The pineal gland is not the 'eye that enlightens'
Our first description of the brain’s pineal gland and speculation about its function appears in a treatise by the surgeon/philosopher Galen of Pergamon written in Rome between 165 and 175 CE. Greek philosophers had long thought that pneuma – a fine, volatile, airy or vaporous substance composed of air (motion) and fire (warmth) – coursed through the human body, bringing consciousness to the brain and animation to the organs and body parts. Stoics had come to believe there was a single pneuma system regulated by the heart. But according to Galen, others believed there were two separate systems: the pineal gland regulated the flow of psychic pneuma for the brain, and the heart regulated the vital pneuma for the body. Galen argued for the double system, though he thought the vermis superior cerebella, rather than the pineal gland, the more likely structure to regulate the brain’s pneuma system.
The relevance of this history to our question is that in addition to sustaining consciousness and animation, the Greeks considered pneuma the ‘breath of life’, the spirit, the substance of the human soul. This is also the meaning often carried by the Greek word pneuma in the Bible (though it's unclear if Bible writers understood it as biological).
Mt.6:22, however, offers no insight on these issues. Within a discussion about the corrupting influence of the envy for earthly treasures, this verse names the eye as the ‘lamp’ (λύχνος, lychnos) by which the whole body may be filled with either light (φωτεινός, phōteinos) or, in the next verse, darkness. While this is surely metaphorical language for a moral lesson, there is no mention of spirit (πνεῦμα, pneuma), soul (ψυχή, psychē), or breath (πνοή, pnoē) in this discussion. Mt.6:22 does not name the pineal gland or even suggest any type of literal, biological function.
But we should note that the previous verse does reference the ‘heart’ (καρδία, kardia). Bible dictionaries often define the heart as the all-encompassing seat of the soul, mind, will, emotions and character, just as the heart was seat of the soul – the regulator of pneuma – for the Stoics.
Helpfully Galen shows there were also other views during the early New Testament era. During the 1st century CE the Pneumatic school of Roman medicine had reasserted old distinctions between the heart (which housed the mind) and the brain (which housed the soul). Galen went a step further and theorized a tripartite soul: the rational soul in the brain, the spiritual soul in the heart, and the appetitive soul in the liver. It’s uncertain what our gospel writer intended by ‘heart’ in Mt.6:21, but it would be interesting to explore whether and how his usage reflects traditional Greek, Gnostic, Galen or uniquely Hebrew or Christian concepts of these ideas.
What is clear is that there are no references to the pineal gland in the Bible. And even though Galen’s treatise reseating the psychic pneuma in another brain structure was highly influential, theories connecting the soul to the pineal gland resurfaced in the Middle Ages, and even in Descartes. Spiritual interest in it today largely stems from New Age appropriation of Madam Blavatsky’s 19th C. Theosophy which claimed the pineal gland is an atrophied vestige of the mystical ‘third eye’, the “organ of spiritual vision” offering awakening, clairvoyance, and higher states of consciousness. Like many other New Age theories, this one has no basis in the Bible.
1 Lokhorst, Gert-Jan, "Descartes and the Pineal Gland", The Stanford Encyclopedia of Philosophy (Fall 2015 Edition), Edward N. Zalta (ed.).