We might think of the eye as God’s camera. If we compare the eyeball to a camera, the retina of the eye would correspond to the photographic film (or the light-sensitive CCD in a digital camera). But unlike film or a CCD, the retina is a living sheet of brain cells and fibers, about the thickness of a sheet of paper.
Figure 1: Blood vessels cover both the front and back surfaces of the retina.
Since living cells need constant nourishment, the retina is supplied by a “sandwich” of blood vessels covering both its front and back surfaces (Figure 1). About 90–95% of the blood supply flows through choroid vessels behind the retina, outside of view, but about 5–10% of the blood supply flows through the retinal vessels in front of the retina (toward the lens and incoming light).
I’m Not Looking at You!
Everywhere we look, we must look through this network of blood vessels on the front of our retinas. Why then do we not notice these vessels in our field of view?
Research studies show that we perceive images only if they move on the surface of our retinas. A fixed image on the retina slowly disappears from the brain’s perception. As a result, we do not notice our retinal vessels because they are fixed to the surface of the retina and our brain simply ignores them.
So How Do We See All the Non-Moving Things Around Us?
If our brain is designed to ignore images that aren’t moving on our retinas, why can we see motionless objects in front of us, like a building? The reason is that the muscles of our eyes make small, rapid, involuntary movements so that everything in our visual field is in constant motion. These slight movements are unnoticeable but help the brain retain the image of any non-moving objects. The retinal blood vessels, of course, move with the eyeball and retina, so their image or shadow remains fixed and thus not noticed.
See for Yourself . . .
It is actually possible to see our own retinal blood vessels if we can make their shadow move on our retinas. You can do this in your own home, using basic materials.
- Stiff business card
- Paper punch
- Brightly illuminated white surface
- Use a paper punch to punch a small hole near the end of a business card.
- Hold the card close to one eye and look through the hole (close the other eye). Now rapidly move the card left and right about an inch while looking through the moving hole at a brightly illuminated white surface, such as a plain white computer screen or even a brightly illuminated white wall.
- If all is done correctly, you should notice silver/grey shadowy lines on the white surface. This is the pattern of your own retinal blood vessels!
- This simple activity works because the light shining through the hole in the card acts as a rapidly shifting light source. The moving light casts a moving shadow on the retinal vessels, just enough to make them visible.
Alternative Way to See Your Blood Vessels
There is another easy way to see the tiny blood vessels that cover your retina.
- Sit in a dark room.
- Cover one eye with your hand.
- Shine the flashlight in the outside corner of the open eye while you look at a blank wall.
- Jiggle the light rapidly (about twenty seconds) until you see a pattern of lines on the wall.
- You are seeing the same blood vessels that your eye doctor examines when he peers into your eyes with a special flashlight (called an ophthalmoscope). He is looking for signs of eye diseases and conditions affecting blood vessels.