Muscles and Human 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. 肌肉和人体 骨骼运动发生在人体的关节部位。这种运动是由称为肌肉的片状或条状的强韧组织拖动而引起的。肌肉组织有一种特殊的收缩能力，可以带动与之相连的骨骼。肌肉在收缩的同时，也会聚成团。肌肉以条状或片状形式布满全身，存在于皮肤与骨架之间。骨骼是身体的支架，而肌肉则构成了人的体形。大多数肌肉接两块骨头，但当肌肉收缩时只有一块骨头运动。肌肉与不动骨相连的一端叫肌起端，与动骨相连的一端叫附着端。有些肌肉不与骨骼直接相连，而是连着附在骨骼上的不能收缩的带状组织或腱上。肌肉不能推动而只能拉动骨骼。要从肘部弯曲手臂，就要收缩上臂正面的肌肉。要伸直手臂，背面的肌肉就得收缩。这两组肌肉，正面的和背面的，称为相反运动肌肉。当一组工作时，另一组就休息。但有时它们也一起工作。有时我们要求肌肉不仅仅做简单的单向拖动。它们需要转动。但只有当肌肉与骨骼形成一定角度时，它们才能转动，从而拖动骨骼绕某个轴旋转。有几块肌肉的作用较特殊，例如隔膜肌。它带动肺部吸入空气。呼吸基本上不是骨骼运动带动的。
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