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    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.

    全文翻譯：肌肉和人體

    骨骼運動發(fā)生在人體的關節(jié)部位。這種運動是由稱為肌肉的片狀或條狀的 強韌組織拖動而引起的。 肌肉組織有一種特殊的收縮能力，可以帶動與之相連的骨骼。 肌 肉在收縮的同時，也會聚成團。 肌肉以條狀或片狀形式布滿全身，存在于皮膚與骨架之間。骨骼是身體的支架，而肌肉則構成了人的體形。  大多數(shù)肌肉接兩塊骨頭，但當肌肉收縮時 只有一塊骨頭運動。 肌肉與不動骨相連的一端叫肌起端，與動骨相連的一端叫附著端。 有 些肌肉不與骨骼直接相連，而是連著附在骨骼上的不能收縮的帶狀組織或腱上。  肌肉不能推動而只能拉動骨骼。  要從肘部彎曲手臂，就要收縮上臂正面的肌肉。  要伸直手臂，背面 的肌肉就得收縮。這兩組肌肉，正面的和背面的，稱為相反運動肌肉。  當一組工作時，另 一組就休息。  但有時它們也一起工作。  有時我們要求肌肉不僅僅做簡單的單向拖動。 它 們需要轉動。  但只有當肌肉與骨骼形成一定角度時，它們才能轉動，從而拖動骨骼繞某個 軸旋轉。  有幾塊肌肉的作用較特殊，例如隔膜肌。它帶動肺部吸入空氣。  呼吸基本上不 是骨骼運動帶動的。