Objectives: The digastric muscles in mammals consist of an anterior belly, posterior belly and an internal tendon in-between. The anterior and posterior bellies are innervated by the trigeminal and facial nerves respectively. Lubosch (1938) proposed that the classification of a muscle as monogastric should not be based solely on whether there is a single muscle belly but also on whether the muscle has a single innervation. He indicated that pig and hare have monogastric muscles with trigeminal innervation. In contrast, kangaroos have a digastric muscle consisting of a single belly but double innervation (Tomo et al. 1998). The digastric muscle in koalas has been referred to being both monogastric (Sontag 1922) and digastric (Charmaine 1990).
Methods: Three heads of koalas (two males, one female) were used for this study. The digastric muscle and its innervations were examined in detail under a stereomicroscope and recorded.
Results: The digastric muscles originated from the medial surface of the ventral border of the mandibular body. They inserted into the paraoccipital process. It is single belly without any intermediate tendon. And the muscle was innervated only by the mylohyoid nerve.
Conclusions: Therefore, we propose that this muscle should be referred to as a monogastric muscle. This study was supported in part by Grant-in-Aid for Scientific Research (c), No. 17590178 from the Ministry of Education, Science, Sports and Culture of Japan.