nearly all arranged after the
last-named plan. As a rule a pair of muscles is so placed, with reference
to a joint, that one moves the part in one direction, and the other moves
it in the opposite direction. From the kinds of motion which the various
muscle pairs produce, they are classified as follows:

1. _Flexors and Extensors._--The flexor muscles bend and the extensors
straighten joints (Fig. 114).

2. _Adductors and Abductors._--The adductors draw the limbs into positions
parallel with the axis of the body and the abductors draw them away.

3. _Rotators_ (two kinds).--The rotators are attached about pivot joints
and bring about twisting movements.

4. _Radiating and Sphincter Muscles. _--The radiating muscles open and the
sphincter muscles close the natural openings of the body, such as the
mouth.

The pupil should locate examples of the different kinds of muscle pairs in
his own body.

*Exchange of Muscular Force for Motion.*--The second difficulty to be
overcome in the use of muscular force in the body is due to the fact that
the muscles contract through _short_ distances, while it is necessary for
most of them to move portions of the body through _long_ distances. It may
be easily shown that the longest muscles of the body do not shorten more
than three or four inches during contraction. To bring about the required
movements of the body, which in some instances amount to four or five
feet, requires that a large proportion of the muscular force be exchanged
for motion. The machines of the skeleton, while providing for motion in
definite directions, also provide the means whereby _strong forces_,
acting through _short distances_, are made to produce movements of _less
force_, through _long distances_. The mechanical device employed for this
purpose is known as

*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
_th