A Book of Exposition | Page 7

man has invented, but
from which the human machine is almost completely free. We can
illustrate the defect best by comparing the movements of the heel with
those of the crank-pin of an engine. One serves as the lever by which
the gastrocnemius helps to propel the body; the other serves the same
purpose in the propulsion of a motor cycle. On referring to Fig. 7, A,
the reader will see that the piston-rod and the crank-pin are in a straight
line; in such a position the engine is powerless to move the crank-pin

until the flywheel is started, thus setting the crank-pin in motion. Once
started, the leverage increases, until the crank-pin stands at right angles
to the piston-rod--a point of maximum power which is reached when
the piston is in the position shown in Fig. 7, B. Then the leverage
decreases until the second dead centre is reached (Fig. 7, C); from that
point the leverage is increased until the second maximum is reached
(Fig. 7, D), whereafter it decreases until the arrival at the first position
completes the cycle. Thus, in each revolution there are two points
where all leverage or power is lost, points which are surmounted
because of the momentum given by the flywheel. Clearly we should get
most out of an engine if it could be kept working near the points of
maximum leverage--with the lever as nearly as possible at right angles
to the crank-pin.
[Illustration: Fig. 7.--Showing the crank-pin of an engine at: A, First
dead centre. B, First maximum leverage. C, Second dead centre. D,
Second maximum leverage.]
Now, we have seen that the tendon of Achilles is the piston cord, and
the heel the crank-pin, of the muscular engine represented by the
gastrocnemius and soleus. In the standing posture the heel slopes
downwards and backwards, and is thus in a position, as regards its
piston cord, considerably beyond the point of maximum leverage. As
the heel is lifted by the muscles, it gradually becomes horizontal and at
right angles to its tendon or piston cord. As the heel rises, then, it
becomes a more effective lever; the muscles gain in power. The more
the foot is arched, the more obliquely is the heel set and the greater is
the strength needed to start it moving. Hence, races like the European
and Mongolian, which have short as well as steeply set heels, need
large calf muscles. It is at the end of the upward stroke that the heel
becomes most effective as a lever, and it is just then that we most need
power to propel our bodies in a forward direction. It will be noted that
the heel, unlike the crank-pin of an engine, never reaches, never even
approaches, that point of powerlessness known to engineers as a dead
centre. Work is always performed within the limits of the most
effective working radius of the lever. It is a law for all the levers of the
body; they are set and moved in such a way as to avoid the occurrence

of dead centres. Think what our condition would have been were this
not so; why, we should require revolving fly-wheels set in all our
joints!
[Illustration: Fig. 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. Some of these are shown in Fig. 8. The foot, then, is a
lever of a very remarkable kind; all the time we stand or walk, its
rigidity, its power to serve as a lever, has to be maintained by an
elaborate battery of muscular engines all kept constantly at work. No
wonder our feet and legs become tired when we have to stand a great