THE EYE
The Structure of the Eye
Figure 1. Structure of eye
Iris - A circular sheet of muscles. It contains a pigment which gives the eye its color. The amount of light entering the eye is controlled by the two sets of involuntary muscles in the iris, the circular muscles and the radial muscles.
​
Conjunctiva - A thin transparent membrane covering the sclera (white of the eye) in front. It is a mucous membrane, that is, it secretes mucous, thus helping to keep the front of the eyeball moist. Continuous with the skin of the eyelids.
​
Sclera - A tough, white outer covering of the eyeball. It is continuous with the cornea. It protects the eyeball from mechanical damage.
Pupil - A hole in the centre of the iris. The pupil allows light to enter the eye.
​
Eyelashes - Help to shield the eye from dust particles
​
Tear gland - A gland at the corner of the upper eyelid. It secretes tears which: wash away dust particles, keep the cornea moist (for atmospheric moisture to dissolve), lubricate the conjunctiva (helps reduce friction when eyelids move).
​
Eyelids - They protect the cornea from mechanical damage, can be partly closed (AKA squinting - to prevent excessive sunlight from entering the eye and damaging the retina inside), and help spread tears over the cornea and conjunctiva while wiping dust particles off the cornea (through blinking).
Ciliary Body - A thickened region at the front end of the choroid. It contains ciliary muscles which controls the curvature or thickness of the lens.
​
Suspensory ligament - A connective tissue that attaches the edge of the lens to the ciliary body
​
Cornea - A dome shaped transparent layer continuous with the sclera or white of the eye. It refracts the light rays into the eye. The cornea causes most of the refraction of light that occurs in the eye.
​
Aqueous chamber - The space between the lens and the cornea. It is filled with aqueous humour, a transparent, watery fluid. Aqueous humor keeps the front of the eyeball firm and helps to refract light into the pupil.
​
Lens - A transparent, circular and biconvex structure. It is elastic and changes its shape or thickness in order to focus light onto the retina.
​
Choroid - The middle layer of the eyeball (between sclera and retina). It has two functions - pigmented black to prevent internal refraction of light and contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabollic waste products.
Figure 2. Vertical section of the eye
Vitreous Chamber - The space behind the lens. It is filled with vitreous humour, a transparent, jelly-like substance. Vitreous humour keeps the eyeball firm and helps to refract light onto the retina.
​
Fovea/yellow spot - A small yellow depression in the retina. It is situated directly behind the lens. This is where images are normally focused. The fovea contains the greatest concentration of cones, but no rods. The fovea enables a person to have detailed color vision in bright light.
​
Blind Spot - The region where the optic nerve leaves the eye. It does not contain any rods or cones, therefore is not sensitive to light.
Optic Nerve - A nerve that transmits nerve impulses to the brain when the photoreceptors in the retina are stimulated.
Retina - The innermost layer of the eyeball. It is the light-sensitive layer on which images are formed. It contains light sensitive cells or photoreceptors. Photoreceptors consist of rods and cones. Cones enable us to see colors in bright light while rods enable us to see in black and white in dim light. The photoreceptors are connected to the nerve endings from the optic nerve.
Figure 3. Actions of the circular and radial muscles
In order for a person to see clearly, only the right amount of light should enter the eye. For example, more light should enter the eye in dim light. The size of the pupil determines how much light enters the eye.
​
The size of the pupil is controlled by the two sets of involuntary antagonistic muscles in the iris.
​
The pupil reflex is when a pupil changes size as a result of changes in light intensity. In any reflex action there must be a receptor and effector. In this case the receptor is the retina, and the effector is the iris.
Photoreceptors
Photoreceptors in the retina consist of rods and cones. They are connected to the nerve endings from the optic nerve.
Cones enable us to see colors in bright light. There are three types of cones: red, blue and green cones. Each cone contains a different pigment which absorbs light of different wavelengths. All the cones together enable us to see a wide variety of colors.
Cones do not work well in dim light.
​
Rods are more sensitive to light than cones. They enable us to see in dim light, but only in black and white. Rods are sensitive to light of low intensity because they contain a pigment called visual purple. When the eye is exposed to bright light, all the visual purple is bleached. Visual purple must be re-formed for a person to see in the dark (this is why it takes a while to get used to the surroundings in the dark). The formation of visual purple requires Vitamin A. Therefore a person deficient in Vitamin A may be unable to see in dim light/ suffer from night blindness.
How we see
When light falls on an object, rays of light are reflected from the object. Some of these reflected rays fall on the eye.
1.The light rays are refracted through the cornea and the aqueous humour onto the lens.
2. The lens causes further refraction and the rays are brought to a focus on the retina.
3. The image on the retina stimulates either the rods or the cones, depending on light intensity.
4. The image formed on the retina is:
- Upside down (inverted)
- Laterally inverted
- Diminished (smaller in size than the actual object)
5. Nerve impulses are produced when light falls on the rods and cones. These impulses are transmitted via the optic nerve to the brain. The brain interprets these impulses so we see the object the right way up, front to back, and the right size.
Figure 4. Figure showing how lens focuses objects from different distances
Focusing is the adjustment of the lens of the eye so that clear images of different objects at different distances are formed on the retina. In focusing, the thickness of the lens is adjusted (Refer to Figure 4).
​
When focusing on a distant object, the lens becomes thin.
When focusing on a nearby object, the lens thickens.
​
Short sightedness - Causes distant objects to appear blurred, while close objects can be seen clearly. This happens when the light focuses in front of the retina instead of on it. Wearing concave lenses helps correct the problem.
Long sightedness - Causes close objects to appear blurred, while distant objects can be seen clearly. This happens when the light focuses behind the retina instead of on it. Wearing convex lenses helps correct the problem.
​
Figure 5. Short-sightedness
Figure 6. Long-sightedness