From memory to the soul
After Galen, the pineal gland remained part of discussions about the brain, although the role assigned to it kept shifting from one period to another.
During the Middle Ages, medicine often divided the powers of the mind into separate functions such as sensation, imagination, reason and memory, then placed each one in its own “chamber” within the ventricular system. In the rather tangled manuscript tradition of the period, conarium¹ and vermis² were sometimes confused with one another, while the area around the pineal gland was also linked to the passage of memory between the ventricles. Anatomy had not yet separated these structures with the precision of modern textbooks, so a term could sometimes refer to a neighbouring region rather than the pineal gland as we define it today.
Even so, centuries before Descartes, the pineal gland and the surrounding area had already appeared in descriptions of memory and other mental functions.
By the seventeenth century, René Descartes shifted the focus sharply.
He was wrestling with the problem of dualism: the body belonged to the material world, while thought and subjective experience belonged to the mind. How could two things so different in nature affect one another?
Looking at the anatomy of the brain, Descartes paid particular attention to the pineal gland’s singularity. The eyes come in a pair, the ears come in a pair, and many neural structures are arranged symmetrically on both sides, yet human experience feels unified. In a brain full of paired structures, the pineal gland was a single organ close to the midline, which made it fit remarkably well with what he was looking for: a point where streams of sensation might come together as one experience.
He also imagined its mechanism in far more detail than the familiar line that the pineal gland was simply “the seat of the soul” suggests. In Descartes’ physiology, the body resembled a machine: animal spirits³ moved through the brain and along tiny nerve tubes, while movements of the pineal gland could redirect their flow and influence different parts of the body.
In modern terms, the mechanism can be pictured rather like a tiny hydraulic lever inside the head. Tilt it one way and the flow of animal spirits follows one route; shift it again and another path opens. Descartes was writing about the soul, yet he still tried to explain how body and mind could affect one another through a distinctly seventeenth-century mechanical vocabulary of tubes, flow, pressure and movement.
In The Passions of the Soul in 1649, he called the pineal gland the principal seat of the soul⁴. Modern neuroscience no longer uses this model, but Descartes’ choice captures one of the great concerns of the period: how a multitude of bodily signals could become the single, unified experience of the self.
From the ancient Greek “valve” regulating pneuma, the pineal gland had now acquired another role: the place where Descartes allowed body and soul to meet.
Then the history took another sharp turn. In the nineteenth century, anatomists began comparing the human pineal gland with related structures in other animals. This time, the word “eye” entered its history in an entirely different sense, through evolutionary biology.
Notes
¹ Conarium: a Latin term historically used for the pineal gland.
² Vermis: the central part of the cerebellum connecting its two hemispheres.
³ Animal spirits: in Descartes’ physiology, an extremely fine material thought to move through the brain and nerves.
⁴ Principal seat of the soul: Descartes’ phrase for the pineal gland as the central location assigned to the soul within his model.
References: