Pineal gland

Function

The pineal gland — the 'third eye' of ancient anatomy — is the body's chronometer, converting the daily rhythm of darkness into a hormonal signal. Its melatonin secretion peaks in the deep of night, opening the gate to sleep and entraining every circadian rhythm to the light-dark cycle via the suprachiasmatic nucleus's relay.

Light suppresses it through a polyneuronal path: retina → SCN → sympathetic fibres to the gland. Blind people's drifting rhythms prove the system's dependence on that light input.

Blood supply

Posterior choroidal arteries.

Nerve supply

Postganglionic sympathetic fibres from the superior cervical ganglion.

Clinical relevance

Jet lag

Melatonin's phase shift lags travel — eastward journeys suffer most; timed light and melatonin re-anchor the rhythm.

Pineal cyst/tumour

Parinaud syndrome — upgaze palsy — flags lesions compressing the tectum below.