The EYE is a spherical structure about an inch in diameter. It has a clear window on the front side called the cornea. The wall of the eye beyond the Cornea consists of three tissue layers. The outermost layer is the Sclera, a tough protective coating that covers most of the outer surface. The middle layer is the choroid, a vascular layer that is continuous with the ciliary body. The iris is the colored part on the front side of the eye. The inner layer is the retina, a light-sensitive tissue that lines the inside wall of the eye. To understand how the eye works it may be helpful to think of a camera. The human eye is more complex than a camera, but the principles are similar. The cornea and lens act like the lens of a camera. They allow for the incoming light rays to be focused on the retina. The retina acts like the film of the camera, processing electrical light pulses into a nerve impulse that is carried to the brain for processing by the optic nerve.
The CORNEA
The cornea is the transparent dome-shaped window covering the front of the eye. It is a powerful refracting surface providing 2/3 of the eye’s focusing power. It allows light to pass into the eye. Like the crystal on a watch, it gives us a clear window to look through.
Because there are no blood vessels in the cornea, it is normally clear and has a shiny surface. The cornea is extremely sensitive – there are more nerve endings in the cornea than anywhere else in the body.
The PUPIL
The pupil is the opening in the center of the iris. It is like the aperture of a camera. Light enters here then passes through the lens. Liquid flowing through the pupil, called aqueous fluid, is absorbed through channels in front of the iris. This fluid provides nourishment to the lens, iris and cornea. Pressure within the eye depends on the delicate balance between the body’s production and absorption of this fluid.
The IRIS
The colored part of the eye is called the iris. It controls light levels inside the eye, similar to the aperture on a camera. The round opening in the center of the iris is called the pupil. The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size depending on light conditions. The sphincter muscle lies around the very edge of the pupil. In bright light, the sphincter contracts, causing the pupil to constrict. The dilator muscle runs radially through the iris, like spokes on a wheel. This muscle dilates the eye in dim lighting. The iris is flat and divides the front of the eye (anterior chamber) from the back of the eye (posterior chamber). Its color comes from microscopic pigment cells called melanin. The color, texture, and patterns of each person’s iris are as unique as a fingerprint.
The LENS
In a healthy eye, the lens is flexible. The ring-shaped ciliary muscle surrounding the lens contracts or relaxes to adjust its shape according to visual need. The lens bends or “refracts” light rays to focus on objects far and near. The light rays pass through the transparent gel (vitreous) that fills the inside of the eyeball and comes into focus on the surface of the retina.
The VITREOUS
The vitreous is a thick, transparent substance that fills the center of the eye. It is composed mainly of water protein and sugars. It comprises about 2/3 of the eye’s volume, giving it form and shape. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed. In children, the vitreous has a consistency similar to an egg white. With age, it gradually thins and becomes more liquid. The vitreous is firmly attached to certain areas of the retina. As the vitreous thins, it separates from the retina often causing floaters.
The RETINA
The retina is a light-sensitive nerve tissue that lines the back of the eye, comparable to film in a camera. The retina receives the picture formed by the light rays and sends the image to the brain through a cable known as the optic nerve. In this way, the retina not only detects light, it also plays a significant part in visual perception. During embryonic development, the retina and the optic nerve originate as outgrowths of the brain.
The MACULA
The central part of the retina, which processes detailed vision is called the macula. It is a small and highly sensitive part of the retina responsible for detailed central and color vision. It is here that the optic nerve collects and transmits visual images to the brain for processing. The brain combines the separate images produced by the eyes into a single composite neural image. The macula allows us to appreciate detail and perform tasks that require central vision, such as reading.
The OPTIC NERVE
The optic nerve transmits electrical impulses from the retina to the brain. It connects to the back of the eye near the macula. When examining the back of the eye, a portion of the optic nerve called the optic disc can be seen. The retina’s sensory receptor cells are absent from the optic nerve. Because of this, everyone has a normal blind spot. This is not normally noticeable because the vision of both eyes overlaps.