Anatomical study of the human omohyoid muscle: regarding intermediate morphologies between normal and anomalous morphologies of the superior belly.

Intermediate morphologies between normal and anomalous morphologies of the superior belly of the omohyoid muscle (Om) were macroscopically and stereomicroscopically observed in 34 cadavers (24 males and 10 females aged between 51 and 97 years; average age 71.0 years) for anatomical practice, which had been preserved in the Department of Morphological Biology, Ohu University School of Dentistry. The intermediate morphologies were classified into four types on the basis of the developmental degree of the muscle fibers and the number and origin of the belly as follows: type 1, the anterior margin of the belly was unclear owing to poor myofiber development; type 2, the superior belly was composed of a posterior large belly and an anterior small belly; type 3, composed of three to five bellies, with the bellies arranged in a roof tile-like morphology; and type 4, the belly was composed of two bellies arranged anterior-posteriorly parallel to each other (the anterior belly was found to be the inferior belly that had developed and reached the superior belly area). For the intermediate morphologies of the Om superior belly observed in the present study, although type 4 was due to the development of an inferior belly, the other three types were considered to be caused by the poor development of the myofibers in the formation process and by the division of the superior belly into two muscles, or secondary lamellar division of the belly with growth.