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 triceps, and the various flexors and extensors of the limbs are antagonistic to one another.

71.  The Tendons.  The muscles which move the bones by their contraction taper for the most part, as before mentioned, into tendons.  These are commonly very strong cords, like belts or straps, made up of white, fibrous tissue.

Tendons are most numerous about the larger joints, where they permit free action and yet occupy but little space.  Large and prominent muscles in these places would be clumsy and inconvenient.  If we bend the arm or leg forcibly, and grasp the inside of the elbow or knee joint, we can feel the tendons beneath the skin.  The numerous tendons in the palm or on the back of the hand contribute to its marvelous dexterity and flexibility.  The thickest and strongest tendon in the body is the tendon of Achilles, which connects the great muscles in the calf of the leg with the heel bone (sec. 49).