part 22

210 In the beginning—as long ago, as far back in the past, as we can talk about—the only sorts of changes which took place, scientists say, were physical changes. They were the sorts of changes which the science of physics is about. Here is a wheel turning round. Its motion is a physical change. Here is water boiling. And here is water turning into ice. Here are clouds forming and rain falling. These are physical changes.

科学家们说，起初——就在很久以前，就在我们所能谈论的过去——发生的唯一变化是物理变化。 这些变化正是物理学所研究的。 这里有一个轮子在转动。 它的运动是一种物理变化。 这是沸腾的水。这是水变成冰的过程。 这是云层形成和降雨。 这些都是生理变化。

211 Much later, other sorts of changes, more complex than these, began to take place. Here is a lighted candle. As it burns the heat melts the candle and the liquid goes up into the flame where it is turned into gas. So far all is physical. But in the flame chemical changes take place. Carbon from the candle and oxygen from the air come together to make carbon dioxide. At the same time water is given off, as you may see if you hold a glass cover over the flame. The cover quickly becomes clouded with little drops of water. This water was not in the wax and the candle flame gives it off however dry the air is. The water and carbon dioxide come into being through the chemical exchanges, between the lighted candle and the air, which make the flame. At the same time there are physical changes taking place. The top of the candle is melting and the liquid fuel is moving up into the flame and being turned into gas, air is coming into the flame, carbon dioxide and water are moving up from it and heat and light are being given out.

很久以后，其他种类的变化，比这些更复杂，开始发生。 这里有一支点燃的蜡烛。当它燃烧时，热量融化了蜡烛，液体上升到火焰中，在那里它变成了气体。 到目前为止，一切都是物理的。但是在火焰中发生了化学变化。 蜡烛中的碳和空气中的氧结合在一起产生二氧化碳。 与此同时，水被释放出来，如果你把一个玻璃杯盖在火焰上，你可能会看到。 盖子上很快就布满了小水滴。 这种水不在蜡里，无论空气多么干燥，蜡烛的火焰都会把它释放出来。 水和二氧化碳通过点燃的蜡烛和空气之间的化学交换而形成，从而产生火焰。 与此同时，物理变化也在发生。 蜡烛的顶部正在融化，液体燃料向上进入火焰，变成气体，空气进入火焰，二氧化碳和水向上移动，热量和光被释放出来。

212 When you step on a scale, the scale goes down and the springs inside it are pulled out, and the pointer moves to a mark. These changes are physical. When you look at the pointer to see what your weight is, rays of light come from it to your eye and go through the lens in your eye to a point on the retina.

当你踩在磅秤上时，磅秤就会下降，里面的弹簧就会被拉出，指针就会移到一个刻度处。 这些变化是生理上的。 当你看着指针看你的体重时，光线从指针射到你的眼睛，并通过你眼睛里的晶状体到达视网膜上的一个点。

213 In a camera, rays of light from whatever you are photographing make an upside down picture on the film. So it is inside your eye. The pictures made on the retina are upside down. Everything going on so far has been physical. But in your retina more complex sorts of changes now take place. Some of these are chemical. Complex materials in the retina go through chemical changes not very unlike those that make photographs on films, and these changes cause other much more complex sorts of changes in the living cells of the retina.

在照相机里，无论你要拍摄什么，光线都会在胶卷上形成倒立的画面。 所以它在你的眼睛里。视网膜上的图像是颠倒的。 目前为止发生的一切都是身体上的。 但是在你的视网膜中发生了更复杂的变化。 其中一些是化学物质。 视网膜中的复杂物质会经历化学变化，这与在胶片上拍摄照片的化学变化没什么不同，而这些变化会在视网膜的活细胞中引起其他更复杂的变化。

214 These nerves, which make us able to see, are an outgrowth from the brain to the retina. From the eye impulses travel to the brain through the living cells of the nerves. There are as many as a million separate telephone lines in these nerves alone. A train of changes goes up these nerves to the back part of the brain. The cells there let us see the things that our eyes are looking at. No one, at present, has any idea of how they do this. The distance from the eye to the back of the brain is not great, but some distances from some parts of the body to the brain are long. The rate at which impulses travel through the nerves is about the same for all of them.

这些使我们能够看见东西的神经是从大脑到视网膜的一个分支。 从眼睛发出的脉冲通过活的神经细胞传到大脑。 仅在这些神经中就有多达100万条独立的电话线。 一连串的变化沿着这些神经传到大脑的后部。 那里的细胞让我们看到眼睛正在看的东西。 目前，没有人知道它们是如何做到的。 眼睛到大脑后部的距离并不大，但身体某些部位到大脑的距离却很长。 冲动在神经中传播的速度对所有人来说都是一样的。

215 The distance from one's toe to one's brain and back may be as much as twelve feet. When you are feeling with your toe for a step to put your weight on, the impulses have to go up your nerves from your toe to your brain and back down again to the muscles which move your foot. When you are walking, almost all the muscles in your body are at work keeping you from falling down. Your muscles and nerves work together to keep you on your feet. These changes in the nerve and muscle cells are dependent on what is going on in other cells in the body in many different ways.

从脚趾到大脑和背部的距离可达12英尺之多。 当你用脚趾来感受迈步的时候，冲动必须顺着神经从脚趾传递到大脑，然后再向下传递到移动你的脚的肌肉。 当你走路的时候，你身体里几乎所有的肌肉都在工作，防止你摔倒。 你的肌肉和神经一起工作来保持你的站立。 神经和肌肉细胞的这些变化以许多不同的方式依赖于身体其他细胞的变化。