Variability in the human m. spinalis capitis and cervicis: frequencies and definitions.

The spinalis muscle is defined as the medial component of the erector spinae muscle group, and is typically subdivided into three regional components: m. spinalis thoracis, m. spinalis cervicis and m. spinalis capitis. Modern authorities, however, differ on the morphology of the cervicis and capitis portions and many claim that these regional distinctions do not commonly exist. This article reports on the validity of that claim. Three morphological conditions are identified for both m. spinalis capitis and m. spinalis cervicis. The distinct condition has the muscle persisting as an independent muscular unit. The blended condition occurs when the spinalis muscle fibers blend with those of the semispinalis muscle. Finally, there are occasions in which no evidence of spinalis muscle fibers can be found. Frequency estimates are provided for each of these morphological conditions. Results of statistical analyses indicate that there are no relationships among muscle varieties and the sex of the individual or the side of the body where the muscle was found.