ing the cells together.]

=188. How muscles act.=--A nerve runs from the brain, and touches
every cell of the muscle. When we wish to move, the brain sends an
order down the nerve. Then each muscle cell makes itself thicker and
shorter. This pulls its ends together, and bends the joint. We can
make muscle cells move when we wish to, but we cannot make any other
kind of cell move. We make all our movements by means of our muscles.

=189. Where you can see muscles.=--In a butcher's shop you can see lean
meat. This is the animal's muscle. White and tough flesh divides the
tender red meat into bundles. Each red bundle is a muscle. You will see
how the muscle tapers to a string or tendon. The butcher often hangs up
the meat by the tendons. You can see the muscles and tendons in a
chicken's leg or wing when it is being dressed for dinner.

Roll up your sleeve to see your own muscles. Shut your hand tight. You
will see little rolls under your skin, just below the elbow. Each roll
is a muscle. You can feel them get hard when you shut your hand. You
can feel their tendons as they cross the wrist.

Open your hand wide. You can see and feel the tendons of the fingers
upon the back of the hand. These tendons come from muscles on the back
of the arm. You can feel the bundles of these muscles when they open
the fingers. There are no muscles in the fingers, but all are in the
hand or arm. You cannot open your hand so strongly as you can close it.

=190. Strength of muscle.=--By using a muscle you can make it grow
larger and stronger. If you do not use your muscles they will be small
and weak. Children ought to use their muscles in some way, but if they
use them too much, they will be tired out. Then they will grow weaker
instead of stronger. Lifting heavy weights, or running long distances,
tires out the muscles, and makes them weaker. Small boys sometimes try
to lift as much as the big boys. This may do their muscles great harm.

=191. Round shoulders.=--The muscles hold up the back and head, and
keep us straight when we sit or stand. A lazy boy will not use his
muscles to hold himself up, but will lean against something. He will
let his shoulders fall, and will sit down in a heap. Sometimes he is
made to wear shoulder braces to keep his shoulders back. This gives
the muscles nothing to do, and so they grow weaker than ever. The best
thing to do for round shoulders is to make the boy sit and stand
straight, like a soldier. Then