Anatomy And Physiology

Anatomy Physiology

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The human eye is one amazing human physical feature. Despite its small size, it is composed of so many parts that work together in order to carry out a very important function which is vision.

The human eye works like a camera. It captures images and sends information to the brain via the optic nerve. The visual process is a complicated mechanism brought about by the concerted effort of the various vital parts of the eye. The human eyeball has the ability to generate detailed three-dimensional images in multiple colors.

To carry out its complex role in the body, the human eye has a sophisticated structure made up of minute parts with special functions. The human eye is composed of the following task-specific parts.

The sclera

Popularly referred to as the white of the eye, the sclera is a tough white tissue covering most of the eyeball. The sclera has several muscles attached to it which allow you to move your eyes around.

The cornea

This is a transparent cup-shaped structure that covers the part where light enters the eye. It acts like a glass cover of a camera’s aperture. The cornea is considered as a window through which eyesight becomes a reality.

The iris

The iris is the part of your eye that appears like a ring of color. It is the basis for eye color, which can be blue, green, hazel, violet, brown or black. It lies below the cornea and can either contract or relax to control the amount of light that gets into the eyeball.

The pupil

This is the circular black hole embedded in the middle of the iris and covered by the cornea. It allows eyesight by controlling the amount of light coming into the eyeball. Acting like a camera’s shutter, the pupil controls the entry of light by enlarging or becoming smaller due which is made possible by the muscles of the iris.

The lens

Another transparent tissue like the cornea, the lens lies behind the pupil. It functions to help focus light onto the back area of the eyeball. The ciliary muscles attached to the lens allow it to change its shape, which in turn, enable you to modify your visual focus.

The retina

The retina is located on the back wall of the eyeball. It contains millions of photoreceptors (photosensitive cells) known as rods and cones. The rods are responsible for the perception of size, shape and brightness of visual images. These photoreceptors are also able to detect movement and work to maintain eyesight in low light, although they are not good for distinguishing colors. The cones are responsible for color distinction and help in processing fine details as well as maintaining eyesight in bright light.  Acting like camera film, the retina reacts to the incoming light and relays a record of it to the brain via the optic nerve.

The optic nerve

One of the 12 pairs of cranial nerves, the optic nerve is a bundle of more than million nerve fibers that takes signals collected by the rods and cones of the retina and transmits them to the brain. These signals are then translated in the brain into the constant flow of images that compose the human eyesight.

The optic nerve head

Otherwise known as the optic disc, this part of the eye is a circular area where the optic nerve enters the retina. It is where the eye’s blind spot is located.

The macula

Lying near the center of the retina, the macula allows you to see objects with great detail.

The fovea

The fovea is a depression in the retina that does not contain rods, only cones.  It functions to provide acute eyesight.

The Choroids

Made up of layers of blood vessels lying between the sclera and the retina, the choroids serve to provide oxygen and nourishment to the outer layers of the retina. Together with the ciliary body and the iris, the choroids form the uvea.

The ciliary body and aqueous humor

The part of the eye that lies between the iris and the choroids, the three structures form the uvea. The ciliary body is composed of the ciliary muscles and ciliary processes. Its main functions are accommodation, production of aqueous humor and holding the lens in place. The aqueous humor is a clear fluid that flows constantly in and out of the chamber and provides nourishment to the surrounding tissues.  This fluid goes out of the chamber at the open angle formed between the iris and the cornea, where it drains from the eye through a spongy meshwork. In the case of open angle glaucoma, the aqueous humor drains slowly through the meshwork causing it to build up. This fluid accumulation leads to increased intraocular pressure (pressure inside the eyeball), which in turn, can cause damage to the optic nerve and loss of vision.

The conjunctiva

This is a mucous membrane lining the visible part of the eye as well as the inner surface of the eyelids.

The suspensory ligament zonules

Otherwise known as zonule of Zinn, this structure is a membrane of zonules (fibers) that keeps the eye’s lens in place.

The lateral rectus muscle and the medial rectus muscle

The lateral rectus muscle works to move the eye away from the nose; while the medial rectus muscle works to move the eye toward the nose.

The vitreous body and vitreous humor

The vitreous body is located between the lens and the retina. It contains a clear, jelly-like substance known as the vitreous gel or vitreous humor which fills up the inner part of the eyeball helping it to maintain its shape.

The anterior and the posterior chambers of the eye

The anterior chamber of the human eye lies at the back of the cornea and in front of the iris and the lens. The posterior chamber, on the other hand, is located at the back of the iris and in front of the lens.

The human eye further contains other structures which play supporting roles in the main visual process.  These include those that carry fluids, such as tears and blood) to lubricate or nourish the eye. It also has parts that protect it from injury, such as the eyelids and the epithelium of the cornea. In addition, it contains other nerves acting as messengers that send sensory information to the brain, such as the pain-sensing nerves in the cornea.

More about this author: Melody St. John

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