*Classes of Levers found in the Body.*—Practically all of the levers of the body belong either to the first class or the third class. In both of these the muscle power can be applied to the short arm of the lever, thereby moving the body weight through a longer distance than the muscle contracts (Fig. 116). In the levers of the second class, however, the weight occupies this position, being situated _between_ the power and fulcrum (Fig. 117). The weight, therefore, _cannot_ move farther than the power in this lever. It must always move a shorter distance. While such a lever is of great advantage in lifting heavy weights outside of the body, it cannot be used for increasing the motion of the muscles. For this reason no well-defined levers of the second class are present in the body.(87) [Fig. 117] Fig. 117—*Weight lifting levers.* Diagrams show relative distances moved by the power and weight in levers having the weight nearer the fulcrum than is the power. _F._ Fulcrum. _P, P’._ Power. _W, W’._ Weight. [Fig. 118] Fig. 118—*Diagram of the foot lever.* _F._ Fulcrum at ankle joint. _W._ Body weight expressed as pressure against the earth. While the muscle power acts through the distance _ab_, the fulcrum support (body) is forced through the distance _FE_. *Loss of Muscular Force.*—Using a small spring balance for measuring the power, a light stick for a lever, and a small piece of metal for a weight, and arranging these to represent some lever of the body (as the forearm), it is easily shown that the gain in motion causes a corresponding loss in muscular power. (See Practical Work.) If, for example, the balance is attached two inches from the fulcrum and the weight twelve inches, the pull on the balance is found to be six times greater than the weight that is being lifted. If other positions are tried, it is found that the power exerted in each case is as many times greater than the weight as the weight-arm is times longer than the power-arm. Applying this principle to the levers of the body, it is seen that the gain in motion is at the expense of muscular force, or, as we say, _muscular force is exchanged for motion_. This exchange is greatly to the advantage of the body; for while the ability to lift heavy weights is important, the ability to move portions of the body rapidly and through long distances is much more to be desired. *Important Muscles.*—There are about five hundred separate muscles in the body. These vary in size, shape, and plan of attachment, to suit their special work. Some of those that are prominent enough to be felt at the surface are as follows: _Of the head_: The _temporal_, in the temple, and the _masseter_, in the cheek. These muscles are attached to the lower jaw and are the chief muscles of mastication.