How does the air get in and out of the lung tubes?
Evidently you do not and cannot swallow it as you would food or drink; and as it will not run down of its own accord when you simply open your mouth, nature has had to devise a special bit of machinery for the purpose of sucking it in and pressing it out again.

This she has done in a rather ingenious manner by causing certain of the muscle-rings in the wall of the chest to turn first into gristle, or cartilage, and then later into bone, making what are known as the _ribs_; these run round the chest much as hoops do round a barrel, or as the whalebone rings did in the old-fashioned hoop skirt.

When the muscles of the chest pull these ribs up, the chest is made larger,--like a bellows when you lift the handle,--air is sucked in, and we "breathe in" as we say; when the muscles let go, the ribs sink, the chest flattens and becomes smaller, the air is driven out, and we "breathe out." FOOTNOTES: [18] This nitrogen, though of no value for breathing, is of great value as a food, forming, as we have seen, an important part of all meats, or proteins, which build the tissues of our bodies.

It can, however, be taken from the air only with great difficulty, by a very roundabout route; the bacteria of the soil eat it first, then they pass it on as food to the roots of plants; animals eat plants, and we eat the animals, and thus get most of our nitrogen..