An anatomical and endoscopic study of the nasopharynx and larynx of the donkey (Equus asinus).

Using histological techniques, anatomical dissection, fibre optic endoscopy and radiography, the nasopharynx and larynx of the donkey were examined and described. Compared with horses, donkeys have a much deeper pharyngeal recess extending 4.0-6.0 cm caudally from a constricted orifice, through which it communicates with the nasopharynx. Movements of the membranous lining of the recess reflect pressure changes in the upper airways. A shallow, thumb shaped depression on the rostral face of each vocal fold corresponds in position to the lateral ventricle in the horse. In the donkey, large paired laryngeal saccules, interposed between the thyroarytenoideus muscle and the thyroid cartilage, open into the laryngeal cavity through small circular orifices near the rostroventral extremity of the vestibular folds. The large thyroarytenoideus muscle is subdivided into vestibular and vocal parts, which are thought to control the expression of secretion from the laryngeal saccules. The unusual characteristics of vocalisation in the donkey may be related to the shape and orientation of the resonance chambers of the upper airways. In comparison with the horse, the asinine nasopharynx is markedly constricted in its middle part and the laryngeal airway has a more acute angulation relative to the nasopharynx. Vocalisation in the donkey was observed to have an inspiratory and an expiratory component.