The basic function of the human eye is to see. Other functions involving the accessory structures and secretions of the eye prevent foreign particles from injuring the ocular organ and pathogens from entering the body through this route.
In its simplest definition, sight is the transmission of light signals through nerve endings to the brain, which interprets the signals and produces visions of what is seen. The light reaches the nerve endings by traveling through the eye, making the brain’s interpretation dependent on the accuracy of the path the light waves travel through the structures of the eye.
Light’s Path Through the Eye
Light enters the eye first through the cornea, a thin clear sheath covering the outer, visible portion of the eye. There are no blood vessels in the cornea because it must remain clear for light waves to pass through uninterrupted. Under the cornea is a cavity containing a nutrient-filled liquid, aqueous humor, which provides nutrients to the cornea and lens. The pupil is the opening that controls how much light passes from the cornea to the lens. The iris, the colored portion of the eye, is actually two sets of muscle fibers that control the size of the pupil.
The lens is the refraction system of the eye; it is located on the other side of the aqueous humor cavity. The lens also contains no blood vessels so that light can pass through unhindered. Light is refracted and focused onto the back of the eye where the nerve endings are present; what is seen may be out of focus if the lens is misshapen or the wrong thickness, resulting in the vision disorders near-sightedness, far-sightedness, and stigmatism. The lens is adjusted by muscles called the ciliary body, which allow light to be focused for distance and movement.
Light travels from the lens through a chamber filled with vitreous humor, a semi-solid that helps the eye keep its shape. The vitreous humor is also what holds the retina in place. The retina is the back of the inside of the eye and contains light receptors. The receptors are nerve endings with a direct link to the brain via the optic nerve. Alterations in the vitreal pressure due to eye injuries or diabetes can affect vision and cause retinal detachment or blindness.
Light Receptors How Vision Works
Receptors in the eye consist of rods and cones. Rods detect light regardless of wavelength and help the brain determine light intensity, whether it is bright or dim. Cones are only active in bright light and are particular to red, blue, or green. Each type of cone detects a particular light wavelength. Color blindness is a disorder that occurs when a person lacks a particular type or types of cones, allowing only bright, dim, and the remaining wavelengths to be seen.
There is a blind spot at the back of the eye at the point where the optic nerve passes through the eyeball and there are no receptors. With two working eyes, the blind spot is compensated for by binocular vision. There is also a spot in the retina with the greatest concentration of cones, called the macula fovea. This specialized pit in the 5-mm wide macula is important for high visual acuity. The macula has two layers of ganglion cells, the nerve endings communicating with the brain via the optic nerve.
Other Functions of the Eye
The eye has accessory structures. The eyelids have hairs called eyelashes that protect the eye and keep out dust and harmful debris. Scratching the cornea would prevent light from passing through properly and hinder vision. Tear ducts, called lacrimal glands, secrete a salt solution to keep the eye from drying out as well as a lysozyme that protects the eye from bacteria.
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