(a) What is a sensory cell? (b) State one function each for the following parts of mammalian eye: (i) lens; (ii) vitreous humour; (iii) optic nerve. (c) (i)...

(a) Sensory cell

A sensory cell (receptor cell) is a specialised cell that is sensitive to a particular stimulus (such as light, sound, heat, touch or chemicals) and converts the stimulus into a nerve impulse which is sent to the central nervous system.

(b) Functions of parts of the eye

• (i) Lens: focuses (refracts) light rays onto the retina and, by changing its shape (accommodation), forms a clear image of both near and distant objects.
• (ii) Vitreous humour: the jelly-like fluid that fills the eyeball behind the lens; it keeps the eyeball firm and in shape and helps to refract light onto the retina.
• (iii) Optic nerve: carries nerve impulses from the retina to the brain, where the impulses are interpreted as sight.

(c)(i) Four structures that protect the eye

Eyelids, eyelashes, eyebrows, and tears (with the conjunctiva/bony eye socket also acceptable).

(c)(ii) How each performs its function

• Eyelids: close over the eye to shut out excess light and to prevent dust and foreign bodies from entering; blinking spreads tears over the eye.
• Eyelashes: hairs on the eyelids that trap dust and small particles, preventing them from reaching the surface of the eye.
• Eyebrows: hairs above the eye that prevent sweat and rain from running down into the eye.
• Tears (from the tear glands): wash away dust and contain an enzyme (lysozyme) that kills bacteria, keeping the eye clean and moist.

(d) Why a person is dazzled in bright sunlight

In a dark room the pupil is widely dilated (opened) to let in as much light as possible, and the light-sensitive pigment in the retina is fully built up. When the person is suddenly exposed to bright sunlight, a large amount of light floods into the eye through the still-wide pupil and over-stimulates the retina, so the person is dazzled and cannot see clearly. After a few seconds the circular muscles of the iris contract, the pupil becomes small (constricts) to reduce the amount of light entering, and the retina adjusts; normal vision then returns.