Parts of the Eye

 

Here I will briefly describe various parts of the eye:

Scleraimage from http://www.ctlens.co.uk/problems.htm

The sclera is the white of the eye. "Don't shoot until you see their scleras."

The Cornea

The cornea is the clear bulging surface in front of the eye. It is the main refractive surface of the eye.

Anterior & Posterior Chambers

Iris/Pupil

Lens

The graph on the right shows the optical density (-log transmittance) of the lens as a function of wavelength. The curves show the change in density with age. More short wavelength light is blocked at increases ages.

Vitreous Humor

Retina

 

Notice the orientation of the retina in the eye. The center of the eyeball is towards the bottom of this figure and the back of the eyeball is towards the top. Light enters from the bottom in this figure.

The light has to pass through many layers of cells before finally reaching the photoreceptors. The photoreceptors are where the light is absorbed and and transformed into the electrochemical signals used by the nervous system. This change is called TRANSDUCTION.

The interior of the eyeball is the "inner" side and the exterior is the "outer" side. The nuclear layers contain cell bodies. The plexiform layers contain the connections between cells in the retina.

This next picture shows a schematic of the cells in the retina:

Again the light in entering from the bottom passing through all these layers before being absorbed in the receptors.

You can see the two types of receptors: the rod-shaped rods and the cone-shaped cones. The signal, after transduction, is passed to the horizontal cells (H) and the bipolar cells via a layer of connections. Lateral processing takes place in this layer via the horizontal cells. The throughput is transferred to another layer of connections with the amacrine cells (A) and the ganglion cells. The amacrine cells also exhibit lateral connections in this inner plexiform layer. The signals pass out of the eye via the ganglion cell axons which are bundled together to form the optic nerve.

The retina has a similar layered structure as the gray-matter top layers of the cerebral cortex of the brain. In fact, the retina is an extension of the central nervous system (the brain and spinal cord) that forms during embryonic development. This is one reason why scientists are interested in retinal processing; the retina is an accessible part of the brain that can be easily stimulated with light.

Speaking of the optic nerve...

The location where the optic nerve is bundled and leaves the retina is known as the optic disk. There are no photoreceptors at the location of the optic disk and hence there is a blind spot. The scientific term for a blind spot is a scotoma. So the blind spot due to the optic disk is a natural permanent scotoma in normal vision. Here is a demonstration of the natural permanent scotoma:

 

 

 

 

 

 

 

 

 

Close your left eye. Fixate on the cross with your right eye. This will cause the image of the cross to fall on your fovea. Adjust the viewing distance until the black spot disappears. When this happens, the image of the spot is falling on your blind spot.

What do you see (or not see) when you do this with the top figure?

What happens when the gap in the bottom figure falls on your blind spot?

You should see the "smiley" in the top figure disappear when it falls in your blind spot. When the gap in the bottom figure falls on the blind spot, the visual system "fills in" the line. So why don't we notice the blind spot in normal vision? For one, we have two eyes and the blind spots are in non-corresponding locations (they are nasally located (towards the nose) on the retina so the blind spots are temporal (towards the temple) in the visual field). In addition, the filling in process makes the blind spot less noticeable especially in a peripheral area of sight that has less visual acuity (the ability to see detail).

As mentioned above, in front of the receptors are layers of cells through which the light must pass. In addition there is vasculature on the front surface of the retina.

You can see this vasculature (or more correctly its shadow) by pressing a pen light to the side of your eyeball and gently wiggling it. What you will see looks like the figure below.

 

 

Why don't we see this regularly? As mentioned previously, the visual system is sensitive to change and when the light enters normally through the pupil, the vessels are stable. They are also small and narrow so they do not block much light however when illuminated from the side they cast a wider shadow.

If you look at a deep blue field or up at the sky (not the sun) on a clear day, you may notice pulsations or squiggles moving around. These are the shadows of the red corpuscles in the blood in these vessels.

The Fovea

The fovea is the location on the retina of central gaze. When you look directly, or fixate, at a stimulus you the retinal locus of this central fixation is the fovea. There are only cones in the human fovea (no rods). They are thinner, elongated, any very tightly packed. Because of this, the fovea is the location of highest visual acuity and best color vision.

In the diagram below you can see that the retinal layers are pulled aside (the axons of the receptors are elongated) leaving a clearer path for the light to reach the receptors. There is actually a little indentation or pit at the location of the fovea due to this and it is a clear landmark in the retina during an ophthalmic examination. The elongated outer segments of the cones (where the photopigment is and where the transduction occurs) increase the sensitivity by increasing the amount of photopigment. There is no vasculature in the central fovea.

The Macula

Covering the fovea is a pigment called the macula. it is thought that the macula serves as a protective filter over the foviea that absorbs blue and ultraviolet radiation. This pigment varies from observer to oberver and is a source of individual variation in color vision. Usually we do not notice the filtering of the macula but under special conditions we can notice its presence causing what is known as Maxwell's spot.

Here is a plot of the density of the macula as a function of wavelength:

To see Maxwell's spot try alternately viewing through a blue and yellow filter. When looking at through the blue filter after adapting through the yellow filter you may see a dark region covering approximately the central 3° of visual angle. Try it by clicking here. No guarantees.

The middle- and long- wavelength sensitive cones are selectively adapted to the yellow so that their response is attenuated while subsequently looking through the blue, thereby enhancing the visual effect of the macula.

Another demonstration of the macula is called Haidinger's Brushes.

Look at a uniform blue field (again the clear sky works well for this) through a linear polarizer. You may be able to see a small yellow hourglass in the central 3° area. As you change the orientation of the polarizer, the orientation of the hour glass changes.

To the right is an artists depiction of Haidinger's Brushes.

The Ophthalmoscope

OK, the ophthalmoscope is not a part of the eye...

If you want to see into someone's eye you have a problem. Your head will block the light entering the eye. Attributed to Helmholtz, the ophthalmoscope solves this problem by shining a small beam of light in to the eye. The reflected light is then available for viewing.

This is a schematic diagram showing how an ophthalmoscope works. An alternative is to use a half silvered mirror that covers the complete entrance area and allows half the light ener the eye and then allows half of the reflecting light to pass through the mirror into the observers eye.

In class, I try to borrow an ophthalmoscope so that the students can look into each other's eyes. Perhaps you can get hold of one or ask your physician or eye doctor to let you try it on him/her.

One other time that one sees the inside of the eye is when you get red-eye in a photograph. What you see here is the reflection off the retina of the rhodopsin, the pink colored photopigment in the rod photoreceptors.

 

Continue on to
Chapter 9