Family Doctor Books
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Published in association with the British Medical
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Family
Doctor Books |
Preview of Eyes: Cataracts, Glaucoma & Macular Degeneration
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Your eyes and how they work |
Your eyes are among the most highly
specialised and sensitive organs of your body. The eye, optic nerve and
brain work together to produce an image. To enable you to see, light
rays must pass through your cornea, pupil and lens to be focused on your
retina (see figure on page 8). An electrical signal is generated in the
retina and passed along the optic nerve to specialist parts of the brain
where the image is interp-reted. Vision is truly the king of all the
senses.
| This
illustration shows the principal features of the eye. The eyeball
is protected by the bony eye socket and cushioned by a layer
of fat. Each eyeball is roughly spherical. |
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The eyelids, orbit and tear film
Your eyes are protected by bony eye sockets (the orbits)
and are cushioned by a layer of fat. Your eyelids provide thick protective
coats with lashes that help to prevent foreign bodies such as dirt and
dust entering your eye. They also spread a special liquid (tears) over
the surface of your eyes at regular intervals. The tear film is produced
by the lacrimal gland, which lies slightly above your eyeball in the
outer upper part of your eye socket. Tears prevent the eyes from drying
out and protect them from infections. The tears are spread by the blinking
action of your eyelids and drain away into two canals (tear ducts) at
the inner part of your eyelids and then through a fine tube to your nose.
We
only notice tears when excess liquid is produced, for example, when we
cry. The tears drain away through your nose and this is one reason why
crying can block your nose. If the tears are reduced in volume or the quality
of the tear film is poor the eyes feel dry and uncomfortable. A large variety
of artificial tears is available at the chemist which can soothe dry eyes
but can never completely replace the natural tear film.
| Tears
are produced by the lacrimal gland. Tears prevent the eyes
from drying out and protect them from infection. Tears drain
away through the nose. |
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Eye movements
Your eyes swivel in their sockets using six delicate
muscles attached to the outside of each eye. These muscles control the
position of the eyes so accurately that, when you are reading a book,
they can pinpoint successive lines of text in less than one hundredth
of a second. The movement of your eye muscles is controlled by three
nerves that come directly from your brain (the third, fourth and sixth
cranial nerves). The front surface of the eye has a clear central portion
(the cornea) and the rest is covered by a waterproof protective layer
(the conjunctiva) which extends from the edges of the cornea to cover
around one-third of the eyeball. Beneath the conjunctiva lies the sclera,
a tough fibrous layer that provides the main structural wall of your
eyeball. The sclera and conjunctiva form the ‘white of your eye’.
Inside your eyes
The cornea is the clear central portion of the front
of your eye. It is strong and allows light to pass through it. The cornea
carries out most of the focusing of the eye. It refracts (bends) light
entering your eye onto the lens behind the pupil, which then fine-tunes
the focusing of the image so that it falls accurately on the retina at
the back of the eye. The iris – the coloured tissue around the pupil – is
made up of fine layers of muscle with the pupil as a central hole. The
pupil is highly sensitive to light, dilating in darkness, or when you
are excited, to allow more light into the eye, but quickly constricting
for protection if the eye is in bright light. The size of your pupil
is controlled by the muscles of the iris. Belladonna eye drops (derived
from the deadly nightshade plant and now called atropine) were used by
ladies in the court of Louis XIV of France to dilate (widen) their pupils
because they believed that it made them look more beautiful. Hence the
name ‘Bella Donna’ which means ‘beautiful lady’.
The colour of the iris, which determines the colour of your eyes, is
inherited from your parents or grandparents.
Each
eye swivels in the eye socket using six delicate and very precise
muscles attached to the outside of the eyeball. |
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The eye is made up of two fluid-filled chambers. The
front chamber, between the lens and the cornea, is filled with a liquid
called aqueous humour which bathes and nourishes this part of your eye.
The fluid circulates continually. It is produced in the region behind
the iris by the ciliary body. The fluid passes around the inside of the
eye and through the pupil to leave the eye mainly through a drainage
angle called the trabecular meshwork, which lies between the base of
the iris and the cornea.
| The
iris is made up of fine layers of coloured muscle with the
pupil as the central hole. The size of the pupil is controlled
by the muscles of the iris in response to the amount of light. |
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The lens lies behind the pupil and is suspended by
a series of very fine threads (the zonules). These threads can tighten
or loosen, under the control of a muscle (ciliary muscle) to which they
are attached, enabling the lens to become fatter or slimmer. The muscle
is circular and shaped like a tyre inner tube with the zonules attached
to the inside surface, so when the muscle contracts the size of the circle
becomes smaller and the zonules relax. The zonules are in turn attached
to the lens of the eye holding the lens in tension so that when the zonules
relax the lens can also relax and become fatter in shape increasing the
focusing power of the lens. This process is called ‘accommodation’ and
enables the eye to vary its focus from a distant object to one that is
closer. Thus, for looking at near objects (such as reading a book) the ‘ciliary
muscle’ as it is called contracts and allows the lens to become
fatter, therefore bending the light rays more and focusing the image
on the retina. Conversely, for viewing objects in the distance the ciliary
muscle will relax thereby tightening the zonules and stretching the lens,
which will then bend the light rays less and focus the light once again
on the retina. This process is demonstrated in the figure below.
The lens
in the eye is translucent and has no direct blood supply as it derives
the nutrients and oxygen it requires from the fluid in which it is bathed
(the aqueous humour). Clouding or opacities within the lens are termed
cataracts and these are discussed in more detail in the chapter on page
24. The lens is shaped like a Smartie sweet and has an outer ‘capsule’ and an inner ‘nucleus’ and ‘cortex’.
| Accommodation
is the process by which the eye can vary its focus from a distant
object to a close object. |
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The second fluid-filled chamber lies behind the lens.
It is large and filled with a clear jelly-like substance called the ‘vitreous
humour’. This is important in the formation of your eye in the uterus
(particularly the lens) but has no known significant function after birth.
In middle age and beyond the vitreous humour tends to shrink down and
lose some of its clarity, forming ‘floaters’ which are commonly
noticed in the vision particularly in bright light looking at a plain
light-coloured surface. The vitreous humour does not circulate like the
aqueous humour which passes around the outside of it.
The retina lines
the inside of the back of the eye. It is a wafer-thin layer containing
light-sensitive cells called photoreceptors (rods and cones). There are
about 130 million photoreceptors in each eye.
The rod photoreceptors (about
123 million) lie in the peripheral (outer) part of the retina and deal
with black and white vision. The rods are sensitive to low-intensity
light, but cannot differentiate between colours, which is why objects
appear to lose colour at night.
Colour vision is served by the cone photoreceptors,
which are fewer in number (about seven million) and work best in high-intensity
light. There are three types of cones; each type responds to a different
primary colour (red, blue and green). Cones are concentrated in the centre
of the retina (the macula). The macula specialises in detailed vision,
such as that used for reading and recognising faces.
Some people are born
with mild defects in one or more of the three kinds of cones and this
leads to colour blindness. For example, eight per cent of men are red/green
colour blind, being unable to distinguish clearly between red and green.
This can mean that they cannot clearly tell the difference between a
red and green traffic light, but can drive safely because they know that
the top light means stop and the bottom light means go.
Nerve fibres connect
the photoreceptors to the brain. The millions of nerve connections from
the retina collect together in the optic nerve; there are two optic nerves,
one for each of the eyes. At the base of the brain, the two optic nerves
join and then divide into separate channels (tracts). After further processing,
the nerves pass to the occipital cortex, a specialist part of the brain
at the back of the head that interprets the visual signals as images.
How you see
The visual system can be likened to two video cameras
(your eyes) connected to a computer (your brain) by connecting electrical
cables (the optic nerves). The cornea and lens form the focusing mechanism
of the camera. They focus light to form an image that is projected onto
the retina at the back of the eye. The photoreceptors in the retina then
translate the light energy into an electrical signal, which is transmitted
along the optic nerves to a specialist part of the brain (the visual
cortices). The picture on the retina is upside down and back to front.
However, your brain learns and becomes programmed from birth to interpret
this image the right way up. The visual cortices interpret the electrical
signals received from your eyes and translate these into the image that
you see in your mind. This immensely complicated process is only partially
understood.
| The
function of the components of the eye in vision. |
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Sight is such an important sense that a large proportion
of the brain is dedicated to interpreting what you see. Interestingly,
the left side of your brain deals with images from your right visual
field (in both eyes) and the right side of your brain with images from
your left visual field. Therefore, people who have a stroke affecting
the visual cortex on one side of their brain find that the opposite half
of their visual field is blanked out in both eyes.
| The
left side of the brain processes images from the right visual
field (in both eyes) and vice versa. |
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KEY POINTS
- Cataracts are common in elderly people but vision
can easily be restored with cataract surgery, which nowadays is a
straightforward and safe procedure
- Glaucoma is a silent condition which causes a
gradual loss of peripheral vision. Your optometrist/optician will
screen you for glaucoma when you have an eye test so that sight-saving
treatment can be commenced early
- Macular degeneration causes central or visual
loss but never causes blindness
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