Human Eye Structure| Eye Parts & Their functions

Human Eye Anatomy

To understand how the human eye works, first imagine a photographic camera – since cameras were developed very much with the human eye in mind. The eye is a wonderful and the most complex organ of the human body. It is strange to learn that such a small organ has so many parts. The following account provides you information on the parts of the human eye and its function along with the particular functions assigned to each part of it.

The main parts of the human eye & Their Functions

• Cornea: transparent tissue covering the front of the eye that lets light travel through. It is composed of 5 layers of tissue. Its outer layer (the epithelium) provides protection for the eye. The epithelium is made up of highly regenerative cells that have the ability to grow back within 3 days, allowing for fast healing of superficial injuries. Most of the inner layers of the cornea provide strength to the eye.
• Ciliary Muscles: The eye lens is held by ciliary muscles. Ciliary muscles help the eye lens to change its focal length. The ciliary muscle changes the shape of the lens - (this is called accommodation). It relaxes to flatten the lens for distance vision; for close work it contracts rounding out the lens. Everyone will develop an eye condition called presbyopia. As we age, the ciliary muscle and crystalline lens lose their elasticity. This is why most people need reading glasses by their 40’s.
• Conjunctiva:  The conjunctiva is a thin, clear membrane covering the front of the eye and inner eyelids. Cells in this lining produce mucous that helps to lubricate the eye. This is the eyes first layer of protection against infection. Inflammation of this membrane is called conjunctivitis, or pink eye.
• Iris: A ring of muscles in the colored part surrounding the pupil of the eye. The primary function of the iris is to control the size of the pupil. This is achieved through contraction or expansion of the muscles of the iris.
• Pupil: An opening in the center of the iris that changes size to control how much light is entering the eye. This is the black circle in the middle of the eye. When you're in a bright environment, the pupil becomes smaller to allow less light through. When it's dark, the pupil expands to allow more light to reach the back of the eye.
• Sclera: The white part of the eye that is composed of fibrous tissue that protects the inner workings of the eye. The sclera's purpose is to provide structure, strength, and protection to the eye.
• Lens: located directly behind the pupil, it focuses light rays onto the retina. The lens is the clear structure located behind the pupil. Its primary function is to provide fine-tuning for focusing and reading. The lens performs this function by altering its shape. At about the age of 45, the lens becomes less flexible. At about the age of 65, the lens becomes cloudy and hard, preventing light from entering the eye.
• Retina: Membrane at the back of the eye that changes light into nerve signals. The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to transmit images to the brain.
• The Uvea: The uvea forms the center of the eyeball. It is made up of three parts, choroid, ciliary body and iris. The choroid is a thin membrane that is placed between the outer protective sclera and retina. Its function is to prevent the rays of light from bouncing off on the back side of the eye. Malfunctioning of the choroid may cause the formation of confusing images. The role of ciliary body is to assist in the adjustment of the shape of the lens. The iris is described as a separate part in this section of the article.
• Fovea: a tiny spot in the center of the retina that contains only cone cells. It allows us to see things sharply. The fovea is an indentation in the center of the macula. Its diameter is only 1.5 mm or about 1/16 inch. This small part of our retina is responsible for our highest visual acuity. It is the center of our central vision.
• Optic Nerve: a bundle of nerve fibers that carries messages from the eyes to the brain. The optic nerve emerges from the back of the eye, travels through the skull and stops inside the skull bone. From the skull bone, the nerves move through the lateral geniculate body, the internal capsule and ends up at the back of the brain. This part of the brain is known as visual cortex. It is responsible for receiving information from the eyes and interpreting it.
• Macula: a small and highly sensitive part of the retina responsible for central vision, which allows a person to see shapes, colors, and details clearly and sharply. This part of the retina is the most sensitive. Its diameter is only 7 mm or about 1/4 inch. It is responsible for our central, or reading vision. This part of the retina gives us 20/20 vision. Without the macula, you would be blind - Legally blind that is. People with eye diseases like Macular Degeneration have vision from 20/200 to 20/800.
•  Vitreous Cavity: The space between the lens and retina filled with the gel like Vitreous Humor.

How Human Eye works

Light reflects off of objects and enters the eyeball through a transparent layer of tissue at the front of the eye called the cornea. The cornea accepts widely divergent light rays and bends them through the pupil – the dark opening in the center of the colored portion of the eye.

The pupil appears to expand or contract automatically based on the intensity of the light entering the eye. In truth, this action is controlled by the iris – a ring of muscles within the colored portion of the eye that adjusts the pupil opening based on the intensity of light. 

The adjusted light passes through the lens of the eye. Located behind the pupil, the lens automatically adjusts the path of the light and brings it into sharp focus onto the receiving area at back of the eye – the retina.

An amazing membrane full of photoreceptors "rods and cones", the retina converts the light rays into electrical impulses. These then travel through the optic nerve at the back of the eye to the brain, where an image is finally perceived.

Since both the eyes are separated by the nose, they have different fields of vision. Due to the difference in the visual fields, each eye sees at different angles of the object, and so gives different information to the brain. Along the way at the optic chiasma, some of the nerves from each optic nerve cross over so as to separately collect the information from the left and right side of the field of vision. 
The swapping of information takes place one more time at the cell station. This connection works in accordance with the reflexes of the pupils. The information is now received by the visual cortex, which interprets the image at this point.