00:01
Let's now move back and
look at the choroid.
00:06
I mentioned earlier it's
supports the retina.
00:09
Shown in this slide,
It's a very it's
a pigmented layer.
00:13
It's got pigment
with in the stroma,
but it's dominated by
capillary by blood vessels
these blood vessels also pass on
to the ciliary body
the ciliary processes
and you can see those blood
vessels when you examine
the internal
structure of the eye
with the ophthalmoscope.
00:33
That pigment and the pigmented
epithelium of the retina.
00:38
Help to stop
scattering of light around
the inside of the eye chamber
and therefore glare.
00:48
On the double pigmented layer
on the posterior aspect of the
IRS that absorbs light coming in.
00:55
And as I said if
it's reflected back,
it can determine the color
of your eyes depending on
the population of melanocytes
in the stroma,
but the choroid essentially
is a vascular layer.
01:08
It's important because as I
mentioned earlier in the lecture,
it provides nutrition
to the avascular retina,
particularly the very important
ganglion cells neural cells
and also the photoreceptors
and below you can see
it's continuous dealing
with the outer fibrous
coat of the eye the sclera.
01:30
On this slide, you can see
on the right hand side
the sclera colored green
and then there's
choreo capillary layer.
01:38
You can just see little red blood
vessels in this capillary layer,
and it's separated from
the pigmented epithelium and
the retina by this membrane
is a very thin faint membrane
that runs between the
pigmented epithelium
and the choroid.
01:54
It forms part of the barrier
the blood retinol barrier.
02:01
Well, let's look at the retina
and goodness me.
02:06
Look how complicated it is.
02:09
On the right hand
side is a picture
taken from a light
microscope of the retina
and all its layers supported
by the pigmented epithelium
and the choroid.
02:22
And that coincides with
a diagram in the center
and then the picture
in the center,
I should say and a diagram
on the left hand side.
02:30
I'm not going to go
through all these cells
and all the process of vision.
02:35
As I said earlier it see
the role I will leave to
the neurophysiologists
and the ophthalmologist,
but I'll just point
out a couple of things.
02:44
On the diagram,
focus on the rods and the cones
they're shaped differently.
02:52
They have outer segments
that are in contact with the
retinal pigment epithelium.
03:00
Those rods and
cones are in various
concentrations
around the retina.
03:05
Cones are specialized for
color, vision,
and rods for vision in dim
light and peripheral vision.
03:16
There is a part of the eye
the macular,
and in the macula,
which is about
four or five
millimeters in diameter.
03:26
There is a central part
of that macular called
the fovea centralis.
03:30
It's about one and a half maybe
two millimeters in diameter.
03:37
And within that the central
part of that is a foveola.
03:41
It's about 200
microns in diameter.
03:45
That's fully concentrated
by cones only
and that's where our
focus of light rays
is perceived to
be the strongest.
03:57
It's where we can perceive
the most detail of structures
and it's where
color is perceived.
04:05
It's a high concentration
as a set of cones
those cones are quite large.
04:10
They can be 80 to 60
microns in height,
the rods a little bit smaller.
04:19
And as I said those cones are
concentrated in that foveola,
there's about 4,000
of them there.
04:27
And they occupy
that space and also
if you look at this
diagram at the foveola
the central spot,
the retina is slightly
tilted on an angle
so that the light actually
hits the photoreceptor cells
without passing through
all the layers of the
ganglion cells and neural
cells supporting those cells
those photoreceptors.
04:53
In that area in the foveola,
there are 4,000 cones
as I pointed out
and the ratio of cones
to the ganglion cells as 1 to 1,
whereas elsewhere,
It's much higher than that.
05:07
There are in fact about a
hundred and twenty million rods.
05:12
And only about 5
to 7 million cones.
05:16
So about a 125 to 127
million photoreceptors.
05:22
There's only a million
ganglion cells.
05:25
So each gang and cell their for
sends impulses
from a combination of
these photoreceptors.
05:34
I want to just turn your attention
now to the pigment epithelium
that pigment epithelium is
a very important structure
that has a number of
different jobs to do.
05:44
The outer segments
of the rods and cones
consist of discs
containing the visual pigment
which are shared continually
and one job of these retinal
pigment epithelial cells
is to phagocytosis.
05:59
Those discs being shed.
06:01
And then to also
re photosynthesize
the visual pigment that
has been dissociated
in contact with light.
06:11
They also form a very important
blood retinol barrier.
06:17
So they're very very
important cells.
06:20
And as I said the pigment
helps to stop light from being
reflected back into the eye
and therefore creating glare.