after its circulation through the lungs." _I._ That must answer our purpose, dear Ann. Of the change which takes place in the blood, and of the renewal of our physical system, which is effected by circulation, I shall say nothing. We will pass to respiration. _E._ Whose popular name is breathing? _I._ Yes. The act of inhaling air, is called inspiration; that of sending it out, expiration. Its organs are the lungs and windpipe. The apparatus employed in the mechanism of breathing is very complex. The windpipe extends from the mouth to the lungs. _A._ How is it that air enters it so freely, while food and drink are excluded? _I._ By a most ingenious contrivance. The opening to the pipe is called glottis. This is closed, when necessary, by a little valve, or lid, called the epiglottis (_epi_ means _upon_.) _E._ And this faithful sentinel is none other than that perpendicular little body which we can see in our throats, and which we have _dubbed_ palate. _I._ You are right, Ellinora. Over this, food and drink pass on their way to the road to the stomach, the gullet. The pressure of solids or liquids tends to depress this lid on the glottis; and its muscular action in deglutition, or swallowing, tends to the same effect. As soon as the pressure is removed, the lid springs to its erect position, and the air passes freely. Larynx and trachea are other names for the windpipe, and pharynx is another for the gullet. The larynx divides into two branches at the lungs, and goes to each side. Hence, by subdivisions, it passes off in numerous smaller branches, to different parts of the lungs, and terminates in air-cells. The lungs, known in animals by the name of lights, consist of three parts, or lobes, one on the right side, and two on the left. _Alice._ The lights of inferior animals are very light and porous--do our lungs resemble them in this?