29 terms

The Human Eye

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What are sense organs?
Receptors that receive stimuli from the environment and inform the body of any changes in the environment
What is the orbit?
A hollow in the skull where the eyeball lies
Rectus muscle
How eyeball is attached to the skull
Cornea
- Continuous with the sclera (white of the eye)
- Specialised form of conjunctiva
- Refracts or bends light rays into the eye
Conjunctiva
- Thin and transparent membrane covering the sclera
- Mucous membrane, keeps front of eyeball moist
Iris
- Circular sheet of muscles
- Pigmented to give the eye its colour
- Controls the amount of light entering the eye
- Includes 2 sets of involuntary muscles: circular muscles & radial muscles
Pupil
- Hole in the centre of iris
- Allows light to enter the eye
Eyelids
- Protects cornea from mechanical damage
- Squinting: occurs when eyelid is partly closed
- Squinting prevents excessive light from entering eye, prevents damage to light-sensitive tissues
- Blinking spreads tears over cornea and conjunctiva
- Blinking wipes dust particles off the cornea
Eyelashes
- Shields eye from dust particles
Tear Gland
- Secretes tear to
1. Wash away dust particles
2. Keep cornea moist for atmospheric oxygen to dissolve
3. Lubricate conjunctiva, reduce friction when eyelids move
Sclera
- Tough, white outer covering of the eyeball
- Continuous with cornea
- Protects eyeball from mechanical damage
Choroid
- Middle layer of the eyeball (between Sclera and Retina)
- Pigmented black to prevent internal reflection of light
contains blood vessles that brings oxygen and nutrients to the eyeball and remove metabolic waste products
Ciliary Body
- Thickened region at the front end of the choroid
- Contains Ciliary muscles which controls curvature or thickness of the lens
Lens
- Transparent, circular and biconvex structure
- Elastic and changes its shape or thickness to refract light onto the retina
Suspensory Ligament
- Tissue that attaches the edge of the lens to the ciliary body
Aqueous Chamber
- Space between Lens and Cornea
- Filled with Aqueous humour
- Aqueous Humour keeps the front of eyeball firm and helps refract light into Pupil
Vitreous Chamber
- Space behind lens
- Filled with Vitreous humour
- Vitreous Humour keeps the eyeball firm and helps refract light into retina
Retina
- Innermost layer of th eyeball
- Light-sensitive layer on which images are formed
- Contains light-sensitive cells or photorecepters (rods and cones)
- Cones: Allows vision of colours in bright light
- Rods: Allows vision of black and white in dim light
- Photoreceptors are connected to nerve-endings from the optic nerve
Fovea (yellow spot)
- A small yellow depression in the retina
- Situated directly behind the lens
- Where images are focused
- Contains greatest concentration of cones but no rods
- Allows a person to have detailed colour vision in bright light
Optic nerve
- A nerve that transmits nerve impulses to the brain when photoreceptors are stimulated
Blind Spot
- Region where optic nerve leaves the eye
- Does not contain any rods or cones
- Not sensitive to light
Cones
- Allows vision of colours in bright light
- Do not work well in dim light
- 3 types: red, blue, green
- Each cone contains a different pigment which absorbs light of different wavelengths, collection of all cones' activity allows vision of wide variety of colours
Rods
- More sensitive to light than cones
- Contains visual purple (sensitive to light)
- Visual purple bleached when exposed to bright light (no image is formed, no nerve impulse will be sent to the brain)
- Formation requires vitamin A (deficiency causes night blindness)
Action of Iris
- Controls the size of the pupil, hence controlling the amount of light entering the eye
- Controlled by 2 sets of antagonistic muscles: Circular and radial
- In bright light: Circular muscles contract, Radial muscles relax, pupil constricts, reduce amount of light entering eye
- Dim light: Radial muscles contract, Circular muscles relax, Pupil dilates, Increase amount of light entering eye
Pupil Reflex
Normal reflex
Stimulus - receptor - sensory neurone - Brain - Motor neurone - effector
Vision
1) Light is refracted throught the cornea, aqueous humour and into the lens
2) Lens causes further refraction, light rays converge further to a focus on the retina (stimulation)
3) Image on retina stimulates activity of rods or cones, depending on intensity of light
4) Image formed on retina is upside down, laterally inverted, diminished
Focusing
- Adjustment of the lens of the eye so that the clear images of objects at differing distances are formed on the eye retina
Focusing on a more distant object (after glancing at a book)
1) Ciliary muscles relax
2) Suspensory ligaments contracts and becomes taut
3) Lens becomes thinner and less convex, increase focal length
Focusing on a near object (after glancing at a distant tree)
1) Ciliary muscles contract
2) Suspensory ligaments relaxes and slacken
3) Lens becomes thicker and more convex, decrease focal length