Page 244 - Biology_F5

Coordination and Irritability

Refraction is greater for light from near ciliary muscle contracts, the tension on
objects than for distant objects. The the suspensory ligaments is reduced, and
refraction is achieved at the air-cornea the lens fattens due to its elastic nature
surface and at the lens when the light which increases the degree of refraction of
passes from one medium to another with a light. When the ciliary muscles relax, the
different refractive index. The function of suspensory ligaments are stretched, pulling
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the lens is to produce the final refraction the lens outwards, making it thinner, and
that brings light to a sharp focus on the decreasing the degree of light refraction.
retina. The lens is elastic and changes the Changing the shape of lens in different
shape by the contraction and relaxation manners causes the lens to focus light rays
of the ciliary muscle which encircles it. from near and distant objects on the retina.
This assists the lens in adjusting the light This process is called accommodation.
from both distant and near objects, which Light rays refraction in an eye at different
cannot be done by cornea. When the distances is shown in Table 4.1.

Table 4.1: Refraction of light rays in human eye at different distances

Light from a distant object Light from a nearby object
1. Parallel light rays reach the eye 1. Diverging light rays reach the eye
2. Cornea refracts (bends) light ray 2. Cornea refracts (bends) light ray
3. Circular ciliary muscle relaxes 3. Circular ciliary muscle contracts
4. Suspensory ligament stretch 4. Suspensory ligament slacks
5. The lens is pulled out 5. The lens becomes more convex
6. Light is focused on the retina 6. Light is focused on the retina

Cilliary muscles
Cilliary contract Loose
muscles Tight
relaxes ligaments Diverging rays ligaments
Parallel rays from a distant object from nearby object Image
Image

Object at infinity Thin Thick
eye Nearby eye
lens object lens

The structure of the retina neurones with synapse, connecting the
The retina is comprised of three layers photoreceptor layer and the inner layer.
of cells. The outermost layer is the Horizontal and amacrine cells are found in
photoreceptor layer containing rods the middle layer. The innermost layer is an
and cones, partially embedded in the internal surface layer containing ganglion
pigmented epithelial cells of the choroid cells with dendrites in contact with
layer. The rods and cones convert light bipolar neurones and axons of the optic
energy into the electrical energy of the nerve (Figure 4.29). It is important to note
nerve impulse. The middle layer is an that cats and some nocturnal carnivorous
intermediate part which contains bipolar mammals possess a reflective layer called

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