A Previously Unreported Accessory Muscle on the Surface of the Trapezius Muscle: A Case Report

Abstract

The trapezius muscle is one of the large muscles of the back and is innervated mainly by the accessory nerve. This muscle is generally divided into three parts composed of ascending, middle and descending fibres. During a study of the course of the accessory nerve, a series of back dissections were initiated by incision of the skin along the dorsal midline between the occipital protuberance and the first lumbar vertebrae. We report a previously unidentified accessory muscle of the back. During dissection of the back of a male cadaver, the skin, underlying fat and fascia were reflected laterally and revealed an accessory muscle superficial to the inferior segment of the left trapezius muscle. The accessory fibres originated from the spinous process of thoracic vertebra 12 and projected superiorly with supero-laterally oriented fibres. It terminated at the level of thoracic vertebra T5. Upon examination of the inferior surface of the accessory muscle, a nerve originating from the thoracic spinal nerve 11 was identified travelling along the medial border of the muscle. All dissections were performed in accordance with the laws regulating anatomic dissection in Ireland. Several surgical procedures are carried out at the inferior portion of the trapezius muscle; therefore, variation of this region is of clinical importance, and the presence of accessory muscles may have an influence on outcomes.