<P> The tube is opened during swallowing by contraction of the tensor veli palatini and levator veli palatini, muscles of the soft palate . </P> <P> The Eustachian tube is derived from the ventral part of the first pharyngeal pouch and second endodermal pouch, which during embryogenesis forms the tubotympanic recess . The distal part of the tubotympanic sulcus gives rise to the tympanic cavity, while the proximal tubular structure becomes the Eustachian tube . It helps transformation of sound waves . </P> <P> Under normal circumstances, the human Eustachian tube is closed, but it can open to let a small amount of air through to prevent damage by equalizing pressure between the middle ear and the atmosphere . Pressure differences cause temporary conductive hearing loss by decreased motion of the tympanic membrane and ossicles of the ear . Various methods of ear clearing such as yawning, swallowing, or chewing gum may be used to intentionally open the tube and equalize pressures . When this happens, humans hear a small popping sound, an event familiar to aircraft passengers, scuba divers, or drivers in mountainous regions . Devices assisting in pressure equalization include an ad hoc balloon applied to the nose, creating inflation by positive air pressure . Some people learn to voluntarily' click' their ears, together or separately, performing a pressure equalizing routine by opening their Eustachian tubes when pressure changes are experienced, as in ascending / descending in aircraft, mountain driving, elevator lift / drops, etc . Some are even able to deliberately keep their Eustachian tubes open for a brief period, and even increase or decrease air pressure in the middle ear . The' clicking' can actually be heard by putting one's ear to another's while performing the clicking sound . This voluntary control may be first discovered when yawning or swallowing, or by other means (above). Those who develop this ability may discover that it can be done deliberately without force even when there are no pressure issues involved . </P> <P> The Eustachian tube also drains mucus from the middle ear . Upper respiratory tract infections or allergies can cause the Eustachian tube, or the membranes surrounding its opening to become swollen, trapping fluid, which serves as a growth medium for bacteria, causing ear infections . This swelling can be reduced through the use of decongestants such as pseudoephedrine, oxymetazoline, and phenylephrine . Ear infections are more common in children because the tube is horizontal and shorter, making bacterial entry easier, and it also has a smaller diameter, making the movement of fluid more difficult . In addition, children's developing immune systems and poor hygiene habits make them more prone to upper respiratory infections . </P>

Which structure is used to channel sound to the ear drum