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 adequa tely described. Its larger size and position, further rostral within t he nasopharynx compared with other species, cannot be easily explained , but could be important for the proposed brain-cooling function of th e guttural pouches: however, unlike other species, it may not be the s ole regulator of auditory tube aeration. A 'second orifice' (represent ing the final entrance to the auditory tube diverticulum) deserves rec ognition in performing a role in air exchange between the auditory tub e, diverticulum and middle ear. The results of this investigation sugg est that regular guttural pouch ventilation during resting respiration does not occur. Previous reports may reflect, at least in part, a mis interpretation of air pressure measurements within the guttural pouche s and nasopharynx. Small decreases in simulated inspiratory pressure c onsistently caused the passive opening of both the pharyngeal orifice and pouch ostium in horse cadavers suggesting that an active counter p rocess 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 acti ve process. The investigation offers theoretical evidence that opening of the equine auditory tubes to ventilate the guttural pouch occurs i n two different ways, active and passive. The active process has two s tages: (1) opening of the pharyngeal orifices by simultaneous contract ions of the levator and tensor veli palatini, and pterygopharyngeus mu scles; then (2) opening of the pouch ostia by contractions of dorsal p ortions of the palatopharyngeus muscles. The alternative passive route involves reduction in tone of the stylopharyngeus and pterygopharynge us muscles accompanied by increased inspiratory pressure.