The five generally recognized senses are: sight, hearing, taste, touch
and smell. These depend upon certain sense-organs, (the eye, ear,
etc.), and also upon specific sense-centers in the brain. The structure
and physiology of these organs can be found in any good text book
on the subject. Suffice it to say that, in the case of the eye, for
example, certain incoming ether vibrations are converted, by the rods
and cones in the retina of the eye (by a wholly mysterious process)
into nerve-currents, which are transmitted to the sight-centers in the
brain, wherein the sensation of sight occurs. The ear converts sound
waves into nerve-currents, which we then perceive as sound, etc. Touch
depends upon nerve-currents from the periphery of the body. Smell and
taste are closely related; in fact, there are only four primary tastes
which are directly conveyed by the nerves of the tongue. These are
sweet, salt, sour and bitter. All our other “tastes” are dependent
upon the sense of smell. That is why, when we have a “cold,” and cannot
smell anything, food seems to have no taste. The primary tastes are
obtained through the “taste buds” in the tongue. Smell is a sense about
which relatively little is known.
In addition of these sensations, arising from the sense organs, there
are also other senses, such as the muscular sense, the temperature
sense, etc. To what extent these may be properly classed as separate
senses has been disputed. The so-called “sixth sense” is theoretically
a sort of “second sight,” of supernormal origin, which orthodox
psychology does not at present recognize.
The sensation of motion should perhaps be mentioned in this place. Such
sensations are of two kinds: (1) sensations of objects moving over our
sensory surfaces; and (2) sensations of our whole person’s translation
through space.
The former of these has been much studied, though little definite can
be said regarding it here. Curiously, while the sensation of a moving
object can be accurately determined, it is most difficult to locate
a number of stationary objects on the skin (if the eyes are closed).
Thus, if one places the finger-tips against the chest, one cannot tell
how many fingers are touching (after the first three). Certain areas
of the skin are also quite insensitive; anaesthetic zones of patches
sometimes existing,--especially in hysterical subjects, and these can
be pricked without involving any reaction. These figure largely in the
“witchcraft trials,” of the middle ages, for it was held that the
witch was insensible to pain wherever the Devil had touched her. Such
spots were, therefore, searched for with long needles!
As to the sensation of movement through space, we often experience
this at night, in our so-called “flying dreams.” (See my little book
in this series on “Dreams.”) The Hindus contend, however, that this
is an actual possibility; that the physical body can really be lifted
or “levitated,” by means of proper breathing exercises, etc. I have
discussed this question, briefly, in my volume in this series devoted
to “Yoga Philosophy.”