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?
I. Yes; they are full of air-tubes and air-cells. These, with the blood vessels and the membrane which connects (and this is cellular, that is, composed of cells,) form the lungs. The process of respiration involves chemical, mechanical, and vital or physiological principles. Of the mechanism I shall say but little more. You already know that the lungs occupy the chest. Of this, the breast bone forms the front, the spine, the back wall. Attached to this bone are twelve ribs on each side. These are joined by muscles which are supposed to assist in elevating them in breathing, thus enlarging the cavity of the chest. The lower partition is formed by a muscle of great power, called the diaphragm, and by the action of this organ alone common inspiration can be performed. Hayward says, "The contraction of this muscle necessarily depresses its centre, which was before elevated towards the lungs. The instant this takes place, the air rushes into the lungs through the windpipe, and thus prevents a vacuum, which would otherwise be produced between the chest and lungs." Expiration is the reverse of this. The chemistry of respiration regards the change produced in the blood by respiration. To this change I have before alluded.
Ann. When we consider the offices of the heart and lungs, their importance in vital economy, how dangerous appears the custom of pressing them so closely between the ribs by tight lacing?
I. Yes; fearful and fatal beyond calculation! And one great advantage in a general knowledge of our physical system, is the tendency this knowledge must have to correct this habit.
A. To me there is not the weakest motive for tight lacing. Everything but pride must revolt at the habit; and there is something positively disgusting and shocking in the wasp-like form, labored breathing, purple lips and hands of the tight lacer.
E. They indicate such a pitiful servitude to fashion, such an utter disregard of comfort, when it comes in collision with false notions of elegance! Well for our sex, as we could not be induced to act from a worthier motive, popular opinion is setting in strongly against this practice. Many of our authors and public lecturers are bringing strong arms and benevolent hearts to the work.