The eye is
designed to bend (refract) light rays and bring them to a focus on the retina.
When we want to focus on near objects, tiny circular muscles around the lens
contract, making the lens fatter; when we focus on distant objects, another set
of muscles, radial muscles, contracts and pulls the lens flatter. All the
transparent elements of the eye - the conjunctiva, the cornea, the fluid-filled
chamber in front of the lens and the gelfilled chamber behind it - contribute
something to refraction but only the lens is capable of 'accommodation',
changing its focusing power.
In a person
who is short-sighted images of distant objects are brought to a focus just in
front of the retina. This happens if the eyeball is too long from back to front,
or if the ligaments which attach the radial muscles to the lens become slack. In
a long-sighted person, the opposite occurs; near objects are focused just behind
the retina, either because the eyeball is too short from back to front or
because the radial muscles become lazy. In either case, the result is a fuzzy
image. These and other refractive errors are relatively easy to correct.
The eyeball
consists of three distinct layers of tissue: a tough, opaque outer layer called
the sclera, in which the cornea is a transparent window; the choroid layer,
liberally supplied with tiny blood vessels and heavily
pigmented to
stop light escaping through the back of the eye or setting up reflections within
the eyeball; and the light-sensitive retina, literally a carpet of nerve fibres
with specialized endings. Pigments in these nerve endings (photoreceptors)
change their chemical composition in response to various wavelengths and
intensities of light, and as these changes take place
electrical
impulses are generated and transmitted to the optic nerve and then to that part
of the brain which makes sense of visual stimuli. Objects are upside down when
projected on to the retina, but 'seen' the right way up because certain fibres
in the optic nerve cross over before they reach the brain. The retina is easily
damaged - by leaking blood vessels in the choroid, for example, or by a build-up
of pressure within the eye.
In dim light,
which stimulates the 125 million or so rod receptors around the edge of the
retina, we see things in monochrome, in shades of black, white, and grey. In
bright light, which stimulates the 7 million or so cone receptors packed in the
central area of the retina, we see things in colour and with great sharpness.
There are three kinds of cones, sensitive to the red, blue, and green wavebands
of the visible spectrum; the marvellous variety of colours we see are the result
of
differential stimulation of these three kinds of
receptors.
The light sensitive pigment inside them, called rhodopsin, is replaced during
sleep, a process which requires Vitamin A. Whereas rod receptors, which contain
a different pigment, wear out every 2 weeks or so, cones remain functional for
9-12 months. At the end of their useful life, rods and cones are replaced, but
after the age of 40 cone replacement becomes less efficient, leading in severe
cases to macular degeneration.
The
pigmented, muscular iris controls the amount of light reaching the retina. In
dim light the iris aperture, the pupil, is large; in bright light it shrinks to
pinhead size. Iridologists diagnose many different ailments from the state of
the iris. Eye movement is
controlled by
three pairs of muscles originating from the bony orbit of the eye and attached
to the sclera. Under normal circumstances both eyes swivel in unison, receiving
two almost identical pictures of the world which we 'see' as one.
The
conjunctiva, a continuation of the epidermis, is continuously cleansed and
lubricated by salty, bactericidal fluid produced in the tear glands just above
the eye. If dust, bacteria, and irritants are not constantly removed, the
conjunctiva becomes scratched and sore, and sometimes infected. The duct which
drains away this cleansing fluid opens into the back of the nose. Small wonder
that our noses run when we start slicing onions! The fluid in the front chamber
of the eye nourishes both lens and cornea; it too is constantly replenished.
Blockage of the duct through which it drains can lead to glaucoma, a rise in
pressure inside the eyeball. |
Back to Ailments & Diseases
