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Subsequently the absorbtion of photons by the visual pigment of the photoreceptors is translated into first a biochemical message and then an electrical message that can stimulate all the succeeding neurons of the retina. Light must, therefore, travel through the thickness of the retina before striking and activating the rods and cones (Fig. A radial section of a portion of the retina reveals that the ganglion cells (the output neurons of the retina) lie innermost in the retina closest to the lens and front of the eye, and the photosensors (the rods and cones) lie outermost in the retina against the pigment epithelium and choroid. The optic nerve contains the ganglion cell axons running to the brain and, additionally, incoming blood vessels that open into the retina to vascularize the retinal layers and neurons (Fig. The retina is approximately 0.5 mm thick and lines the back of the eye. A schematic section through the human eye with a schematic enlargement of the retina The total retina is a circular disc of between 30 and 40 mm in diameter (Polyak, 1941 Van Buren, 1963 Kolb, 1991).įig. Retina as seen through an opthalmoscopeĬLICK HERE to see an animation (from the iris to the retina) (Quicktime movie)Ī circular field of approximately 6 mm around the fovea is considered the central retina while beyond this is peripheral retina stretching to the ora serrata, 21 mm from the center of the retina (fovea). Approximately 17 degrees (4.5-5 mm), or two and half disc diameters to the left of the disc, can be seen the slightly oval-shaped, blood vessel-free reddish spot, the fovea, which is at the center of the area known as the macula by ophthalmologists. From the center of the optic nerve radiates the major blood vessels of the retina. In the center of the retina is the optic nerve, a circular to oval white area measuring about 2 x 1.5 mm across. 65(3): 281–322.When an ophthalmologist uses an ophthalmoscope to look into your eye he sees the following view of the retina (Fig. "Optimization, Constraint, and History in the Evolution of Eyes". "Eye Color Changes Past Early Childhood". Bito, LZ Matheny, A Cruickshanks, KJ Nondahl, DM Carino, OB (1997).Eye color can change over time, mainly due to hormonal changes or chemical reactions in the body.The color is a result of Rayleigh scattering, which is also responsible for the blue color of the sky. Human eyes stay about the same size from birth until death. The reason is that it's too hard to reconnect the million-plus nerve fibers of the optic nerve. Doctors are unable to transplant an entire eye.The hole in vision isn't noticeable because each eye fills in the other's blind spot. This is the point where the optic nerve attaches to the eyeball. There are special cells in the eyes that detect light but aren't involved in forming images. Blind people who still have eyes can sense the difference between light and dark.Insects do perceive ultraviolet light, but their compound eyes don't focus as sharply as human eyes, so the energy is spread out over a larger area. The reason humans evolved to not see UV light is because the light has enough energy to damage the rods and cones. The lens absorbs it before it can reach the retina. People don't see ultraviolet light, but the human retina can detect it.If you wear special goggles that make you view everything upside down, after a few days your brain will adapt, again showing you the "correct" view. When the brain translates the image, it automatically flips it. The eye acts exactly like a camera in the sense that the image formed on the retina is inverted (upside down).The functioning of the eye is fairly simple, but there are some details you might not know: The vitreous humor is a transparent watery gel that supports the eye and allows for this distance. Vitreous Humor: A certain distance is required to focus light.Ciliary muscles surround the lens, relaxing to flatten it to image distant objects and contracting to thicken the lens to image close-up objects. Lens: While most of the focusing of light is done by the cornea, the lens allows the eye to focus on either near or distant objects.

As the pupil dilates (gets bigger), more light enters the eye. The size of the pupil is determined by the iris, the contractile ring that is associated with eye color.

The eyeball is rounded, so the cornea acts as a lens.

Cornea: Light enters through the cornea, the transparent outer covering of the eye.To understand how the eye sees, it helps to know the eye structures and functions: