What should
we think of a metal engine which could reverse its action so that it
could act through its piston-rod at one time and through its cylinder at
another? Yet that is what a great number of the muscular engines of the
human machine do every day.
There is another little point, but an important one, which I must
mention before this chapter is finished. I have spoken of the forearm
and hand as if they formed a single solid lever. Of course that is not
so; there are joints at the wrist where the hand can be moved on the
forearm. But when a weight is placed in the hand, these joints became
fixed by the action of muscles. The fixing muscles are placed in the
forearm, both in front and behind, and are set in action the moment the
hand is loaded. The wrist joint is fixed just in the same way as the
joints of the foot are made rigid by muscles when it has to serve as a
lever. Even when we take a pen in our hand and write, these engines
which balance and fix the wrist have to be in action all the time. The
steadiness of our writing depends on how delicately they are balanced.
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