The Accessory Nerve (XI)
The accessory nerve is curiously formed of two roots. The spinal root is provided by axons that emerge midway between the dorsal and ventral roots of the first five (or so) spinal nerves (Figs.
8.19 and 8.20). These roots combine in a trunk that runs cranially within the spinal subarachnoid space to enter the skull through the foramen magnum; it then approaches the cranial root, which is formed by the most caudal rootlets of the glossopharyngeal-vagus series. There is only brief contact between the two roots, and although some fibers may be exchanged, the cranial root then amalgamates with the vagus to which it probably furnishes the fibers that reach the laryngeal musculature via the recurrent laryngeal nerve. It is the spinal root that forms the accessory nerve of descriptive anatomy. This nerve passes through the jugular foramen to divide within the atlantal fossa into dorsal and ventral branches.The dorsal branch runs caudally over the splenius and serratus ventralis before it supplies the brachiocephalicus, omotransversarius, and trapezius. The ventral branch supplies only one muscle, the sternocephalicus, which it enters close to its cranial attachment.
There is no convincing explanation for the curious detour made by the spinal fibers of this nerve.
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