Anatomy of the Eye

posted on 8 May 2013 by guy
last changed 25 May 2016

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ages: 6 to 99 yrs
budget: $0.00 to $5.00
prep time: 0 to 15 min
class time: 15 to 240 min

This lesson provides an overview of the anatomy of the eye and suggests several simple activites for exploration, including

  • observing pupil response to variations in light
  • verifying the inverted image on the retina
  • comparing central and peripheral vision
  • watching floaters and live blood cells in the vitreous humor

While detailed anatomical descriptions probably don't work well with younger students, many of the suggested activities can be enjoyed by 6-year-olds. Images are included in the attached file for help with a lecture.

 

subjects: Biology
keywords: eye, vision, retina, anatomy, Purkinje image, blind spot

file attachment(s): 



Fig. 1: Overhead diagram of the right eye. By Sathiyamoorthy, modified by ZStardust at Wikimedia Commons.


Fig. 2: Video illustrating eye dilation in response to varying light levels. By Greyson Orlando via Wikimedia Commons.


Fig. 3: Photograph of a human retina. The dark spot in the center of the image is the fovea. The optic nerve connects in the bright area on the right, where the blood vessels converge. Image made available by Alexander Churkin via Wikimedia Commons.


Fig. 4: Diagram of a pinhole camera, showing how the inverted image is formed on the screen. A light ray from the bottom of the tree travels through the pinhole to the top of the viewing screen, while a light ray from the top of the tree travels through the pinhole to the bottom of the screen. The arrangement of the human eye is similar.


Fig. 5: Schematic of layers in the retina. The back of the retina is at the top of the diagram. Adapted from work made available by Peter Hartmann and Marc Gabriel Schmid at Wikimedia Commons.

overview

The human eye is a remarkable device, delicately constructed from a number of complex pieces. Figure 1 shows an overhead schematic of a right eye, which identifies the main parts. The hard white outside layer covering most of the eye is called the sclera, after the greek skleros, which means 'hard'. In the front of the eye is a transparent bulge called the cornea which provides most of the focusing power. Behind the cornea is the anterior chamber, filled with a transparent gelatinous substance called the aqueous humor. The eyelens, which handles the variable focusing of the eye, is behind the anterior chamber.

Just in front of the eyelens, the iris controls the amount of light entering the eye by expanding and contracting in response to light levels, thereby changing the size of the pupil, the opening through which the light passes. Figure 2 shows a video of a pupil responding to changing light levels in the room. Over its full range, the pupil can change diameter by about a factor of 4, thereby controlling the amount of light entering the eye by a factor of 16.

activity - pupil adjustment
The iris responds to light by expanding or contracting in order to control the amount of light entering the eye. As the light intensity increases, the iris closes down, reducing the size of the pupil. You can observe this phenomenon in friend's eye if you put your friend in a dim room and shine a small flashlight in one eye. You should see a pupil response like the one shown in figure 2. You may notice that if you shine the flashlight only in one eye, you still see both pupils contract. This response is a result of the wiring of the visual signal: the light stimulus from one eye is fed to both sides of the brain, which in turn control the irises in both eyes.

The eyelens itself is a transparent, flexible object constructed of many layers, like an onion. The shape of the lens is controlled by the cillary muscle, which changes the tension on the suspensory ligaments attached to the eyelens, and cause the lens to be pulled flatter when the ligaments are taught, or relax into a rounder shape when the ligaments are slack. When the cillary muscle contracts, it closes down on the eyelens, releasing the tension on the ligaments and allowing the lens to assume a rounder shape, which focuses the eye at close distance. When the cillary muscle relaxes and opens, it increases tension on the ligaments, making the lens flatter, which helps the eye focus at far distances. The reason we feel eyestrain after extended periods of close-up work is because the cillary muscle gets tired from long periods of contraction.

Behind the eyelens is a chamber filled with another gelatinous substance called the vitreous humor. Light from the pupil travels through this region to strike the retina at the back of the eye (figure 3). The retina acts as a projection screen, much like the projection screen in a pinhole camera, where an inverted image of the outside world is displayed (see figure 4).

activity - inverted image
It's easy to verify that the image on the retina is inverted. While staring at a bright, uniform scene, press very gently on the outside corner of one eyelid. Don't touch the eyeball directly. As you apply pressure to the retina, you cut off the blood supply to that portion you are touching and wipe out the view for that region. As you push (gently!) on the outside of your eye, you will notice a blank spot appear in the view near your nose. The scene from the nose side of your view is casting an image on the outside of your retina, as you expect for the inverted image.

The retina contains several layers of specialized cells for the absorption of light and transmission of signals to the brain (see figure 5). At the back of the retina are the photoreceptors, which absorb the light and generate chemical signals that are passed to (nearly transparent) layers of nerve cells in front. The signals are processed as they pass through the retinal layers (see the lesson on Shades of Inhibition for more details about retinal structure) until they reach the ganglion nerve cells at the front of the retina, which connect to the brain through the optic nerve.

There are two types of photoreceptors in the human retina: cones, which are responsible for high resolution color imaging, and rods which are responsible for low light sensitivity. A human retina contains about 120 million cones, about 7 million rods, and about 1 million ganglion cells. It follows that, on average, a ganglion cell connects to about 100 different photoreceptors. However, the connections are not uniform throughout the retina. The center of the retina contains the fovea (dark spot in figure 3) where there are no rods, only cones. In this region, only a few cones connect to each ganglion cell, giving the most acute vision. The periphery of the retina is dominated mostly by rods, with very few cones, thousands of which may be connected to each ganglion cell. Where the optic nerve connects to the retina (bright spot on the right in figure 3) there are no photoreceptors at all. This region produces the "blind spot".

activity - fovea vs periphery (acuity & color)
In order to observe the difference in perception between the fovea and the periphery of the retina, have a friend sit in a chair and stare straight ahead. You may want to have an assistant sitting in front to make sure the subject does not cheat and move his eyes. Hold up a random number of colored crayons in the periphery of the subject's vision. Ask how many and what colors are being held up. If you are far enough to the side, your subject will be unable to count the crayons or tell what colors are present.

The eye is fed by a network of blood vessels, which connect both at the back of the retina and in front through a thin network of capillaries. See our lesson on Seeing Blood Vessels in the Eye for tips on how to see these vessels in your own eye. Some blood cells are released into the vitreous humor to nourish the inner portions of the eye. These individual blood cells can sometimes be seen leaving shadows on the retina when looking at a uniformly bright scene. Long chains of dead blood cells floating in the vitreous humor are also visible under similar conditions (see the activity below).

activity - floaters and live blood cells
The blood vessels that lie over the retina also feed the eyelens and other parts of the eye by releasing blood cells into the vitreous humor. You can sometimes see the shadows of these blood cells if you stare at a uniform bright background for a while (like the sky or a white ceiling). These live blood cells appear as whitish dots in your vision that move quickly through your field of view. Even more obvious are collections of dead blood cells floating in the vitreous humor. These "floaters" look like small pieces of lint drifting by. They are most prominent if you lie on your back so that they drift downward, close to the retina.

further resources

Helga Kolb at Webvision provides an in-depth article on the retina.

 


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