= Forebrain
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The \b[forebrain], also called the prosencephalon in embryological terminology, is the anterior major division of the developing vertebrate <brain>. It subdivides into the <telencephalon> and <diencephalon>. The name describes a developmental and anatomical grouping, rather than one nucleus with a single task.<Prosencephalon in NLM MeSH>[Scope note and Forebrain entry term: anterior primitive cerebral vesicle derived from the neural tube and its two subdivisions.]{ref}
The <telencephalon> includes the cerebral-hemisphere system, while the <diencephalon> includes regions such as the <thalamus>, <hypothalamus> and <epithalamus>. Thus the visible <cerebral cortex> is part of the forebrain but does not exhaust it. Deep nuclei and connecting pathways also belong to the region.<Neuroscience, 2nd edition edited by D. Purves and others>[Gross organization of the brain and ventricular-system anatomical landmarks.]{ref}
The ventricular spaces provide an illuminating map. The <lateral ventricles> lie within the cerebral hemispheres, while the <third ventricle> is associated with the diencephalic region. A fluid-filled cavity and its surrounding neural tissue are different structures: locating the cavity helps orient the tissue, but does not make the cavity itself a neuron-processing center.<Neuroscience, 2nd edition edited by D. Purves and others>[The Ventricular System, Figure 1.17.]{ref}
This developmental grouping is useful when comparing brains whose proportions differ greatly. Expansion of one forebrain component does not erase the distinction among its components. Likewise, observing a response in the <lateral habenula> identifies activity in one forebrain circuit, not a computation performed uniformly by the entire <forebrain>.
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