英文科學(xué)讀本 第五冊·Lesson 19 Kinds of Food

Lesson 19 Kinds of Food

Now that we are clear as to the constituents of which the body is formed, we can proceed to examine the different kinds of food necessary to support the body. Suppose we go back once more to the hen's egg, said Mr. Wilson. "I will break it, as before, in a cup. Notice the clear, sticky fluid in the cup, and notice too the change that takes place in this fluid when I pour it into boiling water. It changes to a white, opaque, solid substance. We call it white of egg. Its scientific name, albumen, is given to it because of this white appearance; the Latin word albus means white.

Albumen is a proteid or tissue-former. It was with this as its food that the little chicken built up its tissues while in the egg-shell. Let us next turn our attention to milk, Nature's other food. I have some new milk in this glass. I will pour a little of this rennet into it, so that you may observe the change that takes place. It curdles at once into a white, opaque, solid substance—the curd. The scientific name for this curd is casein. It is the proteid or tissue-forming constituent of milk. It is with this substance that the young sucking mammal builds up its tissues. The food which every animal afterwards seeks for itself must contain proteid or tissue-forming matter of some kind, because its body has to grow, and its tissues can only be formed from these materials.

Let us now go back once more to the little chick. Its body, you know, felt warm immediately it left the egg-shell; the bodies of all mammals and all birds are warm, and must be kept warm. Do you remember whence the chick obtained the heat which warmed its body?

Yes, sir, replied Fred. "You told us that in the yolk of the egg there is laid up a store of oily, fatty matter. It is this fatty matter which supplies the heat. In like manner, there is in milk an oily, fatty constituent. We can separate it from the milk, and when we have done so, we call it cream. The cream in the milk of mammals serves exactly the same purpose as the oily matter in the yolk of the egg. It supplies the little body with the necessary heat."

Just so, said Mr. Wilson, "and I want you to understand clearly that, as soon as the little creatures begin to seek their own living, they must combine with their other food something which will serve the same purpose, in order that the bodily heat may be kept up. We ourselves, in like manner, require heat-giving as well as tissue-forming food, because it is this which supplies all our vital heat and energy."

I remember, sir, said Fred, "you called both the oil of the yolk of egg and the cream of the milk Nature's fuel-foods and you spoke about these fuel-foods burning. You don't really mean that they burn in our bodies; do you, sir?"

Yes, my lad, replied Mr. Wilson. "I mean that these fuel-foods burn in our bodies, as surely as the coal burns in the grate, although there is no flame, no smoke, such as we usually find when things burn. One of these days we will discuss this matter more fully. You must be content now to know, first, that these things and others of a similar nature are called fuel-foods, because, like ordinary fuel, they burn, and secondly, that they actually burn in our bodies, and supply heat and vital energy.

Let us now enumerate some of these fuel-foods. First among them must stand all fat and oily matter of every kind. This, of course, does not surprise you, because these substances are so much like the fuel-food of the egg and milk. I have next some sugar in this spoon. I am going to hold the spoon over the flame of the spirit-lamp. You notice that the sugar soon becomes heated and burns. It is readily combustible and, in burning, gives out great heat. We take sugar as a food, because of this. It is a heat-giving or fuel-food. It burns in our bodies and, like fat, is a source of heat and vital energy.

You remember, no doubt, our early lessons on starch, and you can tell me what kinds of food contain starch.

Yes, sir, replied Fred. "Starch is found more or less in all vegetable foods. It is an important part of the substance of the corn-grains, peas, beans, rice, potatoes, arrowroot, sago, and tapioca. I remember how we used to separate the starch from flour by washing and kneading it in the muslin bag under water."

Quite right, Fred. But I wonder whether you remember what happens to these starchy parts of our food when we take them into the mouth.

Oh yes, sir, said Fred eagerly, for the whole thing flashed through his mind like lightning. "The saliva in the mouth changes these starches into sugar. Now I see it all. Starch is another of the fuel-foods. The saliva changes it into sugar, and this sugar burns in the body to provide the bodily heat.

I remember, you mixed some starch into a paste with water, and made me hold some of it on my tongue for a few minutes, till it began to taste quite sweet. It was the saliva from my tongue that changed the starch into sugar.

Excellent, replied Mr. Wilson, "and now just one thought more, Fred, and we will leave this subject for the present. I am going to take your mind back to that bone which I soaked in the acid. Part of the bone was left—the ossein—but all that had made it hard, firm, and rigid had been dissolved out by the acid. The bones of the little bird, as it was forming in the egg-shell, and those also of the young mammal for some time after its birth, consisted at first entirely of this one substance—ossein. There was, however, stored up in the egg and in the milk, in addition to the substances we have mentioned, a sufficient quantity of earthy matter in each case to change that ossein into actual bone. It could not have become bone without this earthy matter. From the time that the young animals begin to seek their own living, in their own way, Nature supplies the necessary amount of this earthy or mineral matter in the very food they choose.

If we burn a carrot, a cabbage, or a potato, we find that, although the greater part of its substance is consumed in the burning, there is a residue which will not burn away. It forms an ash. This ash is earthy or mineral matter, which the vegetables took up while they were growing in the ground. Whenever, therefore, we and animals in general eat vegetable food, we take in more or less earthy matter, which those vegetables themselves have absorbed from the soil. The very water we drink contains dissolved minerals; salt is a mineral; all our fresh vegetables contain large supplies of mineral matter."