Variable composition of the internal and external branches of the accessory nerve.

The purpose of this study was to clarify the composition of the internal and external branches (IB and EB) of the accessory nerve. Fifty-seven half heads of 34 adult cadavers were used. The IB and EB of the accessory nerve were mixed with the cranial root (CR), vagus nerve, and spinal root (SR). The IB was classified into five types and the EB into four types according to their composition. The IB consisted of only CR in 7.0% of the 57 cases, and of the CR and the vagus nerve in 52.6%; the IB did not exist in 12.3%. The EB was only composed of the SR in 19.3% of cases, the SR and CR in 52.6%, and the SR, CR, and the vagus nerve in 21.1%. There were 14 combinations of IB and EB types. The most common combination was the IB with the CR and the vagus nerve, and the EB with the SR and CR (31.6%). The combination of IB and EB comprising CR and SR, respectively, was not observed. The IB and EB are known to consist of the CR and SR of the accessory nerve, respectively. However, this study shows that there are no IB and EB comprising only the CR and SR, respectively, and the branches have various combinations of the CR, SR, and vagus nerve.