Nature – The Eyes Have It

Have you ever really considered how the eye works?

According to the National Eye Institute, light passes through the cornea and is bent to help the eye focus. Some of this light then enters through the pupil. The iris controls how much of the light is let in, and that light passes through the lens. The lens works together with the cornea to focus light correctly on the retina. And finally, when the light hits the retina, photoreceptors turn it into electrical signals. These signals travel from the retina through the optic nerve to the brain, and the brain turns the signals into an image. And all this happens instantaneously and continuously. Whew! Did you get all that? Source:

Of course, in nature, we find all kinds of eyes, each of which works in ways specific to the creature they dwell in. Let’s look at a few examples.

The eyes of a prairie dog are positioned on the side of its head. This appears to provide it with the ability to focus in a wide arc. The lenses of their eyes are also tinted amber, like having built-in sunglasses.

Good eyesight is essential for a bird’s safe flight. Birds have the largest eyes relative to their size, but have limited movement in the bony eye sockets. Eyes located on the sides of the head have a wide field of view while the eyes located on the front of the head provide binocular vision. Birds of prey have high-density receptors to maximize visual acuity. The placement of their eyes gives them good binocular vision enabling accurate judgment of distances. Nocturnal birds have tubular eyes, with less color detectors, but high-density rod cells that function better in poor light.

Birds’ eyes are protected by two eyelids, and a third transparent, movable membrane. The eyelids are not used for blinking. The third membrane (nictitating membrane) lubricates the eye as it passes side to side across it, much like a windshield wiper. This membrane also covers the eye in many aquatic birds when they are under water. When sleeping, the lower eyelid rises to cover the eye in most birds, but the horned owl is an exception. Its upper eyelid closes down to meet the lower eyelid.

Hippopotamus eyes, along with the ears and nostrils, are placed high on the roof of their skulls so they can submerge and still see above water, but when they swim underwater, nictitating membranes cover the eyes.

Most spiders can have eight, though some have six, eyes, each a single lens above the retina rather than multiple units like the fly or other insects. The specific arrangement and structure of the eyes is one of the features used to identify and classify the different species and genus. Most spiders’ eyes detect little more than brightness and motion, playing a pretty minor role in spider behavior. Species such as jumping spiders or wolf spiders have more developed eyes and can even perceive color.

But Did You Know? Cave spiders have no eyes at all.

One of the defining features of the common house fly is its compound eyes. Their eyes can often be so large that they take almost all the space on their head. These compound eyes comprise an array of tiny sensors—ommatidia—around 3,500 of them. The ommatidia are many tiny lenses all bunched together in a globular shape to form the eye. The large, round shape gives the fly almost a 360-degree view of its surroundings. It contains a corneal lens to focus light and pigments that sense color. The fly eye has the ability to process images at a speed more than six times faster than the human eye. That probably explains why they get away so often when we try to swat them. However, their ability to focus as clearly as the human eye has been sacrificed in order to have such big eyes. They also have excellent peripheral vision, few blind spots, and a better range of focus. And flies can actually see in slow motion. This ability has inspired scientists and engineers to develop cameras that mimic the eye of the fly in the creation of surveillance systems that can act just like the proverbial fly on the wall.

Human eyes, when working together, have a field of view approximately 200 degrees wide and 135 degrees tall, and when they work together correctly, they give you depth perception and 3D vision, as well as color vision.

Sight and vision, terms often used interchangeably, are not necessarily the same thing. Sight is what the eyes do and vision is the process that starts with sight and ends with the brain’s interpretation of what the eyes have seen in a way that it can use and understand. The human eye is one of the most important organs in the body.

The Bible talks about the eyes, too.

“The lamp of the body is the eye. If therefore your eye is good your whole body will be full of light. But if your eye is bad, your whole body will be full of darkness.” Matthew 6:22

“Let your eyes look straight ahead, and your eyelids look right before you.” Proverbs 4:25

“Every way of a man is right in his own eyes, but the Lord weighs the heart.” Proverbs 21:2

“The statutes of the Lord are right, rejoicing the heart; the commandment of the Lord is pure, enlightening the eyes.” Psalm 19:8

“I will lift up my eyes to the hills—from whence comes my help?” Psalm 121:1

“Do not be wise in your own eyes; fear the Lord and depart from evil.” Proverbs 3:7

“And God will wipe away every tear from their eyes.” Revelation 21:4, first part

Yes, we need only to look at the eye, in whatever creature we see, to be assured of the truth in David’s declaration, “I will praise You, for I am fearfully and wonderfully made. And that my soul knows very well.” Psalm 139:14