What thrusts forward the sternum? If you were to watch closely the sides of the chest of a very thin person, you would be able to notice that at every breathing in, at every inspiration, the ribs are pulled up a little way. Now, each rib is connected with the backbone behind by a joint, and is firmly fastened to the sternum in front by cartilage (see Frontispiece). If you were to fasten a piece of string to the middle of one of the ribs and to pull it, you would find you were working on a lever, with the fulcrum at the backbone, with the weight acting at the sternum, and the power at the point where your string was tied. Every time you pulled the string the rib would move on its fulcrum at the backbone, in such a way that the front end of the rib would rise up, and the sternum would be thrust out a little. When you left off pulling, the sternum, which in being thrust forward had been put on the stretch, would sink back, and the rib would fall down to its previous position. [Illustration: FIG. 14.--_View of Four Ribs of the Dog with the Intercostal Muscles._ _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.