ANATOMY AND FUNCTION OF THE EYE
In order to understand disorders of the retina, it is helpful to know the basic anatomy of the eye. The structure and function of the eye is analogous to a camera. The front parts of the eye permit light to enter and focus the image to the surface of the retina. The focusing structures include the cornea, the iris and the lens. The cornea is the clear front surface of the eye. Behind the cornea is the iris (the colored part of the eye) in the center of which is a hole called the pupil. The pupil varies in size depending on the level of illumination just like the aperture of a camera. Behind the iris is the lens which provides additional focusing power. As part of the normal aging process, the lens slowly develops yellowing and clouding which is called cataract. (Cataract surgery involves removing the clouded lens and replacing it with a lens implant.)
How the Eye Sees
The image generated by the cornea, iris/pupil and lens is focused to the surface of the retina. The retina corresponds to the film of the camera and lines the back, inside surface of the eye wall. The retina is a transparent tissue layer and is composed entirely of nerve cells along with blood vessels. It is here that the process of vision takes place. The retina lines the majority of the inside of the eye. The macula is a specialized central region of the retina which is responsible for finer detailed vision. The macula accounts for only a tiny fraction of the entire surface area of the retina (analogous to the bull’s eye of a target), however it is critically important – providing us with the type of detailed vision necessary to read, drive, recognize faces, etc.
The remainder of the retina is responsible for our peripheral (side) vision. The neurological signals generated in the retina are then transmitted to the brain along the optic nerve. The brain provides higher levels of processing of this visual information and the conscious awareness of the sense of vision occurs here.
Additional important structures of the eye include:
- Vitreous – a clear jelly-like substance filling the cavity between the lens and the retina
- Sclera – the external white coat of the eye
- Choroid – the middle coat of the eye (between the sclera and the retina) composed primarily of blood vessels as well as collagen and pigmented cells which create the orange coloration when viewing images of the retina (recall from above that the retina itself is transparent)