Digastric muscle of the kangaroo: a comparative anatomical study

Although the human digastric muscle is classified as a suprahyoid muscle, none of the digastric muscles in other mammals are classed as suprahyoid in textbooks of veterinary anatomy. The aim of this study was to describe the anatomical relationship of the digastric muscle in a marsupial, the kangaroo, and to consider factors thought to be important in leading to the different position of the muscle in quadrupeds compared with humans. Eight heads of the common wallaroo (Macropus robustus) were used in this study. They were fixed by injection of 10% formalin solution into the carotid arteries and dissected under a stereomicroscope. The digastric muscle in the common wallaroo arose from the paroccipital process of the temporal bone and inserted into the mandible but had no intermediate tendon or any connection with the hyoid bone. It was supplied by both the mandibular and facial nerves. The hyoglossus muscle attached to the inferior surface of the hyoid bone and its ventral border overlapped the mylohyoid muscle. The hypoglossal nerve passed between these two muscles. Therefore, in contrast to humans, the digastric, hyoglossus, and mylohyoid muscles in the kangaroo were all located inferior to the hyoid bone. Differences in head posture and the position of the larynx between kangaroos and humans may account for the observed difference in the digastric muscle's position relative to the hyoid bone between these species.