Diencephalon: Epithalamus and Thalamus

iii. Diencephalon : The diencephalon is the part of the forebrain lying below the corpus callosum and above the midbrain, made up of the epithalamus, thalamus and hypothalamus, and enclosing a single, narrow cavity, the third ventricle (diocoel), which connects forward to the two lateral ventricles of the cerebrum through the foramen of Monro and backward, via the narrow duct of Sylvius (iter), to the fourth ventricle.

The epithalamus is the thin, non-nervous roof of the diencephalon; anteriorly it fuses with the pia mater to form the anterior choroid plexus, and from its dorsal wall a short pineal stalk connects to the pineal gland (once thought vestigial, now known to secrete melatonin, covered later in this chapter). The thick lateral walls of the diencephalon form the two thalami, made mostly of grey matter and linked to each other by the habenular commissure. The thalami act as the brain's relay centre: through the reticular activating system (RAS), they tra …