The eyeball is composed of a tough membrane called sclera. This white outer skin of the eye is covered with a thin protective layer of conjunctiva. Light first enters the eye through the cornea. The cornea has five layers; sometimes corneal defects will be managed by removing
one or two layers, rather than full-thickness cornea. The cornea meets the sclera in a ring called the limbus, also known as the sclerocorneal junction. Behind the cornea is the anterior segment of the eye, which is filled with a clear, salty fluid called aqueous humor.
The shape of the eyeball affects the way light is focused and directed (refraction). Any reduction in fluid within the eye will affect the shape of the eye and, thus, refraction. For instance, if the eyeball is too oblong, the patient will be near-sighted. In far-sightedness, the eyeball is
foreshortened, and close-up vision is impaired.
Light from the aqueous humor next enters the crystalline lens, a convex disc suspended on threads just behind the iris. The iris is a muscle that expands and contracts to regulate the amount of light entering the posterior chamber of the eye through the pupil. If the light is too bright, the iris expands so that the size of the pupil shrinks. If there is too little light, the iris contracts
to enlarge the pupil and allow more light into the eye. The threads holding the lens and the ciliary body to which they are connected automatically tug at the lens to change its shape to help focus on items near or far.
After the light has been bent by the crystalline lens, it enters the vitreous humor, a gel-like mass that fills the large posterior chamber of the eye. The vitreous humor presses against the inner layer of the eye, maintaining the eyeball's shape and keeping the blood-rich choroid layer in contact with the retina. The light is placed upon the retina's rods and cones like a projected image at a movie theater, and these images are transmitted via the optic nerve to the brain.
Each eye has six muscles that direct the gaze up and down and from side to side.
To trace again the refraction path: Light travels from cornea to aqueous humor to lens to vitreous humor to retina. The stability of the eyeball and the refractive elements must all be perfect for vision to be 20-20.
The lacrimal system produces tears in glands behind the eyebrows. These tears flow through ducts into the eyes where they drain out the lacrimal puncta, or flow into the nose.
The visual field can be affected by many things: Blood, foreign bodies, or other tissue can obstruct the pathway to the retina. Examples include excessive skin on the eyelids, shielding a portion of the eye from light; a cloudy condition in any of the refractive properties of the eye, or damage to the retina.
The Ear: Introduction and Anatomy Structures of the Ear