Anatomy of the Eye
The eye is a wonderful organ that gives us the ability to see the world around us. Many people think the eye is just one sphere, or globe. It really is composed of two spheres with different radii, one set into the other. The front sphere, called the anterior, is smaller and more curved of the two. This is called the cornea. The second sphere is called the sclera. The cornea sets into the sclera much like the glass on a watch sets into the frame. The cornea and the sclera are the structures that encase and protect the different and more delicate structures within.
The cornea is the clear front surface of the eye. This clear tissue is like a window for the eye. It is composed of 5 different layers. The outer layer is called the Epithelium then comes the Bowman’s Membrane, then the Stroma, then Descemet’s membrane, the fifth layer is called the Endothelium and is on the inside of the eye. The cells and fibers in the cornea are arranged so that light can pass through it with a minimum of diffraction and internal reflection. The cornea contains no opaque substances such as blood vessels that would mar it’s clarity. It receives its nourishment from the vessels surrounding the cornea. It is kept shiny and lubricated by tears that keep its surface moist.
The lens of the eye is a transparent biconvex structure situated between the iris and the vitreous (a clear jelly- like fluid in the back of the eye). Most of the lens is hidden behind iris tissue, only the portion that is directly behind the pupil is visible. It is attached to the eye by means of fine suspensory ligaments called zonular fibers. A capsule, sort of like a transparent highly elastic envelope, surrounds the lens. The lens tissue inside the capsule is rather soft and puttylike in infants. With age it tends to grow harder, especially toward the center of the lens. The lens has the ability to change shape, which helps to bend the light as it passes through the eye. This helps to focus the light rays onto the retina.
The retina is to the eye like film is to a camera. The retina contains all the sensory receptor for the transmission of light. It collects all the information and sends it to the brain. But the retina is also a part of the brain. The retina has two types of receptors, rods and cones. The rods function best in dim light; the cones function best under daylight conditions. There are many more rods than cones in the retina. Color vision is dependent on the integrity of the cones. The cones form a concentrated area in the retina known as the fovea, which lies in the center of the macula. Damage to this area can severely reduce the ability to see directly ahead. The macula is the area of the retina that we use for our fine and straight-ahead vision. If the macula or fovea are damaged, a blind spot appears in the central vision. Rods are in the periphery of the retina, not in the macula. If these are damaged, it can result in night blindness, but with good vision for straight-ahead objects.