Our Spirit-levels

Sources: A Handbook Of Health

The Sixth Sense. Though we usually speak of having five

senses,--sight, smell, hearing, touch, and taste,--we really have also a

sixth--the sense of direction, or of balance. The machine of this

sense is comparatively simple, being made up of three tiny curved tubes,

which, from their shape, are called the semi-circular canals. These

are buried in the same bone of the skull as the internal ear, and

so close to it that they were at one time described as part of it.

These little canals are three in number, one for each of the

dimensions--length, breadth, and thickness,--so that whichever way the

head or body is moved,--backward and forward, up and down, or from side

to side,--the fluid with which they are filled will change its level in

one of them, just as the bead does in the carpenter's spirit-level

that you can find in any tool shop. The delicate nerve twigs that run

out into the fluid in these tiny canals are gathered together into a

bundle, or nerve-cable, which runs back to the part of the brain known

as the cerebellum or hind-brain, which has most to do with controlling

the balance and movements of our bodies.

It is the disturbance set up in these spirit-level canals by the

pitching and rolling of a ship, which makes us seasick. Neither the

stomach, nor anything that we may have eaten, has anything to do with

it. In the same way we sometimes become sick and dizzy from swinging too

long or too high, or from riding on the cars.