VITAMIN A
of light reflected from
the eye, the
strength and type of
g la s s e s
or
c o n ta c t
le n s e s
needed to correct the visual de-
fect can be assessed objectively. Visual
acuity tests are then used to check that
the patient can see clearly using the new
lenses. Further m inor adjustments to
the lenses may be required.
VISUAL FIELD TESTS
Tests of the
v is u a l f ie ld
may be per-
formed to assess disorders of the eye,
such as
g la u c o m a
(increased pressure of
fluid w ithin the eyeball), or disorders
of the nervous system, such as a
s tro k e
.
v is u a l a c u ity
Sharpness of central
v is io n
.
Refractive
errors, such as
m y o p ia
(shortsighted-
ness),
h y p e rm e t ro p ia
(longsightedness),
and
a s tig m a t is m
(abnormal curvature of
the front of the eye), are the most com-
mon causes of poor visual acuity. Poor
visual acuity for near objects occurs in
the
condition
p re s b y o p ia
(age-related
deterioration in focusing power).
v is u a l fie ld
The total area in which visual perception
is possible while a person is looking
straight ahead. The visual fields normal-
ly extend outwards over an angle of
about
90
degrees on either side of the
midline of the face, but are more re-
stricted above and below, especially if
the eyes are deep-set or the eyebrows
are prominent. The visual fields of the
two eyes overlap to a large extent, giv-
ing binocular vision (see the illustrated
box). Partial loss of the visual field may
occur in
g la u c o m a
(raised pressure with-
in the eyeball) or
s t r o k e
.
v ita l s ig n
An indication that a person is still
alive. Vital signs include chest move-
ments that are caused by breathing, the
presence of a pulse, and the constric-
tion of the
p u p il
of the eye when it is
exposed to a bright light.
v ita m in
Any of a group of complex organic sub-
stances essential, in small amounts, for
the normal functioning of the body.
There are
13
vitamins: A, C, D, E, K, B12,
and seven grouped under the
v ita m in B
c o m p le x
.
Apart from vitamin D, w hich
the body can synthesize itself, vitamins
must be obtained from the diet. A varied
diet is likely to contain adequate amounts
of all the vitamins, but
v ita m in s u p p le -
m e n ts
may be helpful for: young chil-
dren, pregnant or breast-feeding women,
or those taking drugs that interfere with
vitamin function.
TYPES
Vitamins
can be
categorized as fat-
soluble or water-soluble.
Fat-soluble vitamins (A, D, E, and K)
are absorbed with fats from the intestine
into the bloodstream and are then stored
in fatty tissue
(mainly in the liver).
Because body reserves of some of these
vitamins last for several years, a daily
intake is not usually necessary. Defici-
ency of a fat-soluble vitamin is usually
due to a disorder in w hich intestinal
absorption of fats is impaired (see
m a l-
a b s o rp t io n
)
or to a prolonged poor diet.
Vitamins C, B12, and those of the B
complex are water-soluble. Vitamin C
and B complex vitamins can be stored
in the body in only limited amounts and
are excreted in the urine if taken in great-
er amounts than needed. A regular intake
is therefore essential to prevent deficiency.
However, vitamin B12 is stored in the
liver; these stores may last for years.
FUNCTION IN THE BODY
The role of all the vitamins in the body
is not fully understood. Most have sev-
eral important actions on one or more
body systems, and many are involved in
the activities of
e n z y m e s
.
(See also indi-
vidual vitamin entries;
R e fe re n c e N u t rie n t
In ta k e
table, overleaf.)
v ita m in A
A fat-soluble
v ita m in
vital for growth, for
bone and teeth formation, for cell struc-
THE VISUAL FIELDS
The field of vision of each eye (with the head and eyes immobile) extends through
an angle of 130 degrees and is divided into an area that overlaps with the visual field
of the other eye (binocular vision) and an area that can be seen only by one eye.
Route of visual signals
Note that all light from the fields left of the
centre of both eyes (grey) falls on the right sides
of the two retinas; and information about these
fields goesto the right visual cortex. Information
about the right fields of vision (blue) goes to
the left cortex. Data about the area of binocular
vision (visible to both eyes) go to both left and
right visual cortices.
V
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