8.--The arch of the foot from the inner side,
showing some of the muscles which maintain it.]
Another property is essential in a lever: it must be rigid; otherwise it
will bend, and power will be lost. Now, if the foot were a rigid lever,
there would be missing two of its most useful qualities. It could no
longer act as a spring or buffer to the body, nor could it adapt its
sole to the various kinds of surfaces on which we have to tread or
stand. Nature, with her usual ingenuity, has succeeded in combining
those opposing qualities--rigidity, suppleness, and elasticity or
springiness--by resorting to her favorite device, the use of muscular
engines. The arch is necessarily constructed of a number of bones which
can move on each other to a certain extent, so that the foot may adapt
itself to all kinds of roads and paths. It is true that the bones of the
arch are loosely bound together by passive ties or ligaments, but as
these cannot be lengthened or shortened at will, Nature had to fall back
on the use of muscular engines for the maintenance of the foot as an
arched lever.
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