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mesencephalon (mez-en-sef´a-lon) [NA]

That part of the brainstem developing from the middle of the three primary cerebral vesicles of the embryo (the caudal of these being the rhombencephalon or hindbrain, the rostral the prosencephalon or forebrain). In the adult, the m. is characterized by the unique conformation of its roof plate, the lamina of mesencephalic tectum, composed of the bilaterally paired superior and inferior colliculus, and by the massive paired prominence of the crus cerebri at its ventral surface. On transverse section, its patent central canal, the cerebral aqueduct, is surrounded by a prominent ring of gray matter poor in myelinated fibers; the periaqueductal gray is ventrally and laterally adjoined by the myelin-rich mesencephalic tegmentum, and covered dorsally by the lamina of mesencephalic tectum. Prominent cell groups of the m. include the motor nuclei of the trochlear and oculomotor nerves, the red nucleus, and the substantia nigra.midbrain vesicle, midbrain; [mes- + G. enkephalos, brain]


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