The alternative passive route involves reduction in tone of the stylopharyngeus and pterygopharyngeus muscles accompanied by increased inspiratory pressure. The active process has two stages: (1) opening of the pharyngeal orifices by simultaneous contractions of the levator and tensor veli palatini and pterygopharyngeus muscles then (2) opening of the pouch ostia by contractions of dorsal portions of the palatopharyngeus muscles. The investigation offers theoretical evidence that opening of the equine auditory tubes to ventilate the guttural pouch occurs in two different ways, active and passive. Intrinsic tone of the stylopharyngeus and pterygopharyngeus muscles may be part of this active process. Small decreases in simulated inspiratory pressure consistently caused the passive opening of both the pharyngeal orifice and pouch ostium in horse cadavers suggesting that an active counter process may exist in the live horse to maintain closure of the guttural pouch openings during rest and light exercise. Previous reports may reflect, at least in part, a misinterpretation of air pressure measurements within the guttural pouches and nasopharynx. The Eustachian tube is also known as the pharyngotympanic tube or the auditory tube. As it grows to double that length, it grows to be at an incline of 45 degrees in adulthood so that the nasopharyngeal orifice. The Eustachian tube measures only 17-18mm and is horizontal at birth. The Eustachian tube opens into the nasopharynx. ![]() The results of this investigation suggest that regular guttural pouch ventilation during resting respiration does not occur. The Eustachian tube is named after the Italian anatomist, Bartolomeo Eustachi, who observed that it was a canal that connected the nasopharynx to the middle ear. Auditory tube: The tube that runs from the middle ear to the pharynx. ![]() ![]() A 'second orifice' (representing the final entrance to the auditory tube diverticulum) deserves recognition in performing a role in air exchange between the auditory tube, diverticulum and middle ear. Its larger size and position, further rostral within the nasopharynx compared with other species, cannot be easily explained, but could be important for the proposed brain-cooling function of the guttural pouches however, unlike other species, it may not be the sole regulator of auditory tube aeration. The pharyngeal orifice of the equine auditory tube has not been adequately described.
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