They are the most familiar things which in our science become the
strangest. _Not_ to know where you are when seasick, still less where
your mind is, is common enough. Our next experiment will trace our
power to know where sounds are to the same origin as seasickness.
Seasickness starts in the ear. In its cavity are three small tubes,
each bent in a circle, and filled with fluid. The three sit at right
angles to each other, like the three sides at the corner of a room or
a box. Consequently, in whatever direction the head is moved, the
fluid in some one of the tubes is given a circular motion. Hanging
out into the tubes, from their sides, are hairs or _cilia_, which
connect with nerve cells and fibres that branch off from the auditory
nerve. When the head moves the fluid moves, the hairs move, the cells
are "fired off," a nervous current is sent up to the brain, and a
feeling of the head's peculiar motion is consequent.
As for seasickness: this nerve current, on its way to the brain, at
one point runs beside the spot or "centre" where the nerve governing
the stomach has its origin. When the rocking of the head is abnormally
violent and prolonged, the stimulus is so great that the current leaks
over into this adjoining "centre," and so excites the nerve running to
the stomach as to cause wretchedness and retching. Deaf mutes, whose
ear "canals" are affected, are never seasick.
But normally the amount of ear-feeling which we get by reason of
moving our head in a particular direction comes in a curious way to be
a measure of the direction of sound. The feelings we get from our skin
and muscles in turning the head play a similar _role_. We turn our ear
to catch a sound. We do this so frequently for every point, that in
time we learn to judge the direction of the sound by the way we would
have to turn the head in order to hear the sound best. Thereafter we
do not have to turn the head to get the direction, for we now remember
the proper feeling and know it. This memory of the old feeling _is_
our idea of the present direction. If we never moved our heads we
never could have any such notion of the location of sounds as at
present--perhaps none whatever.