part 12

110 Our bodies can be looked upon as machines which are kept going by fuel. As food is burned up in the body cells it gives us heat and other sorts of energy. All through our lives—from birth to death—we must be supplied with energy. Even when we are sleeping we are using up some food to keep our bodies warm. Our lungs and heart keep on working. Some animals sleep through the cold winter weather: snakes, for example. When they are asleep their rate of breathing and pulse rate and temperature go down, and less food is needed. Only animals that can store enough fat in their bodies to keep them alive through the winter can sleep through the months when food is hard to get.

我们的身体可以看作是靠燃料维持运转的机器。 当食物在身体细胞中燃烧时，它给我们提供热量和其他种类的能量。 在我们的一生中——从出生到死亡——我们都必须获得能量。 即使当我们睡觉的时候，我们也在消耗一些食物来保持我们的身体温暖。 我们的肺和心脏继续工作。 有些动物在寒冷的冬天睡觉，例如蛇。 当他们睡着时，他们的呼吸频率、脉搏率和体温都会下降，所需的食物也会减少。 只有那些能够在体内储存足够脂肪以度过冬天的动物，才能在食物难以获得的几个月里睡觉。

111 While our bodies are at rest the large muscles are not in use. It is these large muscles which pull on our bones and let us move as we want to. Hard work with our large muscles uses up energy quickly so that we come to feel hungry and want to eat. The more we know about the parts of our bodies and their work, the more new questions come to our minds, questions such as: How is it possible for all the different parts of us to grow from one cell? How is it possible for all the millions of cells in our growing bodies to go on doing what they do without our being conscious of their workings?

当我们的身体休息时，大肌肉就不活动了。 正是这些大肌肉拉着我们的骨头，让我们随心所欲地运动。 我们的大肌肉的艰苦工作消耗能量很快，所以我们来感到饥饿和想吃东西。 我们对身体的各个部位及其作用了解得越多，就会有更多的新问题出现在我们的脑海中，比如：我们的所有不同部位是如何从一个细胞中生长出来的？ 在我们不断生长的身体里，数百万个细胞怎么可能在我们不意识到它们的工作的情况下继续做它们的工作呢？

112 Through about two billion (2,000,000,000) years, scientists say, the development of plants and animals on our earth has been going on. At first both were very small, much less than a pin point in size. We might say that the first plants or animals were bodies living in one-roomed houses, too small for the eye to see. All the housekeeping went on in one room. Such cells take food in and let waste out. They do not give birth to new cells but divide to become two daughter cells. These three pictures show the division of a cell into two separate cells, each of which may in turn divide into two more.

科学家们说，在大约20亿年的时间里，地球上的动植物一直在发展。 一开始，两个都很小，比针尖小得多。 我们可以说，最早的植物或动物是住在一间屋子里的躯体，太小了，眼睛看不见。 所有的家务都在一个房间里进行。 这些细胞吸收食物，排出废物。 它们不会产生新的细胞，而是分裂成两个子细胞。 这三张照片显示了一个细胞分裂成两个独立的细胞，每个细胞又可以分裂成两个。

113 The earth itself is thought to be more than twice as old as life on it. Our picture of what it was like two billion (2,000,000,000) years ago is of a great ball about as hot on its surface as boiling water. In time, as the earth's surface became less hot, the thick clouds round it, from which water went on raining down, got thinner. Then more sunlight could get through to the surface of the earth. Sunlight is necessary for life on earth, as air and water are.

人们认为地球本身的年龄是地球上生命的两倍多。 我们所看到的20亿年前地球的样子是一个巨大的球体，表面的温度和沸水一样高。 随着时间的推移，随着地球表面变得不那么热，它周围的厚厚的云层变得越来越薄，水从云层中不断落下。 这样更多的阳光就能照射到地球表面。 阳光是地球上生命所必需的，就像空气和水一样。

114 Life began, people think, in the salt sea. Near the land, where the waters were not deep, very small one-celled organisms came into being. The cells in our bodies are of many different sorts but they all have water in them. Our bodies are seventy per cent water. This water has in it about the same amount of salt, common table salt, as there was in the water of the sea in which the earliest organisms swam. That water was a little less salt than sea water is today. In billions of years the rivers have washed down more and more salt from the land into the sea. The sea has become more salt. The water in our bodies does not become more salt. Even if we eat a great amount of salt in our food the body automatically controls how much it will keep in the blood stream. And the same is true if we let very much salt out through the skin when we get overheated. (See page 91.) Then we have to eat something that will give us new supplies of salt.

人们认为，生命始于咸海。 在陆地附近，海水不深的地方，出现了非常小的单细胞生物。 我们体内的细胞种类繁多，但都含有水分。 我们的身体70%是水。 这种水的盐含量，普通食盐的含量，和最早的生物在其中游泳的海水中的含量差不多。 那时的海水比今天的海水含盐量要少一些。 数十亿年来，河流把越来越多的盐从陆地冲进了海洋。 海水变得更咸了。 我们体内的水不会变成更多的盐。 即使我们在食物中摄入了大量的盐，身体也会自动控制它在血液中保留的量。 同样的道理，如果我们在温度过高的时候让很多盐从皮肤里流出来。 （见第91页。）那我们就得吃点能补充盐的东西。

115 The first living things, though they were too small to be seen, were eating, growing organisms which divided to become others like them. Under a microscope, a cell looks like a bit of clear jelly with a thin wall round it. Very small openings in the walls of the cell let food in and waste out. In every cell there is a part like a little ball. This is the nucleus, which organizes the work of the cell. Though cells were discovered two and a half centuries ago, it is only in the last hundred years that knowledge of the work of the nucleus has developed. It is this nucleus which keeps the cell working at what has to be done. It is to the cell what London is to Britain, New Delhi to India, Beijing to China. It is the seat of government.

115最初的生物，虽然小得看不见，但它们是吃东西、生长的有机体，它们分裂成和它们一样的生物。 在显微镜下，细胞看起来像一小块透明的果冻，周围有一层薄壁。 细胞壁上非常小的开口让食物进出。 在每个细胞中都有一个像小球一样的部分。 这是细胞核，负责组织细胞的工作。 尽管细胞在两个半世纪前就被发现了，但直到最近一百年，人们才对细胞核的工作有所了解。 正是这个细胞核使细胞在必须完成的工作中工作。 它之于细胞就像伦敦之于英国、新德里之于印度、北京之于中国。 它是政府所在地。