*The Lever.*—The lever may be described as a stiff bar which turns about a fixed point of support, called the _fulcrum_. The force applied to the bar to make it turn is called the _power_, and that which is lifted or moved is termed the _weight_. The weight, the power, and the fulcrum may occupy different positions along the bar and this gives rise to the three kinds of levers, known as levers of the first class, the second class, and the third class (Fig. 115). In levers of the _first class_ the fulcrum occupies a position somewhere between the power and the weight. In the _second class_ the weight is between the fulcrum and the power. In the _third class_ the power is between the fulcrum and the weight. [Fig. 115] Fig. 115—*Classes of levers. I.* Two levers of first class showing fulcrums in different positions. II. Lever of second class. III. Lever of third class. _F._ Fulcrum. _P._ Power. _W._ Weight. _a._ Power-arm. _b._ Weight-arm. *Application to the Body.*—In the body the bones serve as levers; the turning points, or fulcrums, are found at the joints; the muscles supply the power; and parts of the body, or things to be lifted, serve as weights. For these levers to _increase_ the motion of the muscles, it is necessary that the muscles be attached to the bones _near the joints_, and that the parts to be moved be located at some distance from the joints. In other words the (muscle) power-arm must be _shorter_ than the (body) weight-arm.(86) Examining Fig. 116, it is seen that the distances moved by the power and weight vary as their respective distances from the fulcrum. That is to say, if the weight is twice as far from the fulcrum as the power, it will move through twice the distance, and if three times as far, through three times the distance. Thus the muscles, by acting through short distances (on the short arms of levers), are able to move portions of the body (located on the long arms) through long distances. Can all three classes of levers be used in this way in the body? [Fig. 116] Fig. 116—*Motion producing levers.* Diagrams show relative distances moved by the power and weight in levers having the power nearer the fulcrum than is the weight. _F._ Fulcrum. _P, P’._ Power. _W, W’._ Weight.