Body
It is in the joints of the human body that movements of the bones take
place. The movement itself is caused by the pull of sheets and cords of
very tough tissue called muscle. Muscle tissue has the special ability
to shorten itself so that the bone on which it pulls has to move. When
muscle tissue shortens, it also bunches up. Muscle tissue covers the
body in sheets and bands that lie between the skin and the skeleton. The
bones are the framework of the body, but
the muscles fill out the body
shape. Most muscles extend from one bone to another. When the muscle
between the bones shortens, one bone has to move. The point where the
muscle is fastened to the unmoving bone is called the origin of the
muscle, whereas the point where the muscle is not fastened to the bone
that is to be moved is called the insertion. Sometimes the muscle is not
attached directly to the bone but to a tough, nonstretchable cord, or
tendon, that is attached to the bone. Muscles do not push; they can only
pull. To bend the arm at the elbow, the muscle at the front of the
upper arm has to shorten and bunch up. To unbend the arm other muscles
in the back of the arm have to shorten. These two sets of muscles - the
front and the back - are said to act in opposition to each other. When
one set is working, the other set is usually relaxed. But there are
times when both of them work. Sometimes muscles are called upon to do
more than simply pull in one direction. They may have to perform a
turning motion. To be able to do this, the muscle must be attached to
the bone at an angle. By pulling, the muscle can cause the bone to
pivot. A few muscles have special functions. The diaphragm, for example,
forces the lungs to take in air. This part of breathing is not
primarily a bone moving operation.
No comments:
Post a Comment