraction of the ciliary muscle?
Pushing - the lens contracts
What is the retina?
- Very thin layer of tissue that lines the inner part of the eye
- Responsible for capturing light rays that enter the eye
- Light impulses are then sent to the brain for processing via the optic nerve