00:01
So let us look
at the lens now in more detail. On the left-hand
diagram, it is a large picture. You can
see the zonular fibre extending from
the ciliary epithelium of the ciliary
processes and then you see the structure
of the lens. It has got a capsule all
the way around it,
a clear transparent capsule and then
underneath that capsule is an anterior
epithelium, the lens epithelium. It is only
on the anterior surface. The posterior
surface does not have that epithelium and
those anterior lens epithelial cells
produce the lens fibres and those lens
fibres if you look on the right-hand
picture there you see at the equator that
epithelium produces the lens fibres
and then those fibres move towards the
centre of the lens and their nuclei as they
move towards the centre start to
disappear and finally end up as what we call
ghost nuclei. They are just little spaces that are
finally filled up. Now these lens fibres
to be anything up to 8 to 10
millimeters in length.
01:18
They are about 8 t 10 microns in width
and they are packed all together and as
they move towards the nucleus after
being constantly produced at the equator,
they tend to fuse together and its very
hard to separate and will see the different
fibres lined up. And as they move towards
the centre, they also accumulate the
protein crystallines. These lens fibres
are produced
regularly. They consist of type IV
collagen and is very elastic and that
elasticity changes with age.
02:00
It gets reduced as we get older. When you
are in the fourth decade of your life
suddenly the elasticity changes so the lens
can no longer accommodate, change
its shape as well as what it did in the
younger person. That is called presbyopia
and therefore, the person normally has to
wear glasses to accommodate for that, to assist
the lens to focus the light rays on the retina.
You can see here again the zonular fibres
extending from the ciliary processes to the
lens and this summarizes what
happens when the ciliary fibres are stretched
or put tension on or they relax
caused by the ciliary muscle. On the left-hand
side when the ciliary muscle
contracts, the ciliary body squeezes up
and moves closer towards the lens.
02:59
So the zonular fibres become more relaxed and
so the lens will flat out. It will become
wider. Conversely when the ciliary muscle is
relaxed, then the ciliary body moves
away from the lens and the fibres become
stretched or tensed and, therefore,
the lens get thinner change its shape. And
this is how the lens, therefore, can focus
light from distant objects or closer
objects regularly onto the surface of
the retina, the process of accommodation.