Functional anatomy observations of the pharyngeal orifice of the equine guttural pouch (auditory tube diverticulum).

The pharyngeal orifice of the equine auditory tube has not been adequately described. 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. 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. The results of this investigation suggest that regular guttural pouch ventilation during resting respiration does not occur. Previous reports may reflect, at least in part, a misinterpretation of air pressure measurements within the guttural pouches and nasopharynx. 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. Intrinsic tone of the stylopharyngeus and pterygopharyngeus muscles may be part of this active process. 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. 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 alternative passive route involves reduction in tone of the stylopharyngeus and pterygopharyngeus muscles accompanied by increased inspiratory pressure.