Muscles are responsible for many different types of body movements. The action of the muscle is

Figure 1-27.-Types of joints: A. Ball-in-socket joint; B. Condyloid joint; C. Gliding joint; D. Hinge joint; E. Pivot joint; F. Saddle
joint.

determined mainly by the kind of joint it is associated with and the way the muscle is attached to the joint. At one end of some muscles are long white tendons that attach the muscles to bone. The point of fixed attachment of a muscle to bone is called the origin. The more flexible attachments, especially attachments to a movable bone, are termed insertions.

Muscles seldom act alone; they usually work in groups held together by sheets of a white fibrous tissue called fascia. Muscles make up about one-half of the total body weight. Their main functions are threefold:

Providing movement-including internal functions such as peristalsis (rhythmic waves of muscular contraction within the intestines).
Maintaining body posture-through muscle tone, as in the muscles of the head, neck and shoulders, which keep the head up.
Providing heat-through chemical changes that take place during muscle activity, such as exercise that warms the body.
In addition, muscles are involved in such essential bodily functions as respiration, blood circulation, digestion, and other functions such as speaking and seeing.