Abstract: Objective    To explore the anatomical relationship between the accessory nerve and the investing fascia of the posterior border of the sternocleidomastoid, so as to provide a safe and reliable method for avoiding injury of accessory nerve in the posterior cervical triangle surgery.    Methods    Twenty-one formalin-fixed adult cadavers were dissected to expose the sternocleidomastoid and the posterior cervical triangle. The investing fascia of the posterior edge of sternocleidomastoid was cut and observed to identify its integrity and compactness. The anatomical relationship between the investing fascia and the accessory nerve was observed to identify whether the accessory nerve could be easily identified and separated from the tissue structure.    Results    The posterior investing fascia covering sternocleidomastoid muscle was intact and dense. There was no direct contact, fiber connection or other tissues between the accessory nerve and the investing fascia. After the investing fascia was cut and opened at the posterior edge, the accessory nerve could not be observed directly on both sides in the 18 specimens. The accessory nerve was separated from the investing fascia by adipose tissue. In other 3 specimens, the accessory nerve could be seen directly after cutting open investing fascia. There was no direct contact or fibrous connection between the accessory nerve and the investing fascia in all specimens. All accessory nerves were located above the great auricular nerve with a distance of (12.6±5.2) mm.   Conclusions    The investing fascia of the posterior edge of the sternocleidomastoid is intact and dense without significant connection with the accessory nerve. It is easy to identify accessory nerve after sharp dissection with clear surgical field in the posterior cervical triangle surgery.

Key words: Accessory nerve,  ,  , Posterior cervical triangle,  ,  , Investing , fascia,  ,  , Anatomy