What the tradition says. In the classical Tantric and Hatha model, set out in the Ṣaṭ-cakra-nirūpaṇa (translated in Arthur Avalon's The Serpent Power), six lotuses sit along sushumna, the central channel: the Tamil Siddhas' six ādhārams, from moolaadhaaram at the base of the spine to aakkinai between the brows, with the thousand-petalled sahasrara above the head. Each has its petals, element and seed sound (bija). Kundalini sleeps at the base, coiled three and a half times; awakened, she rises up sushumna and pierces each lotus in turn. The teaching on brahmacharya (urdhva-retas, the seed turned upward) adds that retained shukra, or bindu, is refined into ojas and carried up the channel to the crown, bringing radiance and inner vision; the Hatha Yoga Pradipika says that losing bindu is death and keeping it is life. The rainbow colours on the wheels are a modern scheme, popular since the 20th century; the classical Ṣaṭ-cakra-nirūpaṇa gives the lotuses different colours. This is the tradition's model, drawn as the tradition draws it: the wheels are its picture, not organs.

What anatomy shows. No wheel-like organs sit along the spine. Modern writers often line the chakras up with nerve plexuses (sacral, pelvic, celiac or 'solar', cardiac, pharyngeal) and with endocrine glands (gonads, adrenals and pancreas, thymus, thyroid, pituitary, pineal). That is a common modern mapping, not an equivalence, and writers disagree on several of the pairs. Sperm that are not ejaculated are broken down and reabsorbed in the male reproductive tract; nothing travels up the spine to the brain, which is why the bead stays down. The pineal gland is real: about 5–8 mm long, deep in the brain, it makes melatonin, mostly at night. And the 'third eye' has a true echo in evolution: in lampreys, the tuatara and many lizards, a related parietal eye on top of the head really does sense light.