Levers of the first class are those in which the fulcrum is between the power and the weight.  The crowbar, when used to lift a weight at one 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 resistance is the weight of the forearm at the other end.

Experiment 22. To illustrate how the muscles use the bones as levers. First, practice with a ruler, blackboard pointer, or any other convenient object, illustrating the different kinds of levers until the principles are familiar.  Next, illustrate these principles on the person, by making use of convenient muscles.  Thus, lift a book on the toes, by the fingers, on the back of the hand, by the mouth, and in other ways.

  These experiments, showing how the bones serve as levers, may be
  multiplied and varied as circumstances may require.

75.  The Erect Position.  The erect position is peculiar to man.  No other animal naturally assumes it or is able to keep it long.  It is the result of a somewhat complex arrangement of muscles which balance each other, some pulling backwards and some forwards.  Although the whole skeleton is formed with reference to the erect position, yet this attitude is slowly learned in infancy.

In the erect position the center of gravity lies in the joint between the sacrum and the last lumbar vertebra.  A line dropped from this point would fall between the feet, just in front of the ankle joints.  We rarely stand with the feet close together, because that basis of support is too small for a firm position.  Hence, in all efforts requiring vigorous muscular movements the feet are kept more or less apart to enlarge the basis of support.