Sense of Sight
• The eye is the organ of vision. The retina is covered with two basic types of
light-sensitive cells-rods and cones. The cone cells are sensitive to color
and are located in the part of the retina called the fovea, where the light is
focused by the lens. The rod cells are not sensitive to color, but have
greater sensitivity to light than the cone cells. These cells are located
around the fovea and are responsible for peripheral vision and night
vision. The eye is connected to the brain through the optic nerve. The
point of this connection is called the "blind spot" because it is insensitive
to light. Experiments have shown that the back of the brain maps the
visual input from the eyes.
• The brain combines the input of our two eyes into a single three-
dimensional image. In addition, even though the image on the retina is
upside-down because of the focusing action of the lens, the brain
compensates and provides the right-side-up perception. Experiments have
been done with subjects fitted with prisms that invert the images. The
subjects go through an initial period of great confusion, but subsequently
they perceive the images as right side up.
Sense of Smell
• Olfaction or olfactory perception is
the sense of smell. This sense is mediated by
specialized sensory cells of the nasal cavity of
vertebrates, which can be considered
analogous to sensory cells of the antennae of
invertebrates. In humans, olfaction occurs
when odorant molecules bind to specific sites
on the olfactory receptors. These receptors
are used to detect the presence of smell
Sense of Taste
• Humans detect taste with taste receptor cells. These
are clustered in taste buds and scattered in other areas
of the body. Each taste bud has a pore that opens out
to the surface of the tongue enabling molecules and
ions taken into the mouth to reach the receptor cells
• There are five primary taste sensations:salty
Sense of Hearing
• Like your other sense organs, your ears are extremely well-
designed. In fact, they serve two very important purposes. Do you
know what they are? You were probably able to figure out that your
ears help you to hear sounds, but what you probably did not know
is that your ears also help you to keep your balance.How You Hear
When an object makes a noise, it sends vibrations (better known as
sound waves) speeding through the air. These vibrations are then
funneled into your ear canal by your outer ear. As the vibrations
move into your middle ear, they hit your eardrum and cause it to
vibrate as well. This sets off a chain reaction of vibrations. Your
eardrum, which is smaller and thinner than the nail on your pinky
finger, vibrates the three smallest bones in your body: first, the
hammer, then the anvil, and finally, the stirrup. The stirrup passes
the vibrations into a coiled tube in the inner ear called the cochlea.
Sense of Touch
• While your other four senses (sight, hearing, smell,
and taste) are located in specific parts of the body,
your sense of touch is found all over. This is because
your sense of touch originates in the bottom layer of
your skin called the dermis. The dermis is filled with
many tiny nerve endings which give you information
about the things with which your body comes in
contact. They do this by carrying the information to
the spinal cord, which sends messages to the brain
where the feeling is registered.
• The nerve endings in your skin can tell you if something
is hot or cold. They can also feel if something is hurting
you. Your body has about twenty different types of
nerve endings that all send messages to your brain.
However, the most common receptors are heat, cold,
pain, and pressure or touch receptors. Pain receptors
are probably the most important for your safety
because they can protect you by warning your brain
that your body is hurt!
• Some areas of the body are more sensitive than others
because they have more nerve endings. Have you ever
bitten your tongue and wondered why it hurt so much?
It is because the sides of your tongue have a lot of
nerve endings that are very sensitive to pain. However,
your tongue is not as good at sensing hot or cold. That
is why it is easy to burn your mouth when you eat
something really hot.
What part of the body do you use
when you want to smell flowers?
Do you use your ears to hear the
coming of the train?
Draw a check to the part of the body
that you use for seeing.