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The trochlear nerve, also called the fourth cranial nerve or cranial nerve IV, is a motor nerve (a somatic efferent nerve) that innervates only a single muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea. The trochlear nerve is unique among the cranial nerves in several respects: #It is the ''smallest'' nerve in terms of the number of axons it contains. #It has the greatest intracranial length. #It is the only cranial nerve that exits from the dorsal (rear) aspect of the brainstem. #It is the only cranial nerve that innervates a muscle on the opposite side (contralateral) from its origin. Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.〔Maisey JG. Morphology of the Braincase in the Broadnose Sevengill Shark ''Notorynchus'' (Elasombranchii, Hexanchiformes), Based on CT Scanning. American Museum Novitates, Number 3429. New York: American Museum of Natural History, 2004〕 The human trochlear nerve is derived from the basal plate of the embryonic midbrain. ==Structure== The trochlear nerve emerges from the dorsal aspect of the brainstem at the level of the caudal mesencephalon, just below the inferior colliculus. It circles anteriorly around the brainstem and runs forward toward the eye in the subarachnoid space. It passes between the posterior cerebral artery and the superior cerebellar artery, and then pierces the dura just under free margin of the tentorium cerebelli, close to the crossing of the attached margin of the tentorium and within millimeters of the posterior clinoid process.〔Bisaria KK. Cavernous portion of the trochlear nerve with special reference to its site of entrance. J. Anat. 159:29-35, 1988〕 It runs on the lateral wall of the cavernous sinus, where it is joined by the other two extraocular nerves (oculomotor - cranial nerve III and abducens - cranial nerve VI) and the first two branches of the trigeminal nerve (V), Ophthalmic (V1) and Maxillary (V2). The internal carotid artery also runs within the cavernous sinus. Finally, it enters the orbit through the superior orbital fissure and innervates the superior oblique muscle. 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「Trochlear nerve」の詳細全文を読む スポンサード リンク
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