d that injury to these structures brought lack of equilibrium
and inability to walk, swim or fly in a straight course. If, for
example, the horizontal canal in the left ear is destroyed, the animal
continually deviates to the left as he advances, and so is forced into
a "circus movement". They found that the compensatory movements
normally made in reaction to a movement impressed on the animal from
without were no longer made when the canals were destroyed. They found
that something very much like these compensatory movements could be
elicited by direct stimulation of the end-organs in the canals or of
the sensory nerves leading from them. And they found that little
currents of the liquid filling the canals acted as a stimulus to these
end-organs and so aroused the {237} compensatory movements. They were
thus led to accept a view that was originally suggested by the
position of the canals in space.

[Illustration: Fig. 40.--How the sense cells in a semicircular canal
are stimulated by a water current. This current is itself an inertia
back-flow, resulting from a turning of the head in the opposite
direction. (Figure text: water current, nerve to brain)]

Each "semicircular" canal, itself considerably more than a
semicircular tube, opens into the vestibule at each end and thus
amounts to a complete circle. Therefore rotating the head must, by
inertia, produce a back flow of the fluid contents of the canal, and
this current, by bending the hairs of the sense cells in the canal,
would stimulate them and give a sensation of rotation, or at least a
sensory nerve impulse excited by the head rotation.

When a human subject is placed, blindfolded, in a chair that can be
rotated without sound or jar, it is found that he can easily tell
whenever you start to turn him in either direction. If you keep on
turning him at a constant speed, he soon ceases to sense the movement,
but if then you stop him, he says you are starting to turn him in the
opposite {238} direction. He senses the beginning of the rotary
movement because this causes the back flow through his canals; he
ceases to sense the uniform movement because friction of the liquid in
the slender canal soon abolishes the back flow by causing the liquid
to move with the canal; and he senses the stopping of this movement
because the liquid, again by inertia, continues to move in the
direction it had been moving just before when it was keeping pace with
the canal. Thus we