end by the application of power at the other, with a block as a fulcrum,
is a familiar example of this class. There are several examples of this in
the human body. The head supported on the atlas is one. The joint between
the atlas and the skull is the fulcrum, the weight of the head is the
resistance. The power is behind, where the muscles from the neck are
attached to the back of the skull. The object of this arrangement is to
keep the head steady and balanced on the spinal column, and to move it
backward and forward.

[Illustration: Fig. 35.--Showing how the Bones of the Arm serve as Levers.

P, power;
W, weight;
F, fulcrum.
]

Levers of the second class are those in which the weight is between
the fulcrum and the power. A familiar example is the crowbar when used for
lifting a weight while one end rests on the ground. This class of levers
is not common in the body. Standing on tiptoe is, however, an example.
Here the toes in contact with the ground are the fulcrum, the power is the
action of the muscles of the calf, and between these is the weight of the
body transmitted down the bones of the leg to the foot.

Levers of the third class are those in which the power is applied at
a point between the fulcrum and weight. A familiar example is where a
workman raises a ladder against a wall. This class of levers is common in
the body. In bending the forearm on the arm, familiarly known as "trying
your muscle," the power is supplied by the biceps muscle attached to the
radius, the fulcrum is the elbow joint at one end of the lever, and the