eye Flashcards
what makes up the external structure of the eye
sclera, conjunctiva, eyelashes, tear gland, eyelids, iris, pupil
what is the sclera
tough, white outer covering of eyeball that protects the eye from mechanical damage
what is the conjunctiva
thin, transparent membrane covering the sclera and secretes mucus to keep the eye moist
what is the iris
a circular sheet of muscles that controls the size of the pupils and amount of light entering the eye
what is the pupil
hole in the centre of the eye that allows light to enter
function of the eyelashes
shields eyes from dust particles
function of tear gland
secretes tears that wash away dust particles, keep cornea moist for oxygen to diffuse into, lubricate conjunctiva to reduce friction when eyelids move
function of eyelids
protect cornea from mechanical damage, blinking spreads tears over cornea and conjunctiva, wipes dust particles off cornea
what makes up the internal structure of the eye
- retina, lens, blind spot, optic nerve, fovea, vitreous chamber, choroid, ciliary body, suspensory ligament, cornea, aqueous humour
what is the retina
the innermost, light-sensitive layer of the eyeball on which images are formed. contains photoreceptors like rods and cones
what are the lens
transparent, circular, biconvex structure that changes shape or thickness to focus light onto the retina
what is the blind spot
region where optic nerve leaves the eye that does not contain rods or cones
what is the fovea
small yellow depression in the retina directly behind the lens and allows detailed colour vision in bright light. area where images are normally focused, contains highest concentration of cones but no rods
what is the vitreous chamber
space between lens filled with vitreous humour which keeps the eyeball firm and helps refract light onto the retina
what is the choroid
middle layer of eyeball, pigmented black to prevent internal reflection of light, contains blood vessels that bring oxygen and nutrients and remove metabolic waste products
what is the ciliary body
thick region at front of choroid, contains ciliary muscles that control thickness and curvature of lens
what is the suspensory ligament
connective tissue attaching edge of lens to ciliary body
what is the cornea
dome shaped transparent layer at the front of the eye that refracts light rays into the eye
what is aqueous chamber
space between lens and cornea that is filled with aqueous humour to keep the front of the eyeball firm and refract light into the pupil
function of cones and rods
cones: enable us to see colours in bright light
rods: enable us to see in black and white in dim light
what is pupil reflex
automatic adjustment of size of pupil when light intensity changes to ensure the right amount of light enters the eye
how does pupil reflex work
- when light intensity is high, circular muscles contract and radial muscles relax, causing pupils to constrict and less light to enter the eye
- when light intensity is low, radial muscles contract and circular muscles relax, causing pupil to dilate and more light to enter eye
benefits of pupil reflex
- automatic and immediate
- prevents excessive light from damaging retina
- allows enough light to enter eye for us to see
nervous pathway in pupil reflex
- light ray is refracted onto retina
- receptors in retina are stimulated
- sensory neurone in optic nerve transmits nerve impulses to brain
- nerve impulse transmitted to relay neurone in brain
- motor neurone transmits nerve impulses from brain to iris
- iris muscles contract and relax accordingly depending on light intensity
how do we see
- light rays from an object are refracted through the cornea and aqueous humour onto the lens
- lens cause further refraction and rays are brought to focus on the retina
- image on the retina stimulates either rods or cones, and is upside down, laterally inverted and diminished
- nerve impulses are produced and transmitted to the brain where they are interpreted such that we see the object right side up, front to back and the right size
what is focusing / accommodation
adjustment of the thickness and curvature of the lens so that clear images of objects are formed on the retina
process of focusing on distant object (>7m)
since object is far away, reflected light rays that reach the eye are almost parallel to each other
1. ciliary muscles relax, pulling on suspensory ligaments
2. suspensory ligaments become taut, pulling on the edge of the lens
3. lens become thinner and less convex
4. light rays from object are sharply focused on the retina
5. photoreceptors are stimulated
6. nerve impulses are produced and transmitted and brain interprets image correctly
process of focusing on nearby object
when looking at near object, light rays reflected are diverging and lens need to be thicker to bend them more
1. ciliary muscles contract, relaxing pull on suspensory ligaments
2. suspensory ligaments slacken, relaxing pull on lens
3. elastic lens become thicker and more convex
4. light rays from nearby object are sharply focused onto retina
5. photoreceptors are stimulated
6. nerve impulses are produced and transmitted to the brain and interpreted correctly
