The Sensory Organs
The five sensory organs of man
1:- Eye for sight.
2:- Ear for hearing.
3:- Nose for smell.
4:- Skin for touch.
5:- Tongue for taste.
These are marvels of nature indeed. Man can change his environment,
make inventions and advance civilization through the use of these sense organs and
his brain power.
The eye is responsible for one of the greatest miracle of human body-sight.
It gathers 80 percent of all the knowledge for him.
It is the organ that focuses light from external objects on a screen (Retina),
to produce distinctive responses that are transmitted to the brain for interpretation.
Behind a transparent window (Cornea) in the tough outer covering.
(Sclera) of the human eye (Fig) is a lens, whose curvature and hence focal
length can be varied by muscles, This lens is curtained in front by the iris,
which has as its center a circular opening (pupil) able to enlarge or contract in
order to regulate the amount of light passing through the lens to the retina.
Covering an area of about 2 sq. cm, the retina contains 137 million light-sensitive receptor
cells : 130 million shaped like rods for black-and-white vision ; 7 million shaped like cones
for colour vision.
Never fibers from each of the light-sensitive cells lead to
the optic disc, where they unit to form the optic nerve.
The optic nerves from both eyes join at the base of the brain, where some fibers from
the two retinas cross to the opposite sides and so link both eyes to both sides of the brain.
Structural abnormalities of the eyes impair the efficiency of the optical system,
mainly by errors of refraction in focusing, e.g. short-sightedness results from a long
eyeball focusing in front of the retina instead of on it, and long-sightedness from
a short eyeball focusing behind the retina.
The ear is respecter apparatus (Fig.) for the sense of hearing ,
(a) the external ear with its ear drum, beyond which lies
(b) the middle ear, a bony cavity shaped like an oblong box, in which are three
small articulated bones, the malleus (Hammer), the incus (Anvil), and the stapes
(Stirrup), which transmit movements of the drum to
(c) the inner ear, a labyrinth of bone in front of which is the cochlea,
shaped like a minute spiral snail shell, in which lies the organ of Corti
( the actual organ of hearing). The mechanism of hearing is actuated by sound waves,
which cause the ear-drum to vibrate, the vibrations being transmitted by the three bones in
the middle ear to the organ of Corti, whence the auditory nerve carries
the impulses to the brain, where the sensory stimuli are appreciated as sound.
In the inner ear there are also three semicircular canals which have nothing to do with hearing.
They are also filled with a fluid, and they give a sense of balance.
If they are out of order, we become dizzy and cannot walk straight.
Thus the human ear provides a telephone service and prevents him from toppling over.
The Nose And Throat
The human nose is an organ of smell, respiration, and some beauty.
It consists of a few small bones, some cartilage, lined on the outside with skin
and on the inside with a sensitive mucus membrane generously endowed with blood vessels.
Together they form two hollow cavities (Nostrils) divided by a septum of bone and cartilage.
The upper part contains the end organs of the olfactory nerve and provides the sense of smell,
which is closely allied with the sense of taste.
The lower part, lined with hairs, serves to filter and warm inhaled air before it proceeds into
the rest of the respiratory tract. A sensitive nose can recognize up to 10,000 different scents.
The nose communicates with the Eyes, Ears and Throat, as well as the so-called nasal
and paranasal sinuses, which are air spaces in the bones of the skull. (Fig.).
The tear ducts of the eve drain eventually into the nose ; that is why a person who is
crying has to blow his nose. The communication with the ears is by way of the Eustachian Tubes,
which open in the back of the throat.
The back of the nose opens into the upper part of the throat just about at the point
where adenoid tissue commonly grows. When you have a cold, always blow your nose gently in
order to avoid infectious material getting into your throat, the Eustachian tubes (and the ear),
and the eyes.
The tongue (Fig. 4 ) is 10 cm. long and weighs 56 8. It is composed of highly mobile
muscles covered with a special kind of mucus membrane that includes taste-buds.
It is richly supplied with nerves and blood vessels, and is essential to
the production of speech, to the swallowing of food, and to the sense of taste.
The four basic tastes, which everybody can discriminate are
(1) sweets, appreciated at the tip of the tongue,
(2) salt in the middle,
(3) aid on the sides and
(4) bitter at the back of the tongue.
It has five nerve-supply branches on each side, coming from the cranial nerves.
Tongue signs reflect many kinds of diseases.
The skin is really the largest organ in the body and one of the most complex.
It has an area of about 1.581 sq. m (17 sq. ft) and weighs about 2.27 kg.(5 lb).
The epidermis, or outer skin, consists of two layers-the horny top layer of dry dead cells,
constantly being shed, and the growing layer that replaces the dead cells with new cells.
This growing layer includes the pigmented cells called melanocytes, which help to determine
the skin color, depending upon the melanin content. Beneath the epidermis, and several times
thicker, is the dermis, or true skin, consisting of tough, fibrous, living tissue containing
millions of capillary blood vessels, lymph vessels nerve endings, sebaceous glands and sweat
glands. Hair follicles also start here. Fingers and toe nails, the hair on the head and body ,
the callus of your soles, moles and warts are modifications of the skin. Some 2000 diseases of
skin are known. Normally the skin produces vitamin D, activates male sex hormone, keeps water
in and out, maintains body temperature at 37 C excrete sweet, and detect pain. Touch, heat,
cold, and pass on to brain.