Fig. 33—*Lungs and air passages* seen from the front. The right lung shows the lobes and their divisions, the lobules. The tissue of the left lung has been dissected away to show the air tubes. 3. In the lobule the air tube divides into a number of smaller tubes, each ending in a thin-walled sac, called an _infundibulum_. The interior of the infundibulum is separated into many small spaces, known as the _alveoli_, or air cells. The lungs are remarkable for their lightness and delicacy of structure.(30) They consist chiefly of the tissues that form their sacs, air tubes, and blood vessels; the membranes that line their inner and outer surfaces; and the connective tissue that binds these parts together. All these tissues are more or less elastic. The relation of the different parts of the lungs to each other and to the outside atmosphere will be seen through a study of the *Air Passages.*—The air passages consist of a system of tubes which form a continuous passageway between the outside atmosphere and the different divisions of the lungs. The air passes through them as it enters and leaves the lungs, a fact which accounts for the name. [Fig. 34] Fig. 34—*Model of section through the head*, showing upper air passages and other parts. 1. Left nostril. 2. Pharynx. 3. Tongue and cavity of mouth. 4. Larynx. 5. Trachea. 6. Esophagus. The incoming air first enters the _nostrils_. These consist of two narrow passages lying side by side in the nose, and connecting with the pharynx behind. The lining of the nostrils, called _mucous membrane_ is quite thick, and has its surface much extended by reason of being spread over some thin, scroll-shaped bones that project into the passage. This membrane is well supplied with blood vessels and secretes a considerable quantity of liquid. Because of the nature and arrangement of the membrane, the nostrils are able to _warm_ and _moisten_ the incoming air, and to _free it from dust particles_, preparing it, in this way, for entrance into the lungs (Fig. 34). The nostrils are separated from the mouth by a thin layer of bone, and back of both the mouth and the nostrils is the pharynx. The _pharynx_ and the _mouth_ serve as parts of the food canal, as well as air passages, and are described in connection with the organs of digestion (Chapter X). Air entering the pharynx, either by the nostrils or by the mouth, passes through it into the _larynx_. The larynx, being the special organ for the production of the voice, is described later (Chapter XXI). The entrance into the larynx is guarded by a movable lid of cartilage, called the _epiglottis_, which prevents food particles and liquids, on being swallowed, from passing into the lower air tubes. The relations of the nostrils, mouth, pharynx, and larynx are shown in Fig. 34.