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An epiglottis is an anatomical structure that is designed to prevent animals from inhaling foods or liquids while they eat and swallow. You could think of it as a lid or flap which covers the trachea, creating a seal that does not permit anything other than air to enter the trachea. This anatomical structure is very important, as without it, an organism would run the risk of choking and coughing every time it tried to eat.
To use an analogy which may be familiar to you, the epiglottis is a lot like a toilet seat attached to the root of the tongue. During normal activities, it is left in the up position, allowing air to flow freely into the larynx and trachea. However, when an organism starts eating, the epiglottis snaps shut, covering the opening into the trachea. When the organism is done swallowing, the flap pops back up again to allow the organism to breathe.
The function of the epiglottis is made possible by the fact that it is made from elastic cartilage. Cartilage is typically very stiff, but elastic cartilage tissue is made up of tiny bundles of extremely elastic fibers which make it flexible as well as sturdy. When coated with mucus membranes, as in the case of this structure, elastic cartilage tissue is soft enough to form a tight seal, and stiff enough to hold up against the action of swallowing.
Movement of the epiglottis is triggered by movements of the hyoid bone during swallowing, which means that organisms do not need to learn to move it during swallowing, because they are born with the ability to do so. Because the structure is not controlled by brain activity, it is also not dependent on nerve signals; it's basically like a mechanized part that requires no real maintenance on the part of the user.
Sometimes, the epiglottis can become inflamed, in a situation known as epiglottitis. The inflammation causes the flap to swell, and this can potentially be extremely dangerous, as the swollen tissue may inhibit breathing. Prompt treatment is required for such inflammation to ensure that the patient will be able to breathe, and in some cases the patient may need to be intubated to secure his or her airway until the cause of the swelling can be resolved.