ong
bands called _ligaments_ (Fig. 82). These entirely surround the joint.
On their inner sides is a delicate membrane which gives out a slippery
fluid to make the joint work easily.

The ligaments are sometimes strained, stretched, or torn by a fall.
The joint then swells because the watery part of the blood collects
there. A sprained limb should be elevated to prevent swelling. Bathing
it in very hot water is helpful.

=The Muscles.=--The muscles form the lean meat in any animal. They make
up about one half the weight of the body. Each muscle is a bundle of
thousands of little threads held together by other finer threads, while
the whole is surrounded by a thin sheet. Little bundles formed of
several of these threads called fibers may be seen in a piece of cooked
beef picked to pieces. There are over five hundred muscles in the body.

[Illustration: FIG. 83.--Fifty of the muscles just under the skin.
Note the white cords, the tendons in the regions of the hands and feet.]

[Illustration: FIG. 84.--The biceps muscle contracted above and
relaxed or loosened below.]

Some of the muscles are more than a foot long and have the shape of a
ribbon. Some are circular like those around the mouth, eyes, and
stomach, while others are large in the middle and taper toward the ends.

=How the Muscles are fastened to the Bones.=--The two ends of a muscle
are attached to different bones. In many cases the muscle is not
joined directly to the bone, but is connected to a tough white cord
called a _tendon_. The tendon is then fixed to the bone.

Several of the muscles in the forearm run into tendons in the wrist
because if the muscle part were to extend along the wrist, this part
of the arm would be large and clumsy instead of graceful and slender.
Some of these tendons may be seen to move by bending the wrist and
then working the fingers.

=How the Muscles do their Work.=--A tiny nerve thread runs from the
spinal cord or brain to every muscle thread. Messages sent through the
nerve threads to the muscles make them act. A muscle can act in only
two ways (Fig. 84). It can become shorter or longer. When it gets
shorter, we say it _contracts_. When it stretches out, it is said to
_relax_.

A muscle cannot contract more than one fourth of its length. To pull
the forearm up, the brain sends a message to the muscle fixed by one
end at the shoulder and by the other end to a bone at the elbow. The
muscle at once becomes shorter and th