II. WHAT IS THE RETINA?

The retina is a thin film of nervous tissue, about the consistency of wet cigarette paper. It is really a part of the brain, stuck outside the brain, in the eye, like a TV antenna on your house’s roof. It covers the inside wall of the eye, like wallpaper covers the inside wall of a room (Fig. 1). Since the eye is more or less spherical, the retina is also more or less spherical. At the back, it is attached to the optic nerve, which is a ribbon of brain tissue connecting the eye with the brain. In the front, the retina stops around the apparatus that directs light rays to the retina. This apparatus is composed of the cornea and the crystalline lens (Fig. 1). Since it is awkward to represent the retina on paper, because the retina is roughly spherical, doctors sketch the retina as if it were flat (Fig. 2).

Fig. 1 Cut-away of the human eye showing its essential parts. In front are the lenses (cornea and crystalline lens) that focus images on the retina.

Fig. 2 Schematic representation of the retina of a right eye. Doctors sketch the retina as if it were flat when in fact the retina adheres on the inside of a hollow sphere. The sinuous black lines are the retinal vessels, which comes out of the head of the optic nerve. The stippled spot, to the left of the optic nerve head is the macula.

The eye resembles a camera (Fig. 3). The cornea and the crystalline lens act like a camera’s objective; they form an image on the retina, which is like the camera’s film. There, the retina transforms the rays of light into nerve impulses that are carried by the optic nerve to the brain. It is in the brain that "seeing" occurs.

Fig. 3 Comparison between a human eye and a camera.