Observation of the anterior supracostal muscles.

A pair of anomalous supernumerary muscles has been found in a single cadaver which fit the classic description of the Musculus supracostalis anterior. The muscles took origin from the first rib deep to the two pectoral muscles and between the origins of the pectoralis minor and serratus anterior muscles. They were directed inferiorly to insert on the upper margin of the fourth rib. The nerve supply of one muscle of the pair was determined to be from the collateral branch of the second right intercostal nerve. This supports the theory that these rare muscles may be derived from a single external intercostal muscle which grew to span several intercostal spaces.