[Illustration: Fig. 6.--The bones forming the arch of the foot, seen
from the inner side.]
If we had the power to make our heels longer or shorter at will, we
should be able, as is the case in a motor cycle, to alter our
"speed-gear" according to the needs of the road. With a steep hill in
front of us, we should adopt a long, slow, powerful heel; while going
down an incline a short one would best suit our needs. With its
four-change speed-gear a motor cycle seems better adapted for easy and
economical travelling than the human machine. If, however, the human
machine has no change of gear, it has one very marvellous
mechanism--which we may call a _compensatory_ mechanism, for want of a
short, easy name. The more we walk, the more we go hill-climbing, the
more powerful do the muscular engines of the heel become. It is quite
different with the engine of a motor cycle; the more it is used, the
more does it become worn out. It is because a muscular engine is living
that it can respond to work by growing stronger and quicker.
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