midbrain

n.
The portion of the vertebrate brain that develops from the middle section of the embryonic brain.

mesencephalon

A short region of brain between the diencephalon and the pons The midbrain carries a number of tracts of nerve fibres, including the pyramidal tract, involved in the performance of motor skills.

The short part of the brainstem just rostral to the pons. It contains the nerve pathways between the cerebral hemispheres and the medulla oblongata, and also contains nuclei of the third and fourth cranial nerves. The centers for visual and auditory reflexes, such as moving the head and eyes, are located in the midbrain. Called also mesencephalon.

• m. lesion — causes spastic paresis or paralysis, tremor, nystagmus, opisthotonos and depression or coma.
• m. syndrome — mental depression, ipsilateral oculomotor nerve deficits, and spastic weakness or paralysis of all four limbs or those on the contralateral side of the body. May be caused by cranial trauma, thiamin deficiency, brain tumors, and degenerative or inflammatory disorders of the mesencephalon.

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The noun has one meaning:

Meaning #1: the middle portion of the brain
  Synonym: mesencephalon

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