- 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 tecti (tectal plate [TA] or quadrigeminal plate [TAalt]. 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 mesencephalic tectal plate. Prominent cell groups of the m. include the motor nuclei of the trochlear and oculomotor nerves, the red nucleus, and the substantia nigra. SYN: midbrain vesicle, midbrain. [mes- + G. enkephalos, brain]
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* * *mes·en·ceph·a·lon (mez″en-sefґə-lon) [meso- + encephalon] 1. [TA] the part of the brain developed from 2. the middle of the three primary vesicles of the embryonic neural tube; it comprises the tectum, midbrain tegmentum, and basis pedunculi; see Plate 9. See also brainstem. the middle of the three primary brain vesicles in the embryo, lying between the prosencephalon and the rhombencephalon. Called also midbrain.
Medical dictionary. 2011.