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The accessory nerve is a cranial nerve that controls the sternocleidomastoid and trapezius muscles. As part of it was formerly believed to originate in the brain, it is considered the eleventh of twelve cranial nerves, or simply cranial nerve XI. Traditional descriptions of the accessory nerve divide it into two parts: a ''spinal part'' and a ''cranial part''.〔(【引用サイトリンク】title=The Accessory Nerve )〕 However, because the cranial component rapidly joins the vagus nerve, becoming an integral part of said nerve, modern descriptions often consider the cranial component to be part of the vagus nerve and not part of the accessory nerve proper. For this reason, in contemporary discussions of the accessory nerve, the common practice is to dismiss the cranial part altogether, referring to the accessory nerve specifically as the spinal accessory nerve. The spinal accessory nerve provides motor innervation from the central nervous system to two muscles of the neck: the sternocleidomastoid muscle and the trapezius muscle. The sternocleidomastoid muscle tilts and rotates the head, while the trapezius muscle has several actions on the scapula, including shoulder elevation and abduction of the arm. Range of motion and strength testing of the neck and shoulders can be measured during a neurological examination to assess function of the spinal accessory nerve. Limited range of motion or poor muscle strength are suggestive of damage to the spinal accessory nerve, which can result from a variety of causes. Injury to the spinal accessory nerve is most commonly caused by medical procedures that involve the head and neck. Clinically, iatrogenic injury usually results in severe wasting and loss of volume of the trapeizus muscle which is, typically, very noticeable. The accessory nerve is derived from the basal plate of the embryonic spinal segments C1–C6. ==Structure== Like other cranial nerves, the spinal accessory nerve exits the cranium through a specialized hole (or foramen). However, unlike all other cranial nerves, the spinal accessory nerve begins outside the skull rather than inside. In particular, in the majority of individuals, the fibers of the spinal accessory nerve originate solely in neurons situated in the upper spinal cord. These fibers coalesce to form spinal rootlets, roots, and finally the spinal accessory nerve itself, which enters the skull through the foramen magnum, the large opening at the base of the skull. The nerve courses along the inner wall of the skull towards the jugular foramen, through which it exits the skull with the glossopharyngeal (CN IX) and vagus nerves (CN X). Owing to its peculiar course, the spinal accessory nerve is notable for being the only cranial nerve to both enter and exit the skull. Traditionally, the accessory nerve is described as having a small cranial component that descends from the medulla oblongata and briefly connects with the spinal accessory component before branching off of the nerve to join the vagus nerve. A recent study of twelve subjects suggests that in the majority of individuals, this cranial component does not make any distinct connection to the spinal component; the roots of these distinct components were separated by a fibrous sheath in all but one subject.〔 Once the cranial component has detached from the spinal component, the spinal accessory nerve continues alone and heads posteriorly (backwards) and inferiorly (downwards) upon exiting the skull through the jugular foramen. In the neck, the accessory nerve crosses the internal jugular vein around the level of the posterior belly of digastric muscle. Masoud Saman et al. in a study of 84 necks reported that in the anterior triangle of the neck the accessory nerve crossed the internal jugular vein anteriorly in 80% of necks, posteriorly in 19% and in the one case of internal jugular vein bifurcation, the nerve pierced the vein. The average distance traveled by nerve from base of skull to crossing the internal jugular vein was 2.38 cm. As it courses caudally, the nerve pierces the sternocleidomastoid muscle while sending it motor branches, then continues inferiorly until it reaches the trapezius muscle to provide motor innervation to its upper portion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Accessory nerve」の詳細全文を読む スポンサード リンク
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