A Book of Exposition | Page 7

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 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