A muscle consists of a thick, solid middle part, which is called the belly, and two tapering ends. As a rule the muscle is attached to two bones-one fixed, the other movable. It is not joined directly to the bones. The tapering ends usually terminate in tough, whitish,leather-like cords, which we call tendons. It is the tendon which binds the muscle to the bone.
We have now to inquire how the muscles act. The substance of which they are formed is very elastic, but their elasticityis that of sponge or cork, not that of india-rubber. You remember of course that cork and sponge contract or shrink up with pressure, and expand or spring back whenthat pressure is removed. India-rubber, on the other hand, expands when pulled, and contracts when let go. The muscles then contract when they are interfered with.
You will understand the action of the muscles if you straighten the right arm, place the left hand on the fleshy part of it above the elbow, and then bend it upwards to the shoulder.