The eye is a sense organ that reacts to light and processes the emitted or reflected light from the objects around us. That allows us to see the shapes, colors, depth and dimensions of these objects. The eyes are connected with the brain, which interprets what the eyes see.
Anatomy Of the Eye
The eye contains three layers of tissue. The outermost---the sclera---is a tough, white tissue that transforms at the front of the eye into the transparent cornea, which allows light to enter the eye.
The middle layer includes the iris, the ciliary body and the choroid. The iris gives the eyes their color. In bright light the iris shrinks the opening in its center, the pupil, but broadens it in dim light to allow more light to reach the retina. The ciliary body is a ring of tissue that encircles the lens. The lens has the ability to thicken or flatten, allowing light rays to be focused on the retina. The choroid is rich in capillaries that supply nutrients for the retina.
The innermost layer---the retina---is the only structure that gets activated by light to enable vision.
The eye has two liquid-filled chambers that supply it with nutrients, and one filled with a thick, gelatinous substance. This chamber makes about 80 percent of the eye volume and maintains its shape.
Retina
The retina has 10 layers filled with interconnected neurons, and each of them has a specific function that allows the retina to accomplish vision. The photoreceptor layer has two types of light-sensitive cells---photoreceptors: rods and cones. Rods are extremely light-sensitive, allowing us to see in dim light. Cones work only in bright light, but they obtain the sharpest images and enable us to see colors.
Vision
The light passes through the whole retina in order to reach the photoreceptors and activate them. The activated photoreceptors stimulate specialized neurons---the bipolar cells. They further activate the retinal ganglion cells, neurons with long processes---axons. These axons leave the eye and build the optic nerve, which connects the eye with the brain. Various photoreceptors and neurons in the retina work together to collect the information about the world around us. They send this information via the optic nerve to the brain, which combines the information from both eyes into a single three-dimensional image.
Sleep-wake Cycle
The eyes are also important for the maintenance of a healthy circadian rhythm---the body's sleep-wake cycle. They contain specialized cells within the retina that react to the change in light intensity between day and night. They send that information to the hypothalamus, the part of the brain that is the center of the circadian control.
Disorders and Diseases Of the Eye
Eyes suffer through infections, inflammations, blood-supply loss, toxins, age, genetic disorders and metabolic diseases, such as diabetes. The World Health Organization classifies disorders and diseases of the eye and the accessory structures into eleven groups according to the affected structures.
People wear glasses or contact lenses in order to correct nearsightedness or farsightedness, which occur due to the changed length of the eyeball or shape of the lens. The leading cause of blindness in the US, according to the National Institutes of Health, is glaucoma, a disease that damages the optic nerve. The usual trigger is increased pressure of the fluid inside the eyes.
References
- "Anatomy, Physiology & Pathology of the Human Eye"; Ted M. Montgomery, O.D.; 1998-2010
- "Neuroscience"; Dale Purves, M.D., George J. Augustine, Ph.D., David Fitzpatrick, Ph.D., Lawrence C. Katz, Ph.D., Anthony-Samuel LaMantia, Ph.D., James O. McNamara, M.D., S. Mark Williams, Ph.D., 2001
- "Kimball's Biology Pages"; Blood; John W. Kimball Ph.D.; 2010
