VISION, DISORDERS OF
ing what proportion of the population
exposed to the microorganism develops
symptoms of disease, how rapidly the
infection spreads through the body, or
the mortality from the infection.
v ir u s e s
One of the smallest known types of
infectious agent. It is debatable whether
viruses are truly living organisms or
just collections of molecules capable of
self-replication under specific conditions.
Their sole activity is to invade the cells
of other organisms, w hich they then
take over to make copies of themselves.
Outside living cells, viruses are inert.
STRUCTURE
A single virus particle (virio n ) consists
of an inner core of
n u c le ic a cid ,
which
may be either
D N A
or
R N A
,
surrounded
by one or two protective protein shells
(capsids). Surrounding the outer capsid
may be another layer, the viral envelope,
w hich consists mainly of protein. The
nucleic acid consists of a string of
g e n e s
that contain coded instructions for mak-
ing copies of the virus (see
V ir u s e s a n d
d is e a s e
box, overleaf).
VIRAL DISEASES
Common viral conditions include the
common cold (see
c o ld , c o m m o n
) ,
in f lu -
e n z a
,
c h ic k e n p o x
,
c o ld s o r e s
,
and
w a rts
.
A ID S
is caused by the human immuno-
deficiency virus (see
H IV
) .
v is c e ra
A collective term used to describe the
internal organs.
v is c o s it y
The resistance to flow o f a fluid; its
“stickiness” .
The
viscosity
of blood
affects its ability to flow through small
vessels. An increased blood viscosity
increases the risk o f
t h ro m b o s is
(abnor-
mal blood clotting).
v is io n
The faculty o f sight. W hen light-rays
reach the eye, most of the focusing is
done by the
c o rn e a
,
but the eye also has
a c c o m m o d a t io n
,
an automatic fine-focus-
ing facility that operates by altering the
curvature of the
le n s
.
Together, these
systems form an image on the
re t in a
.
The light-sensitive rod and cone cells in
the retina convert the elements o f this
image into nerve impulses that pass into
the visual cortex o f the
b ra in
via the
o p t ic n e r v e s
.
The rods, w hich are more
concentrated at the periphery o f 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
c o lo u r v is io n
) .
The brain coordinates the motor nerve
impulses to the six 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 differ-
ent view o f a given object, the brain
obtains information that is interpreted
as solidity or depth. This stereoscopic
vision is important in judging distance.
(See also
T y p e s o f v is io n te st
box, over-
leaf;
T he s e n s e o f v is io n
box, p.
7 9 2
.)
v is io n , d is o rd e rs o f
The most common visual disorders are
refractive
errors,
such as
a s tig m a t is m
(the abnormal curvature of the front of
LOCATION AND STRUCTURE OF THE VERTEBRAE
The 33 vertebrae are arranged as shown. Apart from the top two, all have a similar
structure. The top cervical vertebra (atlas) has no body. The second (the axis) forms
a pivot on which the atlas can rotate, allowing the head to be turned in all directions.
THE SPINE
CERVICAL VERTEBRAE
THORACIC VERTEBRAE
LUMBAR VERTEBRAE
Arrangement
The vertebrae fall into five groups - cervical,
thoracic, lumbar, sacral, and coccygeal. The
top 24 are separated by discs of cartilage.
Structure
Three typical vertebrae are shown above. The foramen
in each is the channel through which the spinal cord
runs. The processes serve as muscle attachments.
V
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