a. The bony rib; b, the cartilage; c, the junction of bone and cartilage; d, unossified; e, ossified, portions of the sternum. A. External intercostal muscle. B. Internal intercostal muscle. In the middle interspace, the external intercostal has been removed to show the internal intercostal beneath it.
Between the ribs are certain muscles called intercostal muscles ([Fig. 14]). The exact action of these you will learn at some future time. Meanwhile it will be enough to say that they act like the piece of string we are speaking of. When they contract, they pull up the ribs and thrust out the sternum; when they leave off contracting, the ribs and sternum fall back to their previous position.
There are many other muscles which help in breathing, especially in hard or deep breathing, but it will be sufficient for you to remember that in ordinary breathing there are two chief movements taking place exactly at the same time, by means of which air is drawn into the chest, both movements being caused by the contraction of muscles. First, the diaphragm contracts and flattens itself, making the chest deeper or longer; secondly, at the same time the ribs are raised and the sternum thrust out by the contraction of the intercostal muscles, making the chest wider. But as the chest becomes wider and longer, the lungs become wider and longer too. In order to fill up the extra room thus made in the lungs, air enters into them through the windpipe. This is inspiration. But soon the diaphragm and the intercostal muscles cease to contract; the diaphragm returns to its arched condition, the ribs sink down, the sternum falls back, and the extra air rushes back again out of the lungs through the windpipe. This is expiration. An inspiration and an expiration make up a whole breath; and thus we breathe some seventeen times in every minute of our lives.
[43.] But what makes the diaphragm and intercostal muscles contract and rest in so beautifully regular a fashion? The biceps of the arm, we saw, was made to contract by our will. It is not our will, however, which makes us breathe. We breathe often without knowing it; we breathe in our sleep when our will is dead; we breathe whether we will or no, because we cannot help it. We can quicken our breathing, we can take a short or deep breath as we please, we can change our breathing by the force of our will; but the breathing itself goes on without, and in spite of, our will. It is an involuntary act.
Though breathing is not an effort of the will, it is an effort of the brain; an effort, too, of one particular part of the brain, that part where the brain joins on to the spinal cord. Nerves run from the diaphragm and the intercostal and other muscles through the spinal cord, to this part of the brain. And seventeen times a minute a message comes down along these nerves, from the brain, bidding them contract; they obey, and you breathe. Why and how that message comes, you will learn at some future time. When your head is cut off, or when that part of the brain which joins on to the spinal cord is injured by accident or made powerless by disease, the message ceases to be sent, and you cease to breathe.
[44.] At every breath, then, a certain quantity of air goes in and out of the chest; but only a small quantity. You must not think the lungs are quite emptied and quite filled at each breath. On the contrary, you only take in each time a mere handful of air, which reaches about as far as the large branches of the windpipe, and does not itself go into the air-cells at all. This is often called tidal air; and the rest of the air in the lungs, which does not move, is often called the stationary air (see [Fig. 13]).
How then does the carbonic acid at the bottom of the lungs get out? How do the capillaries in the air-cells get their fresh oxygen?