There is only one tube leading from the back of the nasal passage to the lungs. Different parts of this tube are given different names. The pharynx is that part that extends from the back of the nose to the vocal chords. The larynx is the part of the throat where the vocal chords are located. We sometimes call it the "Adam's apple." It is very prominent in some men, but seldom noticeable in women. The trachea is the part of the tube leading down from the larynx. At the lower end of the trachea the tube divides into two parts that we call the bronchi, one leading to each lung. [Sidenote: The bronchi] The bronchi carry the air from the trachea to the lungs. They divide again and again until they become so small that there is a branch for each little air cell in the lungs. [Illustration: FIG. 85. The lungs.] [Sidenote: The lungs] The lungs are the most important part of the respiratory system. They are made up of lobes. There are two lobes in the left lung and three in the right. Each lobe is divided into _lobules_, which means small lobes. Each lobule is divided into air spaces. In these air spaces, or cells, the work of the lungs is performed. [Sidenote: Air cells] The capillaries run in the thin walls of the air spaces. The walls of these fine blood vessels are so very thin that the air in the air cells comes in almost direct contact with the blood in the vessels. While the blood is passing through the vessels in the walls of the air spaces, something happens to both the blood and the air. The air we take into our lungs contains a great deal of oxygen and very little carbon dioxid. The air that comes out of our lungs contains a great deal of carbon dioxid and much less oxygen. In other words, the oxygen from the air goes into the blood, and the carbon dioxid from the blood goes out into the air. [Sidenote: Necessity of pure air] If the air we breathe is not pure and does not contain enough oxygen, the blood cannot get all the oxygen we need, and the cells of the body become sick and die. If, when we breathe, we do not fill each little air cell in our lungs with air, a great deal of the blood sent to the lungs for oxygen cannot get it. If we wear very tight clothing we cannot take a full breath and hence cannot fill all the air spaces with air. If the air spaces in the lungs are not filled, the blood does not get the oxygen it needs, as there is no other place in the body to get it. THE EYE AND THE EAR [Sidenote: The eye compared to a camera] The eye is one of the most important organs in the body and also one of the most delicate. It is very much like a camera. [Sidenote: The cornea and the sclera]