All are familiar with the "five senses" of our elementary physiologies, sight,
hearing, taste, smell, and touch. A more complete study of sensation reveals
nearly three times this number, however. This is to say that the body is equipped
with more than a dozen different kinds of end-organs, each prepared to receive
its own particular type of stimulus. It must also be understood that some of the
end-organs yield more than one sense. The eye, for example, gives not only
visual but muscular sensations; the ear not only auditory, but tactual; the tongue
not only gustatory, but tactual and cold and warmth sensations.
Sight.—Vision is a distance sense; we can see afar off. The stimulus is chemical
in its action; this means that the ether waves, on striking the retina, cause a
chemical change which sets up the nerve current responsible for the sensation.
The eye, whose general structure is sufficiently described in all standard
physiologies, consists of a visual apparatus designed to bring the images of
objects to a clear focus on the retina at the fovea, or area of clearest vision, near
the point of entrance of the optic nerve.
The sensation of sight coming from this retinal image unaided by other
sensations gives us but two qualities, light and color. The eye can distinguish
many different grades of light from purest white on through the various grays to
densest black. The range is greater still in color. We speak of the seven colors of
the spectrum, violet, indigo, blue, green, yellow, orange, and red. But this is not
a very serviceable classification, since the average eye can distinguish about
35,000 color effects. It is also somewhat bewildering to find that all these colors
seem to be produced from the four fundamental hues, red, green, yellow, and
blue, plus the various tints. These four, combined in varying proportions and
with different degrees of light (i.e., different shades of gray), yield all the color
effects known to the human eye. Herschel estimates that the workers on the
mosaics at Rome must have distinguished 30,000 different color tones. The hue
of a color refers to its fundamental quality, as red or yellow; the chroma, to its
saturation, or the strength of the color; and the tint, to the amount of brightness
(i.e., white) it contains.
Hearing.—Hearing is also a distance sense. The action of its stimulus is
mechanical, which is to say that the vibrations produced in the air by the
sounding body are finally transmitted by the mechanism of the middle ear to the
inner ear. Here the impulse is conveyed through the liquid of the internal ear to
the nerve endings as so many tiny blows, which produce the nerve current