left forearm on a table; grasp with the right hand the mass of flesh on
the front of the upper arm. Now gradually raise the forearm, keeping the
elbow on the table. Note that the muscle thickens as the hand rises.
This illustrates the contraction of the biceps, and is popularly called
"trying your muscle" Reverse the act. Keep the elbow in position, bring
the forearm 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