Sense Organs: Structure and Function of the Eye

The eye is the sense organ for vision: it focuses light from objects onto the retina, where light-sensitive cells convert it into nerve impulses that the brain interprets as an image. In brief: light enters through the cornea and pupil, is focused by the cornea and lens onto the retina, and the optic nerve carries the signals to the brain. The iris controls the size of the pupil, and the ciliary muscles change the shape of the lens to focus on near or distant objects.

Sense organs contain receptors that detect the stimuli from our surroundings, and the eye is the most studied of them. This guide gives an overview of the sense organs, describes each part of the eye and its function, explains how pupil size and focusing are controlled, covers common vision problems, works through an example of accommodation and corrects common mistakes.

What are sense organs?

Sense organs are organs that contain receptors for particular stimuli. The five traditional senses and their organs are vision (eyes), hearing and balance (ears), smell (nose), taste (tongue) and touch (skin). Humans also have senses of temperature, pain and body position. Each sense organ converts a stimulus into nerve impulses that travel to a specific region of the brain, where they are interpreted.

What is the structure of the eye?

The human eye is a nearly spherical organ, about 2.4 centimeters in diameter, set in a bony socket and moved by six muscles. Its wall has three layers: the tough outer sclera (the white of the eye) with the transparent cornea at the front, the middle choroid layer with blood vessels, and the inner retina.

• Cornea: the transparent front window. It does most of the bending (refraction) of light, about two thirds of the eye’s focusing power.
• Aqueous humor: the watery fluid between the cornea and the lens that nourishes them and keeps the eye’s shape.
• Iris: the colored ring of muscle that controls how much light enters.
• Pupil: the opening in the center of the iris.
• Lens: a transparent, flexible, biconvex structure that fine-tunes the focus.
• Ciliary body and suspensory ligaments: the ciliary muscle and the fibers that hold the lens and change its shape.
• Vitreous humor: the clear jelly that fills the large space behind the lens and holds the retina in place.
• Retina: the inner layer containing the photoreceptors.
• Fovea: a small pit in the retina with a high density of cones, where vision is sharpest.
• Optic nerve: carries impulses to the brain. The place where it leaves the retina has no receptors and is called the blind spot.

Parts of the eye and their functions

How do we see?

Light reflected from an object passes through the cornea, aqueous humor, pupil, lens and vitreous humor, and forms a small, upside-down image on the retina. The retina contains two kinds of photoreceptor.

• Rods (about 120 million) work in dim light and detect brightness but not color. They are most numerous around the edges of the retina and give us night vision. They contain the pigment rhodopsin.
• Cones (about 6 million) work in bright light and give color vision and sharp detail. There are three types, sensitive to red, green and blue light, and they are concentrated in the fovea. Color blindness is usually caused by a missing or altered type of cone.

When light hits a photoreceptor, it changes the shape of a pigment molecule, which starts a chain of events that changes the electrical signal. Bipolar cells and ganglion cells in the retina process the signal, and the axons of the ganglion cells form the optic nerve. The signals go to the visual cortex in the occipital lobe, where the brain interprets them and turns the upside-down image the right way up. The two optic nerves partially cross, which helps to produce binocular vision and depth perception.

How is light and focus controlled?

Pupil reflex

In bright light, the circular muscles of the iris contract and the radial muscles relax, so the pupil gets smaller and less light enters. In dim light, the radial muscles contract and the circular muscles relax, so the pupil widens. This is a reflex controlled by the autonomic nervous system, and it protects the retina and improves vision in different conditions.

Accommodation (focusing)

The lens changes shape to focus on objects at different distances. This is called accommodation.

• Distant object: the ciliary muscles relax, the suspensory ligaments are pulled tight, the lens is pulled flat (thin) and has less refractive power.
• Near object: the ciliary muscles contract, the tension on the ligaments is reduced, and the elastic lens becomes more rounded (thicker) and bends light more.

With age, the lens becomes less flexible, and most people over 45 need reading glasses (presbyopia).

What are common vision problems?

• Short-sightedness (myopia): distant objects are blurred, because the eyeball is too long or the lens is too curved, and the image forms in front of the retina. It is corrected with a concave (diverging) lens.
• Long-sightedness (hyperopia): near objects are blurred, because the eyeball is too short, and the image forms behind the retina. It is corrected with a convex (converging) lens.
• Cataract: clouding of the lens, usually with age; treated by replacing the lens with an artificial one.
• Glaucoma: damage to the optic nerve, often from high pressure in the eye.

Worked example: looking from far to near

You are looking at a mountain and then look at your phone. Explain each change.

1. Looking at the mountain: light rays are almost parallel when they arrive. The ciliary muscles are relaxed, the ligaments are taut and the lens is thin. The image is focused on the retina.
2. Looking at the phone: rays diverge more from a close object and need more bending. The ciliary muscles contract, tension in the ligaments decreases, and the lens becomes thicker, which increases its power.
3. Reading in bright light also makes the pupils narrower, which increases the depth of focus.

As a numerical check, the power of a lens in dioptres is 1 divided by the focal length in meters. To focus an object at 25 cm, the eye needs an extra 1 ÷ 0.25 = 4 dioptres compared with an object at a large distance. A young person can usually add 10 dioptres or more, so reading is easy, but the available range falls with age, to about 1 dioptre at age 60, which is why reading glasses are then needed. The values are typical and vary between individuals.

How was vision explained?

Ibn al-Haytham (Alhazen) showed around 1000 AD that we see because light enters the eye, and not because the eye sends out rays, as some Greek thinkers had proposed. Johannes Kepler explained in 1604 that the lens focuses an image on the retina. In the twentieth century, George Wald and others worked out the chemistry of rhodopsin, and Wald shared the 1967 Nobel Prize in Physiology or Medicine.

Which mistakes are common?

Mistake 1: Saying that the lens does most of the focusing

The cornea does most of the bending of light. The lens fine-tunes the focus.

Mistake 2: Mixing up the ciliary muscle action

For near objects, the ciliary muscles contract and the lens becomes thicker. For distant objects, they relax.

Mistake 3: Thinking that the image on the retina is upright

It is upside down, and the brain corrects it.

Mistake 4: Confusing rods and cones

Rods work in dim light, without color. Cones need brighter light and give color and detail.