6．几年之后，乳牙开始掉落。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。还有些新的牙齿也会从嘴巴两侧生长出来。这一套新的牙齿共有三十二颗，上下腭各有十六颗。每一侧最远处的牙齿最后才会生长出来。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”⑥。
7．“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。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⑦。请一定要正确使用牙齿来咀嚼食物；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”⑧，那么一定会生病，遭受痛苦。
8．你能否感觉到皮肤下方名为肌肉的红色肉块？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；当小臂放下时，它们又会变得松软。整个人体共有大约四百块这样的肌肉⑨。
9．你还可以感觉到肌肉下方那些更为坚硬的物质--骨头⑩。人体中大约有两百多块独立的骨头，它们全部通过韧带连接在一起。这些骨头就构成了所谓的骨骼。
10．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。当肌肉收缩时，它们就会牵动与自己相连的骨头；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。
译注
①终于说到人类自己了！动物学上，这种神奇的动物属于“脊索动物门-脊椎动物亚门-哺乳纲-灵长目-人科-人属-人种”。全世界人类都是同一种，学名叫“智人”（Homo Sapiens），意为有智慧的人，大概是500万年前起源于东非的。它们最大的特点在于神经系统高度特化，产生了真正的“智慧”，于是就有了你现在看到的这本书。
②有一个很有趣的问题：为什么人体的外表是基本左右对称（Bilateral symmetry）的？其实自然界中的大部分动物也是左右对称的，只有少数辐射对称（r a d i a l symmetry）（比如海星），可见左右对称对动物是一个很大的优势。地球上的生物因为重力影响，下面和上面多少有些不一样，这就有一个“腹背轴”；动物为了取食和逃避敌害，需要尽量快速地向着同一个方向运动，因此有一个“前面”的概念是很有利的，这就出现了“前后轴”（人是直立行走的生物，在自然界中不太普遍，所以咱们最好学老祖宗的样子，四肢着地想象一下）。同样，人之所以长脑袋，是因为身体的前面最先进入新环境，所以如果前面有一个集中了各种感觉器官的部位，就能尽快地了解新环境的情况。此外，为了拥有更好的平衡性，更好地转弯，身体左右对称是最好的办法，这就出现了“左右轴”，三个轴，形成了堪称完美的生物形式。不止是生物，你想一下人类造出的各种能灵活运动的机器，大都也是左右对称的。科幻电影里，外星人开着圆形飞碟跟人类作战，其实是一个相当愚蠢的主意，因为辐射对称的飞碟，在左右对称的战斗机面前会吃大亏的。
③皮肤包括表皮和真皮，广义上还可以包括皮下组织。1/8英寸-1/4英寸相当于3-6毫米。这个数据有点奇怪，因为人类皮肤厚度虽然因为个体、部位、年龄、统计方法而不同，但表皮和真皮的厚度合起来一般都在2-3毫米左右。文中的算法不知是不是加上了皮下组织。皮肤是人体面积最大的器官，大约2平方米，重量大约是体重的1/20。
④指甲是皮肤的角质层，可以理解为从表皮演化而来。毛发则属于皮肤的附属结构。
⑤乳牙的正式英语名称是deciduous teeth。Deciduous原意是“落叶的”（树），这是一个很形象的比喻，因为乳牙的脱落跟落叶很像。
⑥智齿的存在可能会损害口腔卫生，因此很多人都选择把智齿拔掉。
⑦清洁口腔的目的是为了除去牙斑（plaque）和牙垢（tartar），但刷牙无法清洁牙缝，因此现代医学一般认为还需要牙线（floss）。
⑧在喝汤的时候，特别容易不嚼就咽下汤里的固体食物，这种事情一定要避免。
⑨文中提到的肌肉是“骨骼肌”（skeletal muscle），是可以自主控制的肌肉，此外还有无法自主控制的“心肌”（cardiac muscle）和“平滑肌”（smooth muscle）。原文中的400块不准确，骨骼肌的数目，虽然因为统计方法不同而有差异，但一般认为在640-650块左右。
⑩正常人体的骨骼是206块。
THE BODY
人体
1.If you wish to keep your body healthy，you must learn something about its inside as well as about its outside.The trunk or middle part of your body is hollow，and inside it there are several parts or organs，each of which has its own work to do in helping you to live a healthy and happy life.
2.The upper part of your trunk is called the chest.Place your hand near the middle of your chest，and notice whether anything is moving inside it.Yes；that is your heart beating.The heart is a strong hollow bag about the size of your closed hand.It is made of flesh or muscle.
3.The heart is filled with blood from the veins every second.No sooner is it full than it contracts or closes，and so squeezes the blood into pipes or blood-vessels，called arteries，which carry the blood to every part of your body.Then the blood comes back to the heart again through other pipes or tubes，which are called veins.You can easily see some veins under the skin on the back of your hand.
4.Because the blood moves round and round inside our body，starting from the heart and coming back to the heart again，it is said to circulate.For this reason the heart，the arteries，and the veins are called the organs of circulation.
5.In the chest we also find two organs called the lungs，one on each side of the heart.These lungs contain many little tubes which are full of air，and many other tubes which contain blood.In the lungs the bloodgets from the air-tubes the oxygen which it needs，and the air takes from the blood hurtful things which it must get rid of，and whichwe breathe out with our breath.
6.Air flows into the lungs from the nose and the mouth by a strong pipe which you can feel in the front of your throat，and which is calledthe windpipe.The lungs are the organs by which we breathe or respire，and so they are called the organs of respiration.
7.In the lower part of the trunk are several other organs，the most important①of which is the stomach.The stomach is joined to the mouth by a pipe called the gullet or food-pipe.When weswallow our food，it passes down the gullet into the stomach，and there it is dissolved or digested.
8.After this has been done，the food is ready to nourish the body，and it passes through the sides of the stomach，and mixes with the blood in a great many little blood-vessels，which lie all round the stomach.Then the blood carries the digested food with①Most important，chief；principal.
it all over the body，and everywhere it helps to form new bones，and muscles，and nerves.
9.The stomach is aided①in its work of digesting the food bya large organ called the liver，which lies close to the stomach on the right side；and there is a small organ called the pancreas，which also aids in this useful work.All these organs are called the organs of digestion.
10.But what are the organs by which we feel and think ？The chief of these is the brain，which fills the greater part of the head.From the brain a long white cord，called the spinal cord，goes down through the backbone.A great number of fine white threads，called nerves，run from the brain and the spinal cord to all parts of our body.
11.There are so many of these nerves，that we cannot prick our skin without touching one of them.We know this because we cannot prick the skin without feeling pain.The nerves act very much like telegraphwires②，for they carry messages to thebrain which make us feel，and see，and hear.They also carry orders from the brain to the muscles，which make them contract and relax，and thus our limbs are made to move as we wish.
12.There are many other organs in the body，every one of which has some work to do in keeping us alive and well.All the organs work together，and they work so quietly when the body is in good health，that we do not know-except，perhaps，by the beating of the heart-that they are in our body at all.
①Aided，helped；assisted.
②Telegraph wires，stretched wires along which messages are sent by electricity.