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Cranial nerves are the nerves that emerge indirectly from the brain (including the brainstem), in contrast to spinal nerves (which emerge from segments of the spinal cord). Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck. Spinal nerves emerge sequentially from the spinal cord with the spinal nerve closest to the head (C1) emerging in the space above the first cervical vertebra. The cranial nerves emerge from the central nervous system above this level. Each cranial nerve is paired and is present on both sides. Depending on definition in humans there are twelve or thirteen cranial nerves pairs, which are assigned Roman numerals I–XII, sometimes also including cranial nerve zero. The numbering of the spinal nerves is based on the order in which they emerge from the brain, front to back (brainstem).〔 The terminal nerves, olfactory nerves (I) and optic nerves (II) emerge from the cerebrum or forebrain, and the remaining ten pairs arise from the brainstem, which is the lower part of the brain.〔 The cranial nerves are considered components of the peripheral nervous system (PNS),〔 although on a structural level the olfactory, optic and terminal nerves are more accurately considered part of the central nervous system (CNS).〔Board Review Series – Neuroanatomy, Fourth Edition, Lippincott Williams & Wilkins, Maryland 2008, p. 177. ISBN 978-0-7817-7245-7.〕 ==Anatomy== Most typically, humans are considered to have twelve pairs of cranial nerves (I–XII). They are: the olfactory nerve (I), the optic nerve (II), oculomotor nerve (III), trochlear nerve (IV), trigeminal nerve (V), abducens nerve (VI), facial nerve (VII), vestibulocochlear nerve (VIII), glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI), and hypoglossal nerve (XII). There may be a thirteenth cranial nerve, the terminal nerve (nerve O or N), which is very small and may or may not be functional in humans〔〔 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「cranial nerves」の詳細全文を読む スポンサード リンク
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