The eye is an organ that enables sight. What appears to be a simple structure seems more complicated when examining its anatomical structure. The eyes are spherical shaped and placed inside sockets of the cranium. The eyelids provide a cover and form of protection from foreign objects in the environment. The eyelids also contain eye lashes which also aid in protection of the eye. The eyes have a direct connection to the brain optic nerve. The relies on six muscles for basic motion. The reason people have sight is a combination of evolution and how light rays are adjusted to our eyes. The reason animals developed sight was for the purpose of hunting and sensory information gathering. Certain animals do not require perfect sight to function in their environment. Light rays enter the eye through the pupil are then focused by the cornea and lens. The retina then allows for the image to be produced by the eye. The rods and cones which number in the millions are light sensitive cells that convert an image by means of nerve impulses. The impulses are then processed by the optic nerve and the brain produces an image. Like any other human body structure it has certain units that are designed for different functions.
The basic structures of the eye include the pupil, iris, and lens. The lens divides the eye into an anterior chamber. It is filled with a fluid and called the aqueous humour. There also is a posterior chamber. These chambers of the eye allow for enough internal pressure to maintain eye shape. The vitreous humour.
The iris is a membrane structure which is a barrier between the lens and the cornea. The iris contains pigments. The colors ca range from green, blue, gray or hazel. The pigmentation is determined by the amount and positioning of pigment, melanin, including genetic inheritance. The pupil is a hole located at the center of the iris. The pupil appears to be black. Light rays will enter the pupil, then reach the lens. The pupil size monitors the amount of light permitted to enter. The pupil size is mostly controlled by the dilator and sphincter muscles.
The eye would not function without critical photoreceptors. Rods only have the ability to detect dim light including black and white color schemes. They occupy the macula lutea on the retina. Rods are responsible for producing fine vision. Cones are focused more on color vision in bright light. They are distributed in the retina, but the majority are in the fovea. Cones can be classified in three forms. The types are determined by pigments which are red, green, or blue. The reason they are called this is because it is referring to the wavelengths of colors you can see. Cones allow a person to see a vast range of colors. The process of sight also is a production in itself. Light contacts the pupil, then becomes refracted. Following this, it enters the crystalline lens. The lens will make adjustments to change an image to an upside down version projected on to the retina. Each of the human eyes can see different images with visual fields having a tendency to overlap. An example of this is stereoscopic vision , which enables depth perception. Seeing objects at a distance the ciliary muscles relax, while the lens flattens then thins. To see near by objects the ciliary muscles contract. Photoreceptors open a world of visual information.
The eye has an external structure as well. The outer section contains a transparent cornea and sclera. The sclera is the part of the eye that appears white. It is considered an extension or another part of the cornea. The purpose is mostly a protective function. The eyelids are designed to protect the conjunctivae and the eyelashes block debris from the surrounding environment. The eyes must remain moist to prevent infection.Tears act as a washing solution for a person's eyes. The lacrimal apparatus both forms and disposes of tears in the eye. Humans are unique in the sense that they are the only cry extra tears when feeling distraught. Blinking is an action done to remove excess tears to the nasolacriminal duct. The lacrimal gland forms tears which are composed of antibodies and lysozyme. The eye is as complex as other organs working with light, cells, and glands to enable sight.
Further Reading
Brewer, Sarah. The Human Body a Visual Guide to Human Anatomy. London: Quecus, 2011.
DK Ultimate Visual Dictionary 2000. New York: DK Pub., 1999.
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