ngs, which, with other things in it,
fill it quite out, so as to leave no hollow space between themselves and
it. The lungs are a sort of air-sponges, and when you enlarge your chest
to draw breath, they swell out with it, and suck the air in. On the other
hand, you narrow your chest, and squeeze the lungs, and press the air from
them;--that is breathing out. The lungs are made up of a lot of little
cells. A small pipe--a little branch of the windpipe--opens into each cell.
Two blood-vessels, a little tiny artery, and a vein to match, run into it
also. The arteries bring into the little cells dark-colored blood, which
_has been_ all over the body. The veins carry out of the little cells
bright scarlet-colored blood, which _is to go_ all over the body. So all
the blood passes through the lungs, and in so doing, is changed from dark
to bright scarlet."

"Black blood, didn't you say, in the arteries, and scarlet in the veins? I
thought it was just the reverse," interrupted Mr. Bagges.

"So it is," replied Harry, "with all the other arteries and veins, except
those that circulate the blood through the lung-cells. The heart has two
sides, with a partition between them that keeps the blood on the right
side separate from the blood on the left; both sides being hollow, mind.
The blood on the right side of the heart comes there from all over the
body, by a couple of large veins, dark, before it goes to the lungs. From
the right side of the heart, it goes on to the lungs, dark still, through
an artery. It comes back to the left side of the heart from the lungs,
bright scarlet, through four veins. Then it goes all over the rest of the
body from the left side of the heart, through an artery that branches into
smaller arteries, all carrying bright scarlet blood. So the arteries and
veins of the lungs on one hand, and of the rest of the body on the other,
do exactly opposite work, you understand."

"I hope so."

"Now," continued Harry, "it requires a strong magnifying glass to see the
lung-cells plainly, they are so small. But you can fancy them as big as
you please. Picture any one of them to yourself of the size of an orange,
say, for convenience in thinking about it; that one cell, with whatever
takes place in it, will be a specimen of the rest. Then you have to
imagine an artery carrying blood of one color into it, and a vein taking
away blood of another color from it, and the blood changing its color in
the cell."

"Ay, bu