Another set of teeth, called the permanent teeth, is growing, one under each of the milk-teeth, and pushing out the first set. 另一套牙齿从原来的位置上长出, 将乳牙挤掉并取而代之, 这就是恒齿。

　　Some new teeth also grow behind the milk-teeth at each side of the mouth. 还有些新的牙齿也会从嘴巴两侧生长出来。

　　The new teeth are thirty-two in number, there being sixteen in each jaw. 这一套新的牙齿共有三十二颗, 上下腭各有十六颗。

　　The tooth farthest back at each side of each jaw is the last to grow. 每一侧最远处的牙齿最后才会生长出来。

　　You will not have these four back teeth until you are about twenty-one years of age, and so these teeth are called your "wisdom-teeth. " 这四颗牙大约在你二十一岁的时候长出来, 因此, 称之为“智齿”。

　　Permanent means "lasting, " and these second or permanent teeth ought to last you all the rest of your life. “恒”就是“持久”的意思, 而这第二批牙齿或者说恒牙将会在你随后的生命中一直与你相伴。

　　But to make them last, you must take care of them by brushing them at least twice every day with a toothbrush. 然而, 想要让它们持久, 你就必须好好照顾它们：每天至少使用牙刷刷两次牙。

　　Be sure to use your teeth well in chewing your food; 请一定要正确使用牙齿来咀嚼食物；

　　for if you swallow your food half chewed—or "bolt" it, as people say—you will be sure to be ill and to suffer pain. 因为, 如果你没有好好咀嚼便咽下食物或者像人们说的那样“囫囵吞下”, 那么一定会生病, 遭受痛苦。

　　Can you feel under your skin the lumps of firm red flesh which are called muscles? 你能否感觉到皮肤下方名为肌肉的红色肉块?

　　Feel the strong muscle in your upper arm, and notice how it becomes harder and shorter and thicker as your lower arm is raised, 感受一下上臂的强壮肌肉, 并且仔细观察, 当你的小臂抬起时, 这些肌肉如何变得更加坚硬、粗短与厚实；

　　and how it relaxesas the arm is straightened again. 当小臂放下时, 它们又会变得松软。

　　In the whole of your body there are about four hundred such muscles. 整个人体共有大约四百块这样的肌肉。

　　Underneath the muscles you can feel something harder still—your bones. 你还可以感觉到肌肉下方那些更为坚硬的物质——骨头。

　　There are about two hundred separate bones in your body. 人体中大约有两百多块独立的骨头,

　　The ends of many of these bones are joined together by strong bands. 它们全部通过韧带连接在一起。

　　All the bones taken together form what is called the skeleton. 这些骨头就构成了所谓的骨骼。

　　The ends of some of the muscles are fastened to the bones by strong cords or strings called sinews. 有些肌肉的末端通过名为肌腱的结实的索状组织连接在一起。

　　As the muscles contract, they move the bones which are fastened to them; 当肌肉收缩时, 它们就会牵动与自己相连的骨头；

　　and this is the way in which our legs are made to move when we are walking, and our hands when we are working. 我们就是通过这种方法在走路时摆动腿部, 在工作时运用双手。