英文科學(xué)讀本 第五冊(cè)·Lesson 09 Food—Why We Eat

Lesson 09 Food—Why We Eat

The subject which is to engage our attention this morning, said Mr. Wilson, "is an important one, because it comes home to our own every-day life. We are going to commence a series of lessons on food, and I shall endeavour, not merely to fill your memory with facts, but to lead you, step by step, to discover for yourselves such practical truths as will be of inestimable benefit to you in that important but much abused business of life, the duty of eating and drinking,

I have here an ordinary hen's egg. I will break it in this cup, as your mother does when she is going to make a pudding or a custard. Look at the egg in the cup. There is a clear, viscid (sticky) liquid, in the midst of which floats a round yellow ball. I have no doubt you are already aware that we call the clear liquid the white of egg, and perhaps you may have heard its scientific name albumen. The yellow ball is the yolk. For the present I want you to confine your attention to the yolk—the yellow ball. Notice the little round spot in the yolk. This is the germ, or, as it is sometimes called, the embryo. If this egg had been hatched, by keeping it warm for about three weeks, the little embryo would have become an actual chicken; it would have had strength enough to break the shell and set itself free, and would then have been able to run about and seek its own living.

Now, how does it happen that a little speck like this can grow into a chicken? Let us find out. We will first remove the germ. This is the embryo, future chicken. What then is all the other matter? All that we see now in the cup is simply a store of food, laid up within the egg-shell, for the tiny germ to feed upon.

Day by day the little thing absorbs more and more of this food-store into itself, and with it builds up its own body. Day by day, during those three weeks, the little body is growing bigger and bigger, and the store of food in the shell is getting smaller and smaller. At the end of the three weeks this food supply becomes exhausted. What is to happen next? The little creature is now fully formed, and is, moreover, able to look after itself. It has not only built up a body, but it has also accumulated vital energy, or strength, sufficient to enable it to set itself free from its shell. This it does by pecking with its beak all round the inside of the shell until it breaks, and then out it comes. If we were to take it up immediately in our hands, and examine it, we should find the little body to consist of flesh and bones, with feathers already growing on the skin—eyes, bill, feet, everything perfectly formed. Its body, too, would feel warm, and it would show, by its struggles to get free, that it had a certain amount of strength.

Now all these things—the flesh, bones, and blood of its body, with the clothing of feathers, as well as the warmth which you feel, and the strength which it shows— all come from the food which it has taken while in the eggshell. The little chick, moreover, has to grow into a large fowl. How is this brought about? This too is accomplished by the food which it eats, after it is able to run about and seek its own living.

We might follow up the same development in the case of the mammal. The little kitten, the little rabbit, the little baby all grow and become strong by the food— milk—which the mother supplies, until they are able to eat other food. We see, therefore, that Nature, in each of these two foods—the egg of the bird and the milk of the mammal—has supplied everything that is necessary for the life and growth of the little creature. Let us try and find out what these different things are. The little chicken leaves its shell with every part of its body fully formed, and all these tissues—flesh, bones, blood, feathers—it makes from the food which it finds in the shell. We call these parts of its food proteins, or tissue-formers. The milk of the mammal must contain the same kinds of material, for it has also to do the work of building up the little body. When the tiny bird leaves the eggshell, and the young mammal is no longer dependent upon its mother for support, the food which each seeks must continue to do the same work, in order that the natural development of its body may go on.

You remember, moreover, that we said the little chicken's body feels warm, and so does the body of the kitten. Not only so; their bodies must be kept warm. In the yolk of the egg was stored up a quantity of oily, fatty matter. It was this which gave the heat. Our early lessons, too, showed us that milk, in like manner, contains an oily, fatty matter, which we call cream, and you know that we use this cream to make butter.

The fatty matter, then, of the yolk of the egg and the fatty cream of the milk are both for the same purpose—that of supplying the necessary heat to the little body. We call them heat-givers or fuel-food. But why is heat necessary? It is the heat produced by the food that supplies all the vital force, and power, and energy, of both body and mind. Think for a moment of a steam-engine ready for work, but powerless to move a wheel till force is put into it. Whence comes this force? It comes from the fuel which is burnt in its furnace.

So it is in the body. This heat-giving food is the fuel, and without fuel to burn, the fire would go out, the body would become cold and powerless; it would die.