We will now consider the apparatus by means of which we breathe, the way in which that apparatus works, and the result of that working.

The principal organs that are used in breathing go by the name of the respiratory tract. This begins with the nostrils, passes along through the lower part of the cavities of the nose, on each side of the partition or septum, as it is called, over the hard and soft palate of the mouth, into a cavity which we call the pharynx, situated behind the mouth, opening into this cavity by two apertures called the posterior nostrils; then it passes on through the pharynx, down into a box made of cartilage or gristle in the front of the throat, which we call the larynx, and in which box the voice is formed; it passes through an opening in that box, called the glottis, between two cords, called the vocal cords, into the trachea or windpipe, which lies in the front part of the neck. Now the windpipe is a tube which must be always open; it would not do if the windpipe were closed, even for a short time, for reasons that we shall soon see, and so its walls are provided with incomplete rings of cartilage or gristle, something like the letter C, the ends being joined by membrane which forms the back part of the windpipe, where it touches the gullet, down which the food passes, and these rings being not complete, but joined by membrane, offer little or no resistance to food passing down the gullet. This windpipe passes down the front of the neck, into the thorax or chest, and there divides into two branches, still with rings. These two branches go by the name of the bronchi. You will remember the name, because it is from this name that the word bronchitis comes; bronchitis being inflammation of the lining membrane of these tubes, and each of these tubes is called a bronchus; these bronchi go one to each lung, the lungs being two organs, situated one on each side of the cavity of the thorax or chest. When each bronchus gets into the lung, after going a little distance in the structure of the lung, it divides into a great number of branches. Now inside the lung the rings do not remain, but the tubes require to be still wide open, and so in their walls there are patches of cartilage sufficiently stiff to keep the tubes open. At last they become very fine, and the stiff cartilage is lost, but there is a certain amount of muscular fibre in their walls; then each of these fine tubes has at its extremity a large number of small bags or sacs made of very elastic membrane sticking out from its walls; these are called the air-sacs of the lungs, and are about one-fortieth of an inch in diameter.

All this system of tubes, beginning with the nose and going down to the fine bronchi, is of course lined with a form of the internal skin, because it is a system of tubes communicating with the external air.

The peculiar character of the epithelial lining of the mucous membrane of the respiratory passages is that it has on its surface an enormous number of very fine hair-like bodies, called cilice, which have a property of their own of continually moving, lashing towards the outlet of the respiratory passages in the nose, and it is by the movement of these little ciliae that the respiratory passages do not get clogged up with the moist secretion of the mucous membrane.

Such is a brief description of the air-passages of the lungs.

You will remember I told you that the pulmonary artery which leaves the right ventricle of the heart divides into two branches, one of which goes to each lung, and then subdivides into a very large number of branches, ending at last in very fine capillary vessels -just in the same way as the branches of the large artery of the body end in capillaries in tissues of the body-which entwine themselves round the little air-sacs of the lungs, and which are so near together, that the capillaries generally have an air-sac on each side of them; the capillaries then run together and form little veins. These little veins join and form larger veins, which ultimately unite in two large veins for each lung, the pulmonary veins, and these run into the left auricle, the upper chamber of the left side of the heart.

The chief things that we have to consider in the lungs are the air-passages and the blood-vessels.

Now the lungs are contained in the cavity which we call the thorax or chest. Before we can understand how the lungs work, and what they do, we must understand the cavity in which they are contained, and the movements of the walls of that cavity. This cavity has the root of the neck at its upper part; the spinal column and back part of the ribs behind; the ribs, and the things which connect the ribs at its sides, and partly in front; the breast-bone and the cartilages of the ribs, and the structures connecting them in front, and the partition mentioned in the first chapter as being between the thorax or chest and the abdomen, which we called the diaphragm, below; this partition is partly muscular, and partly tendinous; it rises by muscular fibres from the bodies of the lumbar vertebrae; these go by the name of the pillars of the diaphragm; they run upwards from the bodies of the vertebrae in an arched way, and are attached to the edges of the lower ribs and the cartilages in front, and to the lower part of the breast-bone, so that the diaphragm forms a complete partition between the thorax or chest and the abdomen.

I have mentioned that there are structures between the ribs; the ribs are connected partly by fibrous tissue, and more particularly there are muscles that pass between the ribs, from one rib to the next, and there are two important layers of muscles that pass between the ribs that are of great use in respiration ; these muscles, because they run between the ribs, go by the name of intercostal. The outside ones are called the external intercostal muscles, and their fibres run from each rib downwards and forwards to the rib below it; the inner ones are called the internal intercostal muscles, and their fibres run downwards and backwards from each rib to the one below it.