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.
- A multichambered, almost spherical structure
- Located in the anterior portion of the orbit
*orbit is the bony structure of the skull that surrounds & protects the eye
I. EXTERNAL LAYER
- Fibrous coat that supports the eye
a. SCLERA - a tough, leather-like tissue, also extends around the eye. Just like an eggshell surrounds an egg and gives an egg its shape, the sclera surrounds the eye and gives the eye its shape.The extraocular muscles attach to the sclera. These muscles pull on the sclera causing the eye to look left or right, up or down, and diagonally. * opaque white tissue normal – white
* systemic problems – icteric sclera
b. CORNEA – dense transparent layer. The cornea is the transparent, clear layer at the front and center of the eye. It is well supplied with nerve endings. The main purpose of the cornea is to help focus light as it enters the eye. If one wears contact lenses, the contact lens rests on the cornea. normal – transparent, smooth, shiny & bright
accident or eye injury – cloudy areas or specks on the cornea
II. MIDDLE LAYER
- Vascular & heavily pigmented
- lines most of the sclera & is attached to the retina
- contains many blood vessels
- supplies nutrients to the retina
b. CILIARY BODY
- connects the choroid with the iris
- secretes aqueous humor which gives the eye its shape
- Colored portion
- Located in front of the lens
- The iris has a ring of muscle fibers around the pupil, which, when they contract, causes the pupil to constrict (become smaller). This occurs in bright light. A second set of muscle fibers radiate outward from the pupil. When these muscles contract, the pupil dilates (becomes larger). This occurs under reduced illumination or in darkness.
- Has a central opening called PUPIL
* pupil – control the amount of light that enters the eye
darkness – dilatation (mydriasis)
light – constriction (miosis)
* lens – lies behind the iris. It bends the rays of light entering through the pupil so that they focus properly on the retina
- The lens is a clear, flexible structure that is located just behind the iris and the pupil. A ring of muscular tissue, called the ciliary body, surrounds the lens and is connected to the lens by fine fibers, called zonules.
III. INNER LAYER
- thin, delicate structure made up of sensory receptors that transmit impulses to the optic nerve - The retina acts like the film in a camera to create an image. When focused light strikes the retina, chemical reactions occur within specialized layers of cells. These chemical reactions cause electrical signals, which are transmitted through nerve cells into the optic nerve, which carries these signals to the brain, where the electrical signals are converted into recognizable images. Visual association areas of the brain further process the signals to make them understandable within the correct context.
- Contains blood vessels & 2 type of photoreceptors:
rods – work at low light & for peripheral visions; more sensitive to light; therefore, they allow one to see in low light situations but do not allow one to see color. cones – active at bright levels & provide...