A case of the scalenus anterior muscle lying behind the fifth cervical nerve root.

Muscle slip composition and innervation was minutely examined in the case of the scalenus anterior muscle lying behind the 5th cervical nerve root. This minor change of topography deeply involved the muscle-nerve relationships of the entire muscle. Critical observations were as follows: (1) lower and unusual origins of the muscle; (2) irregular innervation with regard to the nerve elements and their approach to the muscle. This evidence suggested that abnormal migration, splitting and fusion of the muscle anlage had occurred during the development of the variant.