orearm slowly to the table, and the biceps appears to become softer
and smaller,--it relaxes.

Experiment 20. Repeat the same experiment with other muscles. With
the right hand grasp firmly the extended left forearm. Extend and flex
the fingers vigorously. Note the effect on the muscles and tendons of
the forearm. Grasp with the right hand the calf of the extended right
leg, and vigorously flex the leg, bringing it near to the body. Note the
contractions and relaxations of the muscles.

70. Arrangement of Muscles. Muscles are not connected directly with
bones. The mass of flesh tapers off towards the ends, where the fibers
pass into white, glistening cords known as tendons. The place at
which a muscle is attached to a bone, generally by means of a tendon, is
called its origin; the end connected with the movable bone is its
insertion.

There are about 400 muscles in the human body, all necessary for its
various movements. They vary greatly in shape and size, according to their
position and use. Some are from one to two feet long, others only a
fraction of an inch. Some are long and spindle-shaped, others thin and
broad, while still others form rings. Thus some of the muscles of the arm
and thigh are long and tapering, while the abdominal muscles are thin and
broad because they help form walls for cavities. Again, the muscular
fibers which surround and by their contraction close certain orifices, as
those of the eyelids and lips, often radiate like the spokes of a wheel.

Muscles are named according to their shape, position, division of origin
or insertion, and their function. Thus we have the _recti_ (straight), and
the _deltoid_ ([Greek: D], delta), the _brachial_ (arm), _pectoral_
(breast), and the _intercostals_ (between the ribs), so named from their
position. Again, we have the _biceps_ (two-headed), _triceps_
(three-headed), and many others with similar names, so called from the
points of origin and insertion. We find other groups named after their
special use. The muscles which bend the limbs are called _flexors_ while
those which straighten them are known as _extensors_.

After a bone has been moved by the contraction of a muscle, it is brought
back to its position by the contraction of another muscle on the opposite
side, the former muscle meanwhile being relaxed. Muscles thus acting in
opposition to each other are called antagonistic. Thus the biceps serves
as one of the antagonists to the tri