Plural of diencephalon; the plural form referring to multiple diencephalons (a major division of the vertebrate brain).
From Greek 'dia-' (through) + 'enkephalos' (brain). The term was coined in the 19th century to describe the brain region between the midbrain and cerebrum, literally meaning 'between-brain.'
The diencephalon includes your thalamus and hypothalamus—basically the brain's relay station and control center that manages everything from hunger to body temperature, yet most people have never heard of it because it's deep inside and tiny.
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