5.16 - The Eye - Structure

The structure of the eye and its parts

• The eye is a sense organ containing receptor cells that are sensitive to light - to both its brightness (intensity) and its colour.
• The rest of the eye focuses the right amount of light onto these receptors so a clear image can form.

The main focusing parts

• Cornea - The transparent outer layer at the front. It refracts (bends) light as it enters and does most of the focusing.
• Iris - The coloured ring of muscle surrounding the pupil. By changing shape it controls the pupil's diameter, and so how much light gets in.
• Pupil - The hole in the middle of the iris that light passes through.
• Lens - Sits just behind the iris and pupil. It fine-tunes the focusing, bringing light to a sharp focus on the retina.
• Ciliary muscle - A ring of muscle that adjusts the lens's shape. It is attached to the lens by thread-like suspensory ligaments.

The retina and its receptor cells

• The retina lines the inside back of the eye and is the light-sensitive layer.
• It is covered in two kinds of receptor cell - rods and cones:
  1. Rods - Very sensitive to light, so work well in dim conditions (e.g. at night), but give only black-and-white information, no colour.
  2. Cones - Less sensitive, work best in brighter light, and are responsible for colour vision. There are three types of cone, each sensitive to light of a different colour.
• When light lands on these receptors, the information is turned into electrical impulses.
• The optic nerve carries these impulses from the retina to the brain, which interprets them as an image.

How rods and cones are spread across the retina

• The retina is not uniform - the two receptor types are spread differently:
  1. Rod cells - Mainly towards the edges (peripheral parts) of the retina, where their high sensitivity is good for seeing in dim light and detecting movement at the side of the field of view.
  2. Cone cells - Concentrated and packed tightly in a small central area called the fovea, which gives the sharpest, most detailed colour vision.
• This is why we turn the eye to look directly at something we want to see clearly.

Supporting and protective structures

• Sclera - The tough, white outer wall that gives the eye its shape and protects it. The cornea is the transparent part of this wall at the front.
• Conjunctiva - A thin, transparent layer covering the front surface, helping keep microorganisms out.
• Aqueous humour - Clear fluid in front of the lens.
• Vitreous humour - Clear fluid behind the lens.
  • Both humours help the eye keep its shape and let light pass straight through to the retina.
• Choroid - A dark, pigmented layer between the sclera and retina. It carries blood vessels that supply the retina and absorbs stray light, stopping it reflecting around inside so the retina forms a clearer image.
• Blind spot - The small area where the optic nerve leaves the eye. It has no receptor cells, so cannot detect light. It isn't usually noticed, because the brain fills in the missing part from the surrounding information and from the other eye.

The iris (pupil) reflex

• The amount of light entering the eye must be carefully controlled:
  • Too much light in very bright conditions could damage the light-sensitive cells of the retina.
  • Too little light in dim conditions would mean not enough reaches the retina to form a clear image.
• The eye adjusts automatically to the brightness of its surroundings through a reflex action that changes the size of the pupil - the hole through which light passes.

How the pupil responds to light

• Bright light - The reflex makes the pupil smaller (constricted), so less light gets into the eye and the retina is protected.
• Dim light - The pupil is made wider (dilated), so more light can enter and reach the retina.

Because this happens as an automatic reflex, the eye keeps the amount of light reaching the retina within a safe and useful range without any conscious effort.

The reflex can be investigated by photographing a person's eye in light of different brightness and measuring the diameter of the pupil on each photograph. Holding a ruler close to the eye to measure it is unsafe.