the human eye
The retina lines the back of your eye, fitting neatly despite its thin, paper-like form. Inside, over 100 million nerve cells sort signals in a careful arrangement. The Living Body calls the retina “one of the most remarkable pieces of tissue in the human body.” Sandra Sinclair writes in her book How Animals See, “The retina is the envy of the computer scientist, performing approximately 10 billion calculations every second.”
Think of how a camera directs light onto film. Your eye works in a similar way, winding each image onto the retina. Yet as Dr Miller points out, camera film “does not even begin to compare with the versatile sensitivity of the retina.” With the same “film” in your eye, you can see by moonlight and adapt to daylight 30,000 times brighter. The retina also helps you make out fine details in a scene, even when part stands in shadow and the other in direct light. As Professor Guyton states in his Textbook of Medical Physiology, “The camera cannot do this because of the narrow critical range of light intensity required for proper exposure of film.” This is why photographers often need to use flash.
You have this adaptability mainly due to about 125 million rods in your retina. Even in dim light, they help you see shapes and movement. Alongside them are 5.5 million cones sensitive to brighter light, which allow you to see sharp detail and colour. Some cones respond best to red light, others to green, and others to blue. These cells work together so you see the full range of colours in this magazine. When all three cone types react at once, you perceive pure white.
Many animals do not share this ability to see colour, and some have no colour vision at all. Surgeon Rendle Short notes, “Colour vision adds immensely to the joys of life.” He also says, “Of all the organs of the body not absolutely essential for life, the eye may be considered the most wonderful.”
Working in Step
Every image that falls on the retina lands upside down, much like an image forming on camera film. Dr Short asks, “Why is the world not upside down to us?” He explains, “The brain has developed the habit of reversing the impressions.”
To test this, researchers gave people special glasses that made everything look inverted. At first, these people saw the world upside down. After a few days, however, their vision adjusted to see normally again. The Body Book describes this as, “The miraculous teamwork of your eye and your brain is exhibited in a number of ways.”
As your eye glides across this page, cones pick out the black ink from the white background. Yet, your retina alone cannot interpret words or letters. You learn to make sense of text through your brain, not your eye. The information needs to move from the retina to other areas of your mind.
The retina carries its coded messages along about one million nerve fibres to a part of your brain towards the back of your head. The Brain publication explains, “The projections from the retina to the cerebral cortex are highly organised and orderly. . . . If a small light is shone on each different part of the retina, a corresponding part of the visual area [in the brain] will respond.”
