We have now a clear notion of the kind of work that goes on in the lungs-that incessant exchange, by osmosis, through the delicate walls of the air-cells, and the capillaries which surround them, enabling the blood to give up its impurities, and at the same time to take in fresh supplies of oxygen. Think for a moment of those air-cells. The air which they contain is being perpetually deprived of its oxygen, and loaded with carbonic acid and water-vapor in place of it. In a very short time that air, if it were not constantly renewed, would lose the whole of its oxygen, and become completely saturated with carbonic acid. This is why the work of breathing never stops, sleeping or waking. The impure, vitiated air must be driven out, and pure air taken into the lungs to supply its place.
A healthy man, resting calmly in a sitting position, breathes from thirteen to fifteen times every minute, but as soon as he begins to exert himself with exercise of any kind, the breathing becomes more rapid, and continues to increase in rapidity in proportion to the exertion he makes.
Each breathing consists of three distinct acts, followingeach other in regular order. First, a quantity of air is drawn in. This is called an inspiration-that is, a breathing in. As soon as the inspiration is over the second act commences, and the air is driven out, or expelled, from the mouth and nostrils. This is called an expiration, or a breathing out.
Following the expiration comes a pause, after which the same process is perpetually repeated, and always in the same order-inspiration, expiration, pause-like some beautiful piece of machinery.
The mechanical work of breathing is due to three causes-the pressure of the atmosphere, the elastic nature of the lungs themselves, and the character of the air-tight box which contains them.
The thorax is absolutely air-tight; no air can get in between the lungs and the outer walls of this chamber. The lungs themselves, however, are full of air, which is brought to them by the windpipe and other air-passages, and presses with a force of nearly 15 lbs. on every square inch. This enormous pressure from within the lungs forces them outwards against the sides and floor of the chamber. The chest itself is protected by a hard, resisting, bony cage, and not by soft, flabby walls, so that, although the outer air is pressing with the same force, it cannotsqueeze that chamber, nor affect the lungs inside. The lungs, therefore, are pressed outwards against the walls of the chest by the air within them, but they feel no pressure from without.
The ribs, which form the walls of the thorax, are jointed to their vertebrae by joints which allow them a limited up-and-down movement. The spaces between the ribs are occupied by stout, strong muscles, whose sole duty it is to move the ribs. One set of these musclesraises them, the other set depresses them; and we have already seen that these respective movements enlarge and diminish in turn the capacity of the chest.
Now let us turn to the diaphragm, which forms the floor of the chamber. It is attached to the ribs all round, and the central part of it rises in the form of a hollow dome or arch. It is clear that, if such a floor could be forced down in any way, it would considerably enlarge the chamber above.
You already know that this diaphragm is a stout, strong, muscular partition. Like all other muscles, it has the power of contracting, and when it contracts its muscular fibres are shortened, so that the dome or arch is pulled down and flattened. This contraction, and consequent pulling down of the diaphragm, always takes place at the very moment when the ribs are being raised; hence the chest is enlarged on all sides at once.
While this is going on the elasticity of the lungs enables them to stretch out, to fill the larger space, and the air rushes in down the windpipe to fill all the little distended air-cells, or there would be a vacuum. This is the whole mechanism of an inspiration.
Immediately after the inspiration the diaphragm ceases to contract, and springs back by its own elasticity, while the ribs too are being depressed, so that the chest is diminished in size from both causes at the same time. Meanwhile the lungs themselves assert their power by contracting, so that each little air-cell is squeezed intosmaller space. In this way some of the air is driven outfrom the lungs, and we have an expiration.