The chest cavity is entirely enclosed by these walls, and is completely shut off by them from the external air, so that there is no communication between the external air and the cavity of the chest itself. The chest is filled by the organs that it contains-these organs I have mentioned before, the heart and the lungs. As the heart is surrounded by a serous bag, so each lung is surrounded, except at the place which is called its root, which is where the bronchus and the pulmonary artery enter it, and the pulmonary veins leave it, by a serous bag. One layer of this serous bag is against the lung, and the other layer is against the chest-wall; this serous bag is called the pleura.

That is a rough description of the organs of breathing, and the walls of the cavity in which they are.

How does this contrivance work ? In the first place, the walls of this cavity, the chest, are movable; the diaphragm is for the most part muscle, and we know that muscles are capable of moving by contracting; the sides and part of the back, and part of the front of the chest are movable, the walls as a whole are movable, because the ribs are attached by true joints to the spinal column; so that the whole set of ribs and the structures which join them and the breast-bone can be moved as a whole upon the spinal column.

Now suppose the diaphragm to contract, what would happen ? the muscular fibres of the diaphragm lie in a curved line; if these fibres contract, they become shorter; if they become shorter, the distance that the fibres run must be less; they must approach more nearly to a straight line between the two points, so that when the muscular fibres of the diaphragm contract, the diaphragm as a whole becomes flatter, and so the chest cavity is made longer from above downwards.

When the external intercostal muscles contract (remember that the fibres of these run from each rib downwards and forwards to the next rib) the distance between their points of attachment must be made less, and this results in the raising of the ribs, thus the ribs, by the contraction of the external intercostal muscles, are raised upon the vertebral column, and so the breastbone is raised and tilted forwards.

The chest is in this way made deeper from before backwards, and wider from side to side: so that by the contraction of the diaphragm the chest is made longer from above downwards; and by the contraction of the external layer of muscles between the ribs, the chest-walls are raised, and the chest is made deeper from before backwards, and broader from side to side. You might say at once, But, then, do these things happen at the same time ? Of course it might so happen that when the diaphragm contracted these external muscles should not contract, but, on the contrary, other muscles might contract and make the chest cavity smaller in other directions. But the answer is this, that when the diaphragm contracts and makes the chest cavity deeper from above downwards, at the same time the external layer of muscles between the ribs contracts and makes the chest wider in the other dimensions, so that the chest cavity is at the same time made larger in all directions.

Now the result is that, by lifting the chest-walls away from the lungs, a certain amount of pressure has been taken off the lungs, while the pressure inside remains the same, and so is sufficient to expand the little air-sacs, and thus to inflate the lungs, until they are sufficiently large to fill the enlarged cavity of the chest; this process is called inspiration, or breathing in.

It is not the pressure of air inside the lungs that pushes the chest-walls out.

When we inspire, the chest-walls are lifted away from the contents of the chest, and the cavity of the chest is made larger. The diaphragm contracts and makes it larger in one direction; the external muscles between the ribs make it larger in the other directions, so that it is the chest cavity that is made larger first, and then, as that cavity is not connected with the external air, the air must come in through the tubes that lead into the lungs, and makes them larger so as to fill the enlarged cavity, and they are made larger because they are capable of being extended, consisting, as they do for the most part, of little exceedingly elastic bags.

Then when this has been done, i.e., when inspiration has taken place, the next thing that happens is that these little elastic air-bags begin to collapse, because when the diaphragm and the external intercostal muscles stop contracting, the pressure of the external air upon the walls of the chest is no longer resisted by the contraction of the muscles, so the external air presses upon the walls of the chest and its contents, and gradually brings them back to the original position. The instant that begins, these little elastic air-sacs partly collapse and squeeze the air out of them; at the same time the internal intercostal muscles, the fibres of which run in the opposite direction to those of the external ones, begin to contract and help the external air which is pressing upon the ribs by pulling them back towards their former position. This is called expiration, and thus about as much air is driven out of the lungs as was drawn in during inspiration.

So you see the chief forces at work in inspiration are active muscular forces, by which the chest cavity is made larger, and the chief agents in expiration are passive agents, the pressure of the air upon the walls of the chest and abdomen (because the pressure of the air upon the walls of the abdomen push it back and help the diaphragm into its position), one passive force, and the elastic recoil of the air-sacs, another passive force, and these forces are aided, and that is all, by the contraction of the internal muscles between the ribs, which help to bring the ribs down again.

This, then, is how the lungs are inflated, how the lungs collapse, and how the chest cavity is reduced to its original size.