must take place during our most common movements. Look at a person running
or leaping, or watch the motions of the eye. How rapid, how delicate, how
complicated, and yet how accurate, are the motions required! Think of the
endurance of such a muscle as the heart, that can contract, with a force
equal to sixty pounds, seventy-five times every minute, for eighty years
together, without being weary."]--The muscles are nearly all arranged in
pairs, each with its antagonist, so that, as they contract and expand
alternately, the bone to which they are attached is moved to and fro. (See
p. 275.)

If you grasp the arm tightly with your hand just above the elbow joint,
and bend the forearm, you will feel the muscle on the inside (biceps,
_a_, Fig. 14) swell, and become hard and prominent, while the outside
muscle (triceps, _f_) will be relaxed. Now straighten the arm, and
the swelling and hardness of the inside muscle will vanish, while the
outside one will, in turn, become rigid. So, also, if you clasp the arm
just below the elbow, and then open and shut the fingers, you can feel the
alternate expanding and relaxing of the muscles on opposite sides of the
arms.

If the muscles on one side of the face become palsied, those on the other
side will draw the mouth that way. Squinting is caused by one of the
straight muscles of the eye (Fig. 17) contracting more strongly than its
antagonist.

KINDS OF MUSCLES.--There are two kinds of muscles, the _voluntary_,
which are under the control of our will, and the _involuntary_,
which are not. Thus our limbs stiffen or relax as we please, but the
heart beats on by day and by night. The eyelid, however, is both
voluntary and involuntary, so that while we wink constantly without