Among the intrinsic laryngeal muscles, the arytenoid muscle (AR) displays greatest morphological variations among species. There are two major types : unpaired and paired type. The former has been considered to be more highly evolved than the latter. Dual innervation by the bilateral recurrent laryngeal nerves has been demonstrated in unpaired AR of some species. This study consisted of two portions. The first portion macroscopically confirmed that AR was of a typical unpaired type in man, monkey and guinea pig (Cavia porcellus), of an atypical paired type in dog, and of a typical paired type in cat and rat (Rattus norvegicus). In the second portion, the localization of motor endplates in AR was investigated with the use of acetylcholinesterase staining in three guinea pigs, three rats and four dogs. In the guinea pigs, a broad band of motor endplates was observed in the middle portion of the unpaired AR. In rats, a band of motor endplates was noted in each of the paired AR. The canine AR consisted of two muscles transverse arytenoid and ventricular muscles. The former is paired whereas the latter is partly paired and partly unpaired. Motor endplates of the ventricular muscle were localized in the bilateral muscle bellies and the middle of the unpaired portion. The findings indicate that AR of unpaired type receives dual innervation.

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