The eyes and vision
The structure and functioning of the eye are complex and fascinating. The eye constantly adjusts the amount of light let in, focusing on objects near and far and generates continuous images that instantly relayed to the brain.
Eye Structure and Function
The front part of the relatively strong external white layer of the eye (sclera, or white of the eye) is covered by a thin membrane (conjunctiva). Light enters through the cornea, a transparent dome that is on the surface of the eye. Besides acting as a protective layer of the front of the eye, the cornea also helps focus light onto the retina at the back of the eye. After passing through the cornea, light enters the pupil, a black area is in the middle of the iris (the colored circular area of the eye). The iris controls the amount of light entering the eye opening and closing like the opening of the lens of a camera. The iris allows more light in the eye when the environment is dark and let the less when the environment is lots of light. The size of the pupil is controlled by the pupil sphincter, a muscle that opens and closes the iris.
Interior Eye
Behind the iris is the lens. Shapeshifting, the lens focuses light onto the retina. For the eye to focus on close objects, a small muscle called the ciliary contracts, causing the lens to bulge and become stronger result. For the eye to selectively focus on distant objects, the same muscle relaxes, decreasing the thickness of the lens and therefore making it weaker. Over the years, the lens tends to become less flexible, less able to increase its thickness, and therefore le
ss able to focus on nearby objects, a condition called presbyopia.The retina contains the nerves that perceive light and the blood supply that nourishes them. The most sensitive part of the retina is a small area called the macula, which has hundreds of nerve endings very close together. A high density of nerve endings generates an accurate visual image, just as high-resolution film containing cells more closely together. Then the retina converts the image into electrical impulses that are transmitted to the brain by the optic nerve.
The optic nerve connects the retina to the brain divided in two. Half of the nerve fibers cross to the opposite side at the optic chiasm, an area just below the most anterior (front) of the brain. The bundles of nerve fibers then join again, just before reaching the back of the brain, where it receives and interprets the vision.
The globe is divided into two segments, each of which contains liquid. The front segment (anterior) extending from the cornea to the lens, the dorsal segment (posterior) extending from the rear edge of the lens to the retina. Anterior segment contains a fluid called aqueous humor that nourishes its internal structures, the posterior segment contains a gelatinous substance called vitreous humor. Both fluids allow the eyeball maintain its shape. The anterior segment is divided into two houses. The camera front (anterior) extends from the cornea to the iris, the camera dorsal (posterior) extending from the iris to the lens. Typically, the aqueous humor is generated in the posterior chamber, passes through the pupil and reaches the anterior chamber, and then leaves the eye through specific channels for that purpose are at the edge of the iris.
Muscles, nerves and blood vessels
There are several muscles that, working together, move the eyes. Each muscle is stimulated by a particular cranial nerve. The bony orbit that protects the eye also contains many other nerves. As mentioned above, the optic nerve exits the back of the eye and transmits nerve impulses created in the retina to the brain. The lacrimal nerve stimulates the tear glands to produce tears. Other nerves transmit sensations to other parts of the eye and stimulate the muscles of the orbit.
The ophthalmic artery and retinal artery supplying blood to each eye and the ophthalmic vein and the draining retinal vein. These blood vessels enter and exit through the back of the eye.
Structures that protect the eye
Eye protection
The structures surrounding the eye to protect it, while allowing it to move freely in all directions. These structures protect the eye, which is constantly exposed to dust, wind, bacteria, viruses, fungi and other potentially harmful substances, and at the same time, allow you to remain open enough to receive the rays of the sun.
The orbits are bony cavities that contain the eyeballs, muscles, nerves, blood vessels, fat and structures that produce and drain tears. The eyelids, a thin fold of skin covering the eyes. Close quickly and reflected to protect the eye from foreign objects, wind, dust and bright light. With the blink, the eyelids help to spread liquid on the surface of the eye and, when they close, they help keep the surface moist. Without this moisture, the cornea is normally clear, can dry, damaged and become opaque.
The inner surface of the eyelid is a thin membrane (conjunctiva) that bends backwards to cover the surface of the eye. The tabs are short hairs growing on the edge of the eyelid and helps protect the eye by acting as a barrier. Small glands located on the edge of the eyelid secrete an oily substance that improves the tear film and prevents tears from evaporating.
The lacrimal glands, located in the upper outer end of each eye, produce the watery part of tears. The tears flow from eyes to nose through both nasolacrimal ducts, each of which has openings at the end of the upper and lower eyelids, and are close to the nose. Tears keep the eye surface moist and healthy, in addition, catch and carry small particles that enter the eye. On the other hand, the tears are rich in antibodies that help prevent infection.
Causes of poor eyesight
Both injury and disease in the eye can impair vision. The clarity of vision is called visual acuity, which ranges from full vision and lack of vision. As the acuity decreases, the vision becomes increasingly blurred. Acuity is usually measured on a scale that compares a person's vision to 6 meters with someone who has maximum sharpness. Consequently, a person with 20/20 vision sees objects at 6 feet away with complete clarity, while a person with 20/200 vision sees at 6 meters what a person with the highest acuity is 60 meters .
Legally, blindness is defined as visual acuity less than 20/200 even after correction with glasses or contact lenses. Many people who are considered legally blind, can distinguish shapes and shadows, but not the normal details.
Causes
Blindness can occur due to any of the following reasons:
- The light does not reach the retina.
- Light rays do not focus properly on the retina.
- The retina can not perceive light rays normally.
- Nerve impulses from the retina are not transmitted to the brain normally.
- The brain can not interpret the information sent by the eye.
There are several disorders that can cause these problems that result in blindness. A cataract can block light entering the eye so that it never reaches the retina. The targeting errors (refraction) in general can be corrected with lenses that the doctor prescribes, but not always gets full correction. The detachment of the retina and inherited disorders such as retinitis pigmentosa may affect the ability of the retina to perceive light. Diabetes or macular degeneration can also damage the retina. Nervous system disorders such as multiple sclerosis or an inadequate blood supply can damage the optic nerve that transmits impulses to the brain. Tumors near the brain structures, such as the pituitary gland, also can damage the nerve. The areas of the brain that interprets the nerve impulses can be damaged by a sudden stroke, tumors and other diseases.
