英文科學(xué)讀本 第三冊·Lesson 27 More about the Frog

Lesson 27 More about the Frog

Have you got over your dislike for the poor little frog, Norah? asked Fred. "We have been learning some wonderful things about him today. Would you like to know how he breathes? It is all very strange."

Yes, said Norah, "I should like to hear all about it."

Well, said Fred, "before you can understand the breathing of the frog, I must make you think about the way in which we and most of the animals around us breathe."

You can feel the ribs, which, pass right round the upper part of your body. They form a sort of airtight box, which we call the chest. The chest holds the lungs, and the lungs are the breathing organs.

Our ribs are constantly rising and falling. You can feel them rise and fall, if you put your hand on your chest. When they rise they make the chest larger, and the lungs inside expand; when they fall they make it smaller, and squeeze the lungs together again. As the lungs expand air rushes in at the mouth and nostrils to fill them, and when they are pressed together the air is driven out. All this goes on without any effort on our part, and this is the way we breathe. The great thing for us to remember is that the frog has no ribs, and cannot therefore breathe in this way.

He must have air, however, and he is compelled to swallow it in gulps. He closes the mouth, and sucks up a quantity of air through the nostrils, swallowing each draught with a special effort. He cannot take in much air in this way, but he is assisted in breathing by his moist, porous skin, which also absorbs a small quantity of air.

During the winter, when flies and grubs are not to be found, the frog betakes himself to the bottom of the pond, scoops out a hole for himself in the mud, and there sleeps till the frost and snow have gone.

All this time his skin has to do the entire work of breathing by taking in air from the water, for the frog cannot breathe under water with his lungs any more than we could.

Teacher showed us that, even with lungs and skin too, the frog takes in very little air, so little, indeed, that it is not enough to warm his blood. The blood is always cold. We call the frog a coldblooded animal.

This explains why the body is not provided with a warm coat, for there is no need to keep the heat in. It has nothing but a naked skin, which always feels cold and clammy to the touch.

SUMMARY

The frog has no ribs; he does not breathe as we do. He shuts his mouth, sucks the air up through his nostrils, and swallows it in gulps. The frog can breathe through his moist skin. He spends all the winter in the mud at the bottom of the pond. His blood is always cold.