Though many of René Descartes’ anatomical and physiological assumptions were vastly off target, he was the first to make a convincing case for a purely physical, nonspiritual view of life. Instead of seeing the mind and body as intimately intertwined, Descartes viewed them as interacting but separate entities. Animals, he reasoned, did not have minds, but were still capable of functioning, much like machines. Performing dissections, often on live animals, Descartes theorized about visual perception, learning, and voluntary and involuntary motor skills. Sometimes he even got it right.
1 - According to Descartes, when light from external stimuli reflected onto the retina, tubules behind the eye that connect to the pineal gland opened, triggering an influx of fluid Descartes called “animal spirits,” which allowed the eye to focus. The separate retinal images from each eye then converged back in the pineal gland to form a single visual picture.
Scientists now know the retinas send a series of fragmented images via electrical signals along the optic nerve to the brain, where they are translated into a sharp and detailed image.
2 - Prior to Descartes, his contemporary, Johannes Kepler, hypothesized that the lens moved forward and backwards to change focus.
Descartes was the first to correctly hypothesize that muscles change the shape of the lens of the eye to focus on objects that are closer or farther in distance, says Gary Hatfield, historian of philosophy at the University of Pennsylvania.
3 - During one of his dissections, says Gideon Manning, assistant professor of philosophy at California Institute of Technology, Descartes removed the eye from an ox, carefully cut away its retinal membranes, and placed it on a window sill.
Looking through the back of the eye, Descartes saw an inverted image of the landscape outside, and confirmed that the image on the retina was inverted by the eye’s lens.
4 - Descartes believed that fluid from the pineal gland was involved in all animal motion and in involuntary human motions. Once activated by a stimulus, the pineal gland would flood muscles with animal spirits, allowing muscles to move without the involvement of the mind, much like a hydraulic machine.
Today scientists know that, upon stimulation, motor neurons release neurotransmitters that bind to receptors in muscle fibers, causing them to contract.