Eye structure and function

The eye is the sense organ for sight. Its parts work together to focus light onto light-sensitive cells and to convert that light into nerve impulses the brain can interpret.

Parts and functions

How structure suits function

To focus on a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes more rounded, increasing its refractive power; to focus on a distant object, the ciliary muscles relax, the ligaments become taut, and the lens flattens. This ability to change shape, called accommodation, lets the eye form a sharp image at different distances.

The iris also adjusts to light intensity: in bright light its circular muscles contract to narrow the pupil and protect the retina, while in dim light its radial muscles contract to widen the pupil and let in more light.

An examiner marking a structure-to-function answer looks for a feature, a consequence and a link to vision. The adaptations below follow that pattern.

• The cornea is transparent and curved, so it lets light through and refracts it, providing most of the eye's fixed focusing power before the light reaches the lens.
• The lens is elastic and biconvex, so it can round up or flatten during accommodation to fine-focus light from near or far objects sharply onto the retina.
• The iris holds circular and radial muscles, so it can widen or narrow the pupil and control how much light reaches the retina, protecting it in bright light.
• The fovea is densely packed with cone cells, so it produces a sharp, colour image at the centre of the field of view where light is focused most precisely.
• The choroid is dark and rich in blood vessels, so it absorbs stray light to stop blurring by internal reflection and supplies the retina with oxygen and nutrients.

Related processes

The eye is the receptor in the reflex arc for sight. Light focused on the retina stimulates rod and cone cells, which generate nerve impulses; these travel along the optic nerve to the brain, where the image is interpreted.

This links the eye to the neuron and the brain studied in the same coordination chapter.

The pupil reflex is a rapid, involuntary response coordinated without conscious thought. A sudden increase in light intensity is detected by the retina; impulses pass to the brain and back to the iris muscles, which adjust the pupil size.

Because it protects the retina automatically, it is a good example of a reflex action alongside the knee-jerk and withdrawal reflexes.

Accommodation shows the antagonistic action of muscles: the ciliary muscle and the elastic lens work against each other so that the lens is pulled thin for distant vision and allowed to thicken for near vision. Sight defects such as short sight and long sight, and their correction with concave or convex lenses, extend directly from this section.

Common labelling errors

Worked question

Question. Diagram 1 shows a section through the human eye. Structure X is the transparent front layer, structure Y is a biconvex body behind the pupil, and region Z is the light-sensitive inner layer.

(a) Name structures X, Y and Z. (b) Describe how structure Y changes when a person looks up from a book to a distant tree.

(c) Explain how region Z sends information about the image to the brain. (d) A person moves from a dark room into bright sunlight.

Describe and explain the change in the pupil.

Model answer. (a) X is the cornea; Y is the lens; Z is the retina. (b) The ciliary muscles relax and the suspensory ligaments become taut, so the lens is pulled thinner and flatter; it refracts light less, which focuses the more parallel rays from the distant tree onto the retina (accommodation).

(c) Light focused on the retina stimulates receptor cells (rods and cones), which generate nerve impulses; these travel along the optic nerve to the brain, where the image is interpreted. (d) The circular muscles of the iris contract and the radial muscles relax, so the pupil becomes smaller; this reduces the amount of light entering the eye and protects the retina from damage.

This is a reflex action.

Source:SRC-DSKP-EN

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