chapter 14 human eye Flashcards
function of eyelid
- Protect cornea from mechanical damage
- Carries out blinking: spreads tears over cornea and conjunctiva/wipes dust particles off the cornea
function of eyelashes
Shield the eyes from dust particles
function of tear duct
Carries tears produced in tear gland to eyes to wash away dust particles
Keep cornea moist for atmospheric oxygen to dissolve → dissolved oxygen diffuses in cornea
function of pupil
Hole in centre of iris - allows light to enter eye
function of iris
Controls amount of light entering eye by the 2 muscles it has: circular and radial muscles
Contains pigment that gives eye its colour
structure and function of conjunctiva
⭐ A thin transparent membrane continuous with skin of eyelid - covers inside of eyelid and covers sclera
Secretes mucus: helps to keep front of eyeball moist
structure and function of sclera (outermost layer of eyeball)
⭐ Continuous with cornea (‘white of the eye’) - covered by conjunctiva
Protects eyeball from mechanical damage
structure and function of cornea
⭐ Dome shaped transparent layer continuous with sclera
Refracts/bends light rays into eye
function of aqueous chamber
Space between lens and cornea
Filled with aqueous humour (a fluid): keeps front eyeball firm and helps to refracts light into pupil
function of retina (innermost layer of eyeball)
Light sensitive layer where images are formed
Contains photoreceptors: connected to nerve endings from optic nerve
function of fovea
Small yellow depression in retina - situated directly behind lens
1.Where images are usually focused
2. Has most cones but no rods = provides detailed colour vision
function of vitreous chamber
Space behind the lens
Contain vitreous humour (jellylike fluid): keeps eyeball firm and helps to refract light onto the retina
function of optic nerve
Nerve that transmits nerve impulses to brain from photoreceptors when stimulated
what are photoreceptors and its 2 types
detect changes (stimulus) near the eye = generate and send nerve impulses to brain by optic nerve
→ made up of rods and cones:
Cones: enable us to see colours in bright light = do not work well in dim light
Three types of cones: red, blue and green - contains a different pigment which absorbs light of different wavelengths
Rods: enable us to see in dim light but only in black and white
Contain the pigment visual purple: allows a person to see in the dark - gets bleached in bright light
Moving from a brightly lit to dimly lit room = takes a while for visual purple to reform from being bleached in bright light = takes a while to see in dimly lit room
describe pupil reflex in bright light (CCRR)
pathway: change in light intensity (stimulus) - photorecpetors in retina - sensory neurone in optic nerve - relay neurone in brain - motor neurone - radial and circular muscles of iris (effector)
actions by effector:
Circular muscles of iris contract
Radial muscles of iris relax
Pupil constricts/becomes smaller = diameter of pupil decreases
reduces amt of light entering eye = prevents excessive light from entering and damaging the retina
describe pupil reflex in dim light (CRRC)
pathway: change in light intensity (stimulus) - photorecpetors in retina - sensory neurone in optic nerve - relay neurone in brain - motor neurone - radial and circular muscles of iris (effector)
actions by effector:
Circular muscles of iris relax
Radial muscles of iris contract
Pupil dilates/enlarges = diameter of pupil increases
increases amt of light entering eye
how are images formed
- light rays are reflected off an object and fall on the eye:
- Refract through cornea and aqueous humour through pupil onto lens
- Lenses further refract light rays
- Lenses adjust thickness so that clear images of objects at different distances are formed on the retina = light rays brought to a focus on the retina
- Image on the retina stimulates the photoreceptors depending on the intensity of light of surroundings
Image formed on retina is inverted (upside down), laterally inverted (flipped right to left) and diminished (smaller) - to see a proper image: photoreceptors are stimulated = produce nerve impulses and transmitted via optic nerve to brain = brain interprets impulses = images are changed to right way up, front to back and right size
how do we focus on far objects
- light rays are almost parallel when they reach the eye = little refraction is needed = lens is thinner
- Ciliary muscles relax = pull on suspensory ligaments
- Suspensory ligaments become taut/tight = pull on edge of lens
- Lens become thinner and less convex = increase focal length
- Light rays from distant object are sharply focused on retina = stimulate photoreceptors
- Nerve impulses produced, transmitted by sensory neurone in optic nerve to relay neurone in brain = brain interprets impulses = person sees distant object
how do we focus on near objects
- light rays are diverging when they reach the eye = more refraction is needed = lens need to be more convex
- Ciliary muscles contract = relax pull on suspensory ligaments
- Suspensory ligaments slacken = relax pull on lens
- Lens become thicker and more convex = decrease focal length
- Light rays from near object are sharply focused on retina = stimulate photoreceptors
- Nerve impulses produced, transmitted by sensory neurone in optic nerve to relay neurone in brain = brain interprets impulses = person sees near object
pathway of blinking action
Blinking done by eyelids
Eyes are dry/foreign substance in eye/pain (stimulus) → Receptor (photoreceptors in retina) → sensory neurone (optic nerve) → relay neurone (brain) → motor neurone → effector (muscles of the eyelids) = blinks
function of choroid (middle layer of eyeball)
Pigmented black to prevent internal reflection of light - if transparent, light would exit the eye and images would be blurry
Contains blood vessels that bring oxygen and nutrients to eyeball and remove metabolic waste products
function of lens
Transparent, circular, biconvex and elastic structure
Changes its shape/thickness to focus light onto retina
