VISCERA
VISION, LOSS OF
VIRUSES
(genetic material)
envelope
STRUCTURE OF A VIRUS
by
1
or
2
protective protein shells (cap-
sids). Surrounding the outer capsid may
be another layer, the viral envelope,
which consists mainly of protein. The
nucleic acid consists of a string of
genes
that contain coded instructions for
making copies of the virus.
Common viral diseases include the
common
cold, influenza,
and
chickenpox
(caused by the varicella-zoster virus).
AIDS
is caused by the human immuno-
deficiency virus (
HIV
).
viscera A
collective
term
used
to
describe the internal organs.
viscosity The resistance to flow of a fluid;
its “stickiness”. The viscosity of blood
affects its ability to flow through small
vessels. An increase in the viscosity of
blood increases the risk of
thrombosis.
vision The faculty of sight. When light-
rays reach the
eye
, most of the focusing
is done by the
cornea
, but the eye also
has an automatic fine-focusing facility,
accommodation
, that operates by altering
the curvature of the
lens
. Together, these
systems form an image on the
retina.
The light-sensitive rod and cone cells in
the retina convert the elements of this
image into nerve impulses that pass
into the visual cortex of the
brain
via
the
optic nerves
. The rods, which are
more concentrated at the periphery of
the retina, are highly sensitive to light
but not to colour. The colour-sensitive
cones are concentrated more at the
centre of the retina (see
colour vision
).
The brain coordinates the motor nerve
impulses to the
6
tiny muscles that
move each eye to achieve alignment of
the eyes. Accurate alignment allows the
brain to fuse the images from each eye,
but because each eye has a slightly dif-
ferent view of a given object, the brain
obtains information that is interpreted
as solidity or depth. This stereoscopic
vision is important in judging distance.
vision, disorders of The most com-
mon visual
disorders are
refractive
errors, such as
myopia
,
hypermetropia
,
and
astigmatism
, which
can
almost
always be corrected by
glasses
or
con-
tact lenses.
Other disorders include
amblyopia
;
double vision
; and disorders
of the
eye
or
optic nerve
, of the nerve
pathways connecting the optic nerves
to the
brain
, and of the brain itself.
The eye may lose its transparency
through corneal opacities,
cataract
, or
vitreous haemorrhage.
Defects near the
centre of the retina cause loss of the
corresponding parts of the
visual field
(see
macular degeneration
).
Floaters
,
which are usually insignificant, may in-
dicate a
retinal tear
or haemorrhage, or
they may herald a
retinal detachment.
Optic neuritis
can cause a blind spot in
the centre of the visual field.
Damage to the brain (for example, from
a
stroke
) may cause visual impairment
such as
hemianopia
,
agnosia
, visual pers-
everation (in which a scene continues
to be perceived after the direction of gaze
has shifted), and visual hallucinations.
vision, loss of Inability to see. This may
develop slowly or suddenly and may be
temporary or permanent, depending on
the cause. Vision loss may affect 1 or
both eyes. It can cause complete
blind-
ness
or may affect only peripheral, or
only central, vision.
Progressive loss of visual clarity is
common with advancing age and may
be due to a number of disorders (see
vision, disorders of
).
Sudden loss of vision may be caused
by disorders such as
hyphaema
, severe
uveitis
, v
itreous haemorrhage
, or
retinal
haemorrhage
.
Optic neuritis
can reduce
vision in 1 eye. Damage to the nerve
V
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