Medical Definition of brain
1: the portion of the vertebrate central nervous system enclosed in the skull and continuous with the spinal cord through the foramen magnum that is composed of neurons and supporting and nutritive structures (as glia) and that integrates sensory information from inside and outside the body in controlling autonomic function (as heartbeat and respiration), in coordinating and directing correlated motor responses, and in the process of learning—see forebrain, hindbrain, midbrain
2: a nervous center in invertebrates comparable in position and function to the vertebrate brain
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