[Illustration:  Fig. 10.—­Showing the action of the brachialis anticus in the arm of an anthropoid ape.]

In the human machine the levers of the arm have been fashioned, not for climbing, but for work of another kind—­the kind which brings us a livelihood.  We must have perfect control over our hands; the longer the lever of the forearm is made, the more difficult does control of the hand become.  Hence, in the human machine the forearm is made relatively short and the upper arm long.

We have just seen that the brachial muscle could at one time move the forearm and hand, but that when they are fixed it could then use the humerus as a lever and thereby lift the weight of the body.  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.  Like the muscles of the foot, the fixers of the wrist may become overworked and exhausted, as occasionally happens in men and women who do not hold their pens correctly and write for long spells day after day.  The break-down which happens in them is called “writer’s cramp,” but it is a disaster of the same kind as that which overtakes the foot when its arch collapses, and its utility as a lever is lost.

FOOTNOTES: 

[Footnote 1:  From The Engines of the Human Body, Chapters VI and VII.  J.B.  Lippincott Company, Philadelphia, 1920; Williams and Norgate, London, 1920.]

THE EXPOSITION OF A MACHINE

THE MERGENTHALER LINOTYPE[2]

Philip T. Dodge

The Mergenthaler Linotype machine appeared in crude form about 1886.  This machine differs widely from all others in that it is adapted to produce the type-faces for each line properly justified on the edge of a solid slug or linotype.

These slugs, automatically produced and assembled by the machine, are used in the same manner as other type-forms, whether for direct printing or for electrotyping, and are remelted after use.