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brain

Side view of the brain showing its major structures. The large cerebrum is divided into two halves, …—© Merriam-Webster Inc.

Concentration of nerve tissue in the front or upper end of an animal's body. It handles sensory information, controls motion, is vital to instinctive acts, and in higher vertebrates is the centre of learning. Vertebrate brains consist of the hindbrain (rhombencephalon), midbrain (mesencephalon), and forebrain (prosencephalon). The hindbrain comprises the medulla oblongata and the pons, which connects the spinal cord with higher brain levels and transfers information from the cerebral cortex to the cerebellum. The midbrain, a major sensory integration centre in other vertebrates, serves primarily to link the hindbrain and forebrain in mammals. Large nerve bundles connect the cerebellum to the medulla, pons, and midbrain. In the forebrain the two cerebral hemispheres are connected by a thick bundle of nerve fibres (corpus callosum) and are divided by two deep grooves into four lobes (frontal, parietal, temporal, and occipital). The cerebrum, the largest part of the human brain, is involved with its more complex functions. Motor and sensory nerve fibres from each hemisphere cross over in the medulla to control the opposite side of the body.

This entry comes from Encyclopædia Britannica Concise.
For the full entry on brain, visit Britannica.com.