Positional relationships between the masticatory muscles and their innervating nerves with special reference to the masseter and zygomaticomandibularis in Suncus murinus

In the present study, we investigated the structure and nerve innervation of the masseter, temporalis and zygomaticomandibularis of Suncus murinus which has no zygomatic arch. Detailed dissection of eight head halves of four 5. murinus was performed. In S. murinus, small muscle bundle was observed to be adjoined with the lateral surface of the temporalis. This muscle bundle was completely separated from the masseter. Based on the positional relationships between the muscle bundle and supplying nerves, we conducted that the bundle corresponded to the zygomaticomandibularis of human described in our previous study (Shimokawa et al., 1999). In addition, some differences in the nerve distribution to the masticatory muscles were observed in 5. murinus as compared with humans with respect to the following points: 1) The additional supplying branch to the masseter originated from the auriculotemporal nerve: 2) The common trunk of the masseteric nerve and the nerve to the posterior part of the temporalis penetrated the superior head of the lateral pterygoid. A possible model to account for these differences based on the positional relationships among the muscles and supplying nerves is presented.