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Physiology of Vision

Anatomy of the eye:
Even though the eye is small, only about 1 inch in diameter, it serves a very important function -- the sense of sight. Vision is by far the most used of the five senses and is one of the primary means that we use to gather information from our surroundings. More than 75% of the information we receive about the world around us consists of visual information. The eye is often compared to a camera. Each gathers light and then transforms that light into a "picture." Both also have lenses to focus the incoming light. Just as a camera focuses light onto the film to create a picture, the eye focuses light onto a specialized layer of cells, called the retina, to produce an image.
The eye is essentially an opaque eyeball filled with a water-like fluid. In the front of the eyeball is a transparent opening known as the cornea. The cornea has the dual purpose of protecting the eye and refracting light as it enters the eye. After light passes through the cornea, a portion of it passes through an opening known as the pupil. Rather than being an actual part of the eye's anatomy, the pupil is merely an opening. The pupil is the black portion in the middle of the eyeball. Its black appearance is attributed to the fact that the light that the pupil allows to enter the eye is absorbed on the retina (and elsewhere) and does not exit the eye. Thus, as you sight at another person's pupil opening, no light is exiting their pupil and coming to your eye; subsequently, the pupil appears black.
Like the aperture of a camera, the size of the pupil opening can be adjusted by the dilation of the iris. The iris is the colored part of the eye - being blue for some people and brown for others (and so forth); it is a diaphragm that is capable of stretching and reducing the size of the opening. In bright-light situations, the iris adjusts its size to reduce the pupil opening and limit the amount of light that enters the eye. And in dim-light situations, the iris adjusts so as to maximize the size of the pupil opening and increase the amount of light that enters the eye.
Light that passes through the pupil opening, will enter the crystalline lens. The crystalline lens is made of layers of a fibrous material that has an index of refraction of roughly 1.40. Unlike the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process. The lens is attached to the ciliary muscles. These muscles relax and contract in order to change the shape of the lens. By carefully adjusting the lenses shape, the ciliary muscles assist the eye in the critical task of producing an image on the back of the eyeball.
The inner surface of the eye is known as the retina. The retina contains the rods and cones that serve the task of detecting the intensity and the frequency of the incoming light. An adult eye is typically equipped with up to 120 million rods that detect the intensity of light and about 6 million cones that detect the frequency of light. These rods and cones send nerve impulses to the brain. The nerve impulses travel through a network of nerve cells. There are as many as one million neural pathways from the rods and cones to the brain. This network of nerve cells is bundled together to form the optic nerve on the very back of the eyeball.
Vision:
Vision begins when light rays are reflected off an object and enter the eyes through the cornea, the transparent outer covering of the eye. The cornea bends or refracts the rays that pass through a round hole called the pupil. The iris, or colored portion of the eye that surrounds the pupil, opens and closes (making the pupil bigger or smaller) to regulate the amount of light passing through. The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye. The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells called rods and cones, which are named for their distinct shapes. Cones are concentrated in the center of the retina, in an area called the macula. In bright light conditions, cones provide clear, sharp central vision and detect colors and fine details. Rods are located outside the macula and extend all the way to the outer edge of the retina. They provide peripheral or side vision. Rods also allow the eyes to detect motion and help us see in dim light and at night. These cells in the retina convert the light into electrical impulses. The optic nerve sends these impulses to the brain where an image is produced.
Diseases:
* Cataract: A cataract is cloudiness in the usually clear lens of the eye. Common cataract symptoms include poor night vision, sensitivity to light, a painless blurring of vision and a fading or yellowing of colors. * Amblyopia, "lazy eye": Amblyopia cannot be corrected by glasses or contact lenses, and is not caused by any eye disease. This eye condition, however, is noted by reduced vision. *
Ocular Albinism: This condition is inherited. Ocular Albinism results in a condition where a patient's eyes lack melanin pigment, but skin and hair retain normal or near-normal coloration. * Blepharitis: Blepharitis is a chronic or long term inflammation of the eyelids and eyelashes. It affects people of all ages.

Samer Eid
201101709
Anatomy and physiology term paper