We report, for the first time, a unilateral agenesis of the thyrohyoid muscle in an adult dog. The thyrohyoid muscle in the dog normally originates in the lamina of the thyroid cartilage, where it is limited dorsally by the insertion of the thyropharyngeal muscle and caudally by the sternothyroid (dorsally) and cricothyroid (ventrally) muscles. The thyrohyoid fibres pass obliquely forward and downward, over the surface of the thyroid lamina, and are inserted at the caudal part of the thyrohyoid bone. The thyrohyoid muscle draws the hyoid apparatus caudally, and the larynx rostrally (Schaller, 1992; Evans, 1993) cooperating with the sternothyroid and sternohyoid muscles to stabilize the hyoid apparatus and to retract it towards the thorax during deglutition.
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