In the equids (horse family) and some species such as the desert hyrax, the guttural pouch is one of a pair of air chambers in the neck just behind the skull and below the ears. These pouches are on both sides of the head and are air-filled out-pouchings or evaginations to the Eustachian tube, leading into the nasopharynx.
While the location of the pouches is generally understood, their purpose is not. Currently, researchers believe the pouches serve to cool blood flow to the brain during prolonged physical exertion. 
Due to the general inaccessibility of the pouches in horses, they can be an area of infection by molds and bacteria, and these infections can be extremely severe and hard to treat. The condition guttural pouch tympany affects several breeds, including the Arabian horse. The condition predisposes young horses to infection, often including severe swelling and often requires surgery to correct.
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