SUMMARY A horse with rostral displacement of the palatopharyngeal arch was found to have a bilaterally symmetrical deformity of the laryngeal area. Both left and right cricopharyngeal muscles were absent. The shape of the thyroid cartilage was grossly abnormal and vestiges of the cricothyroid muscles were attached only to the cricoid cartilage. It was suggested that such an anomaly could have resulted from aberrant development of the fourth branchial arch. RÉSUMÉ Un cheval présentait un déplacement rostral de l'arc palato‐pharyngien. Il présentait également une déformation bilatérale et symétrique de la région laryngée. Les muscles crico‐pharyngiens droit et gauche étaient absents. La forme du cartilage thyroïde était anormale et des vestiges des muscles crico‐thyroïdiens étaient insérés sur le cartilage cricoïde. Une telle anomalie peut résulter d'un développement perturbé du 4ème arc branchial. ZUSAMMENFASSUNG Ein Pferd mit einer Verlagerung des Gaumen‐Rachen‐bogen nach rostral wies eine bilateral symmetrische Deformation der Larynxgegend auf. Die beiderseitigen Cricopharyngealmuskeln fehlten. Die Gestalt des Thy‐roidknorpels war stark verändert und Ueberreste von Cricothyroidmuskulatur fanden sich nur am Cricoid‐knorpel befestigt. Es wird vermutet, dass sich eine derartige Anomalie aus einer Fehlentwicklung des vierten Kiemenbogens ergeben haben könnte.

An endoscopic survey was performed to assess arytenoid cartilage movement during quiet respiration in 48 Clydesdale horses. Physical characteristics which could influence the length of the recurrent laryngeal nerves were also recorded from these horses and the relationship of these characteristics to the presence of abnormal left arytenoid cartilage movements was assessed statistically. Some degree of abnormal arytenoid movement was observed in 50% of the animals over one year of age. No significant correlation between sex, age, estimated weight, height, or neck length could be found although affected animals were, on average, slightly taller, lighter and longer necked.

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The nerve supply to the intrinsic laryngeal muscles of the horse was studied by gross dissection and by electromyography which was carried out before, during and after section of various intralaryngeal nerve branches. The anatomical relationships and passage of the laryngeal nerves throughout the larynx were defined. Unlike the dog and man there was no evidence of the passage of motor nerve fibres from one side of the larynx to the other.

Some clinical features of laryngeal hemiplegia in 127 horses are described. Possible aetiologic factors were found in only 11% of affected animals. The onset of clinical signs was either sudden or insidious. The majority of cases were presented because of an abnormal respiratory noise made at exercise. Other clinical signs, particularly those usually attributed to laryngopalatal dislocation, were observed in a substantial proportion of affected animals. In all cases surveyed the left arytenoid was affected, although in 3 animals a bilateral laryngeal dysfunction was noted. Thirty of 65 animals examined with a fibreoptic endoscope had other upper respiratory tract abnormalities in addition to abnormal arytenoid movements. Electrocardiographic examination of 45 affected horses did not reveal a significantly higher incidence of abnormalities than that observed in the racing population. The majority of haemograms taken from affected animals were also within normal limits.