c. Two Types of Senses. They are differentiated by the type of sensation they
cause. Special senses are produced by receptors limited to small areas such as the
tongue, nose, balance, hearing, vision, smell, and taste. General senses are produced
by receptors scattered throughout the body such as pressure, temperature, pain,
position, and touch.
d. Sensation and Perception. In its broadest meaning, sensation refers to
man's state of being aware of external or internal conditions of the body. The state of
being aware of something through the senses is perception. Four conditions must take
place for a sensation to occur.
(1) A stimulus--a change in the environment which causes a response by
the nervous system.
(2) A receptor or sense organ-- picks up a stimulus and converts it to a
(3) Conduction--the impulse must be conducted from the receptor or sense
organ along a pathway to the brain.
(4) Translation--the impulse must be translated into a sensation when the
impulse is in a region of the brain.
Section II. THE SPECIAL SENSE OF VISION (SIGHT)
ANATOMY OF THE EYE
a. Structure/Location. The size of the adult eyeball is about 2.5 cm (1 inch) in
diameter. The eyes are recessed in the bony cavities (the orbital cavities) of the skull
with only the front one-sixth of the eyeball's total surface area exposed. Bones protect
each eyeball: the frontal, maxillary, zygomatic, lacrimal, sphenoid, and palatine bones.
b. Tunics (Layers) of the Eyeball. The eyeball is composed of three layers
called tunics: the fibrous tunic, the vascular tunic, and the retina or nervous tunic.
(1) Fibrous tunic. The fibrous tunic is the outer layer of the eye. This layer
is formed by the sclera (the white of the eye) and the cornea (the window of the eye).
The sclera is a white coat of fibrous tissue that covers the entire eyeball except the front
colored portion. The sclera gives the eyeball shape and protects the inner parts of the
eyeball. The cornea covers the iris, the colored part of the eye. The cornea is a
transparent fibrous layer which has no blood vessels. An epithelial layer covers the
outer surface of the cornea. At the point where the sclera and the cornea meet is a
venous sinus called the Canal of Schlemm.