A Book of Exposition | Page 8

deal. Some of these engines, the larger ones, are kept in the leg, but
their tendons or piston cords descend below the ankle-joint to be fixed
to various parts of the arch, and thus help to keep it up (Fig. 8). Within
the sole of the foot has been placed an installation of seventeen small
engines, all of them springing into action when we stand up, thus
helping to maintain the foot as a rigid yet flexible lever.
We have already seen why our muscles are so easily exhausted when
we stand stock-still; they then get no rest at all. Now, it sometimes
happens in people who have to stand for long periods at a stretch that
these muscular engines which maintain the arch are overtaxed; the arch

of the foot gives way. The foot becomes flat and flexible, and can no
longer serve as a lever. Many men and women thus become
permanently crippled; they cannot step off their toes, but must shuffle
along on the inner sides of their feet. But if the case of the overworked
muscles which maintain the arch is hard in grown-up people, it is even
harder in boys and girls who have to stand quite still for a long time, or
who have to carry such burdens as are beyond their strength. When we
are young, the bony levers and muscular engines of our feet have not
only their daily work to do, but they have continually to effect those
wonderful alterations which we call growth. Hence, the muscular
engines of young people need special care; they must be given plenty
of work to do, but that kind of active action which gives them alternate
strokes of work and rest. Even the engine of a motor cycle has three
strokes of play for one of work. Our engines, too, must have a liberal
supply of the right kind of fuel. But even with all those precautions, we
have to confess that the muscular engines of the foot do sometimes
break down, and the leverage of the foot becomes threatened. Nor have
we succeeded in finding out why they are so liable to break down in
some boys and girls and not in others. Some day we shall discover this
too.
We are now to look at another part of the human machine so that we
may study a lever of the third order. The lever formed by the forearm
and hand will suit our purpose very well. It is pivoted or jointed at the
elbow; the elbow is its fulcrum (Fig. 9 B). At the opposite end of the
lever, in the, upturned palm of the hand, we shall place a weight of 1 lb.
to represent the load to be moved. The power which we are to yoke to
the lever is a strong muscular engine we have not mentioned before,
called the brachialis anticus, or front brachial muscle. It lies in the
upper arm, where it is fixed to the bone of that part--the humerus. It is
attached to one of the bones of the forearm--the ulna--just beyond the
elbow.
In the second order of lever, we have seen that the muscle worked on
one end, while the weight rested on the lever somewhere between the
muscular attachment and the fulcrum. In levers of the third order, the
load is placed at the end of the lever, and the muscle is attached

somewhere between the load and the fulcrum (Fig. 9 A). In the
example we are considering, the brachial muscle is attached about half
an inch beyond the fulcrum at the elbow, while the total length of the
lever, measured from the elbow to the palm, is 12 inches. Now, it is
very evident that the muscle or power being attached so close to the
elbow, works under a great disadvantage as regards strength. It could
lift a 24-lb. weight placed on the forearm directly over its attachment as
easily as a single pound weight placed on the palm. But, then, there is
this advantage: the 1-lb. weight placed in the hand moves with
twenty-four times the speed of the 24-lb. weight situated near the elbow.
What is lost in strength is gained in speed. Whenever Nature wishes to
move a light load quickly, she employs levers of the third order.
[Illustration: Fig. 9A.--A chisel used as a lever of the third order. W,
weight; P, power; F, fulcrum.]
We have often to move our forearm very quickly, sometimes to save
our lives. The difference of one-hundredth of a second may mean life
or death to us on the face of a cliff when we clutch at a branch or
jutting rock to save a fall. The quickness of a blow we give or fend
depends on the length of our reach.