Vision

Vision is a complex process that involves a photochemical reaction converting light into electrical impulses. Light photons enter the eye through the cornea and pass through the transparent media of the aqueous humor, the lens and the vitreous body. After passing through the sensory retina and the retinal photoreceptors. Some of these photons are absorbed and converted to an electrical potential. The remaining light passes through the RPE and choriocapillaris to the tapetum. The tapetum acts to reflect light back to the retina and allow more photons to be absorbed by photoreceptors and converted to an electrical potential.

These electrical impulses are transmitted to the bipolar neuron, modified then transmitted to the ganglion cell. The axons of ganglion cells become part of the optic nerve. The optic nerve perforates the sclera at the lamina cribrosa and proceeds toward the brain. The two optic nerves meet at the optic chiasm.

Certain species possess a highly specialized area of the retina, the macula, which contains the fovea.  This is found in humans, primates, birds, and some fish and reptiles.  In addition, an area centralis is described in several other laboratory animals such as the cat, but these animals do not possess a true fovea.  In the center of the macula is an area, the fovea, which is the area of highest visual acuity.  It is completely rod free, containing only cone photoreceptors.  The fovea is an avascular area and lacks all retinal layers except the photoreceptors and their nuclei.