THE OTHER SENSES
Man has about 3,000 taste buds. They are
mainly on the tongue, although there are a few on the palate, tonsils, and
pharynx. There are four primary or basic tastes sensations-sweet, bitter, sour
or acid, and salt. You can't taste all flavors on all parts of the tongue.
Sweet flavors register near the tip, sour on the sides, bitter on the back, and
salty all over. The sense of smell is located in odor receptors in the upper
passage of the nasal cavity.
The size of the membrane containing the odor receptors
is only about one-fourth square inch in man as against an area 40 times as
great in the dog. The organ of smell, which can detect things at a distance, is
obviously more important as a danger warning system in animals than in man.
It's because of the location of the receptors that you may not smell delicate
odors at first.
It takes several whiffs to get the odor into the upper nasal
passage. Before you can taste anything, the substance must be moistened, and
the salivary glands supply the moisture. And to be smelled, an odor must be
dissolved in the mucus secreted by the nasal membranes. Smell receptors in man,
although they do not have the same capacity as in lower animals, still are
sensitive enough to allow you to detect a substance diluted to as much as one
part in 30 billion.
No special care is required to guard the senses of taste
and smell. You may wish to read, in connection with these senses, the section
dealing with care of the mouth and the nose. Touch, sometimes called the fifth
sense, is actually five senses: touch, pressure, pain, heat, and cold. Skin:
sensations are registered in nerve endings all over the body. Nerve fibers
carry them as impulses to the spinal cord and then to the brain where all these
feelings register. If you place your hand lightly on any object, the first
sensation is touch.
Press harder and you sense pressure. And if the object has a
rough surface and you press hard enough, you may feel pain. The senses are
closely related though distinct from each other. Also in the skin are separate
nerve endings to register heat and cold, which is absence of heat. A discussion
of sense organs could go much further but would serve no useful purpose here.
For example, you can feel the pain of a stomach- ache, but you can also feel
hunger, which is quite different. You can also feel thirst, which is not among
the sensations classically classified. Some investigators have suggested that
the senses might well be divided into a dozen or more categories. In addition
to the usual five-sight, hearing, taste, smell, and touch-pressure, heat, cold,
and pain deserve individual categories, and so, too, the ability to sense
vibration, position, and equilibrium.
