The eye is a complex organ though which we perceive vision. The round shape of the eye is maintained by its outermost layer which is called the sclera (Figure 1). Light enters the eye through the part of this layer that is called the cornea. Underneath the sclera is the choroid, the second layer that carries blood vessels. It delivers the necessary blood supply to all the structures of the eye. The colored part of the eye, the iris, is the frontal part of the choroid. Its function is to control the aperture size of the pupil, the opening through which light enters the eye. The innermost layer is the retina. Its function is to convert light that enters the eye into bioelectric signals. The inside of the eye is filled with two fluids, separated into different compartments by the lens. The larger section contains the gel-like vitreous humor, and the smaller front section contains a clear fluid called the aqueous humor.
Structure of the eye. There are three main layers. The outermost layer is the sclera, underneath which lies the choroid, and the innermost layer is the retina.
The retina itself consists of several structural and functional layers. The layer closest to the sclera consists of photoreceptors. These are rod and cone cells. The first are optimized for low light conditions, and the latter are primarily used during high-luminance environments, and they are in charge of color vision. In the center of the retina is the macula which contains only cones, present in much higher density then in other parts of the retina. In the next layer the bipolar and amacrine cells preprocess the signals received from photoreceptors. The final layer in this retinal signal processing chain consists of ganglion cells. The network of these cells eventually encodes the visual information as trains of action potentials that are sent to higher brain centers via the optic nerve.