The general receptors distribute all over the skin. They are usually
grouped together as sensation. The special receptors locate only at
certain part of the body in the head. Altogether, they are referred
to as the five senses. The followings present a further break down
into components, and functions.
Sclera - This is the white of the eye. It is a tough
whitish sheath covering most of the eye (see Figure 11).
The inter-cellular space contains a kind of loose
proteins composed of collagen and elastic fibers, which
make the wall of the eyeball more pliant.
Cornea - It is almost perfectly transparent at the
front of the sclera for the protection of the eye. Light
rays is refracted through this dome-shaped structure.
Choroid - The soft layer inside the sclera. It has
many blood vessels that nourish the eye. It also absorbs
Ciliary body - The choroid becomes suspensory
ligaments near the opening. It holds the lens in place,
and controls the shape of the lens for near and far
Iris - Further toward the opening, the choroid becomes a
thin, circular, muscular diaphragm known as the iris, which
regulates the size of a center hole (the pupil) and thus
controls the amount of light entering into the eye. The pupil
gets larger in dim light and smaller in bright light.
Aqueous humor - This is the anterior cavity between the
cornea and the lens. It is filled with an alkaline, watery
solution secreted by the ciliary body. Normally, the solution
leaves the anterior cavity by way of tiny ducts that are located
where the iris meets the cornea. When a person has glaucoma,
these drainage ducts are blocked, and aqueous humor builds up.
The resulting pressure compresses arteries that serve the nerve
fibers of the retina. The nerve fibers begin to die due to lack
of nutrients, and the person becomes partially blind.