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Biology for Secondary Schools

Ciliary body is a circular structure that cones. Rods are very sensitive to light of
is an extension of the iris and it is the low intensity, and are used for vision in
coloured part of the eye. The ciliary body dim light while cones are sensitive to light
produces the fluid called aqueous humor. It of high intensity (bright light) and enable
also contains the ciliary muscles. When the us to see colours. Cones are highly packed
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eyes focus on an object they contract and together in a region called fovea which
relax, and they change the shape of the lens. increases the visual acuity. The fovea
centralis (yellow spot) is the region that
Pupil is an opening at the centre of the iris is directly opposite to the lens and is the
through which light enters the eye. The most sensitive part of the retina.
pupil changes in size as the iris changes
in shape when one is in dim or bright Blind spot is the area found in the retina
light. It appears black because light rays through which the optic nerves leave the
entering the pupil are absorbed by the eyeball. The blind spot is not light sensitive
black pigment of the choroid. because it does not contain rods or cones.

Lens is a biconvex transparent structure The eyeball contains two cavities; a large
behind the iris held in position by the posterior portion which is filled with
suspensory ligaments. The shape of the transparent jelly-like fluid called vitreous
lens is altered by the action of the ciliary humour and a small anterior portion filled
muscles. The process of changing the with watery fluid called aqueous humour.
lens shape is called focusing. The lens is The humours maintain the shape of the eye.
transparent; therefore, it allows light to Vitreous humour is replaced continuously.
pass through it. Both vitreous and aqueous humour refract
light and since they are transparent they
Retina is the inner layer of the allow light to pass through. Some parts of
eye. It contains light sensitive cells mammalian eye have similar functions as
(photoreceptors) which are the rods and the parts of a camera as shown in Table 2.3.

Table 2.3: Comparison between mammalian eye and camera

Part in mammalian eye Part in camera Function

Iris Diaphragm Regulate amount of light entering
Lens Convex lens Focuses light
Retina Sensitive film Formation of image

Choroid Black surface Prevent internal reflection of light.

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BIOLOGY FORM 3 FINAL EDITED 16.09.2025.indd 49 KAMISHNA 23/10/2025 11:52:37 KAMISHNA