Bma Illustrated Medical Dictionary

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).
AIDSis caused by the human immuno-
deficiency virus (HIV).
visceraA collective term used to
describe the internal organs.
viscosityThe 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.
visionThe 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 brainvia
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 ofThe most com-

mon visual disorders are refractive

errors, such as myopia, hypermetropia,

and astigmatism, which can almost

always be corrected by glassesor con-

tact lenses. Other disorders include

amblyopia; double vision; and disorders

of the eyeor 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 tearor haemorrhage, or

they may herald a retinal detachment.

Optic neuritiscan 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 ofInability 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-

nessor 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, vitreous haemorrhage, or retinal

haemorrhage. Optic neuritiscan reduce

vision in 1 eye. Damage to the nerve

VISCERA VISION, LOSS OF

V

Surface
protein

Protein shell

Nucleic acid
(genetic material)

VIRUSES

Viral

envelope

STRUCTURE OF A VIRUS