A Very Unjust Whiz through Early Understandings of the Nervous System
(1700 BC to the 17th Century)
If we were to be accurate, our journey should probably date back several thousand years earlier, but let us begin around 1700 B.C., in Egypt. The Egyptians apparently didn't think too much of the brain. In mummifying their deceased, the brain was scooped out and thrown away, whereas the heart, thought to be the seat of intelligence and thought, was carefully preserved. Despite this, the Egyptians are credited with the first written record of the nervous system. The document, now known as the "Edwin Smith Surgical Papyrus" was probably written by the physician Imhotep, and has mentions of the brain and its convolutions, the meninges, and the cerebrospinal fluid. (1).
Now let us move forward in time and westward in direction, to classical Greece. The Greeks thought that the world was composed of four Pythagorean elements: fire, earth, water, and air; and that each of these four elements was composed of ideal solids with distinct geometries. (2). These ideas lay the foundations for thinking about the world, and the mind, in a mathematical context. In about 500 BC, Alcmaion of Crotona had dissected sensory nerves, and roughly 150 years later, Hippocrates (of Hippocratic Oath fame) stated that the brain is involved with sensation and is the seat of intelligence. (1). Not long afterwards, Plato was teaching at Athens, and "his belief in the perfect sphere of heaven led him to postulate that the rational soul of man 'the divinest part of us' must be in the brain, because the head is spherical in shape." (2). As much as we admire Plato today, he apparently had some pretty bizarre ideas about the brain. He thought of the brain as "a gland, and that it produced semen which flowed down through the spinal chord, out through the phallus, and into the female vagina, thus rendering women as 'flower pots for male seeds', surely a major factor in the exclusion of women from the governance of academic, religious, and governmental institutions for 2,000 years or longer." (2).
Some 50 years later, Aristotle proposed three forms of the 'soul': the vegetative, (possessed by plants-allows for growth, decay, and nutrition, but not sensation or motion), the animal (possessed by animals-allows for motion and sensation), and finally, the rational (unique to man, it accounts for the conscious and intellectual aspects of us). The hierarchy was constructed so that the higher souls had the benefits of the lower, leaving only human beings at the top with the rational soul. Aristotle believed that it was the heart, not the brain, that was the seat of the rational soul. He believed we have a fifth element, which was termed the 'pneuma', and this pneuma was the fuel of intelligent thought, circulated around the body by the heart. Around 300 BC, Herophilus of Chalcedon, sometimes called the 'Father of Anatomy", located human intelligence in the head on the basis of neuroanatomy for the first time (unfortunately, I was unable to find further details on this neuroanatomical basis). Herophilus was also the first to distinguish between the motor and sensory nerves. (2). Sometime between 300 and 260 BC, the anatomist Erasistratus of Chios found three tubular structures going to every body organ-today we know these structures as arteries, veins and nerves-and concluded that Aristotle's 'pneuma' was carried via the carotid arteries from the heart to the brain, and from the brain via the nerves to all body organs. Thus, the brain was "seen as the vessel for distilling the pneuma." (2). In 280 BC, Erasistratus is also supposed to have noted the divisions of the brain. (1).
Then came Galen, who incorporated earlier ideas to his own findings, such as his finding of blood in the arteries (he believed arterial blood was intermixed with pneuma) and wrote, "In the universe there are four elements-fire, air, water and earth; and in the living body there are four humors, black bile, yellow bile, sanguine and phlegm. Out of the excess or deficiency or misproportion of these four humors there arise disease; by restoring the correct proportion diseases are cured." (2). Galen also housed the three Aristotlean souls in three prominent organs: the vegetative soul in the liver, the animal soul in the heart, and the rational soul in the brain. According to Galen, the pneuma came from the world into the lungs through the trachea. It was carried by the pulmonary artery to the heart. Food was converted to natural spirits in the liver, and these natural spirits were taken to the heart via the hepatic vein, which cleansed and returned most of it to the lungs, but some of it mixed with the pneuma and went to the brain as vital spirits. In the brain, the vital spirits were distilled into animal spirits, an "ethereal substance passed into the (then thought to be) hollow nerves and distributed throughout the body, thus animating the body." (2). Since Galen believed the brain's primary functions were to distill and distribute animal spirit throughout the body, he thought that the fluid filled ventricles, not white matter or grey matter, were the major functional units of the brain. Galen further put forth that this fluid animal spirit, which travelled to the muscles and organs through nerves, controlled bodily activity, and the brain, the seat of the rational soul, was "responsible for imagination, reason, and memory." (2).
In the 14th century, nervous system structures were more thoroughly looked into. In 1536 Nicolo Massa described the cerebrospinal fluid, and in 1543 Andreas Vesalius discussed the pineal gland and drew the corpus striatum. In 1564 Aranzi coined the term 'hippocampus', in 1573 Varolio named the pons, and in 1586 Piccolomini distinguished between the cortex and white matter. By 1590, Janssen had invented the compund microscope. (1).
Into the scene then entered French mathematician, philosopher and physiologist Rene Descartes, who "reduced the humoral description of the body/brain with its variety of souls to a mechanical/hydraulic model", using the "most celebrated technological achievements of his time as his analogy." (3). Descartes used intricate systems of waterworks of fountains and water driven clocks as models to explain "how the brain operated the muscles and the general description of nerve processes." (3). Residues of this tradition are evident in our language when we speak of the nervous system: our (arguably) most celebrated technological achievements (i.e., computers) still serve as templates in understanding and explaining nervous system structure and function.
Descartes also established Dualism, a way of thought that holds the mind and the body as two separate entities. A purely mechanical model of the mind-seat would not fare well with the Church, and Descartes' Dualism was less offensive (Descartes had heard of Galileo's fate with the Inquisition, so his treatise on physiological psychology was not published until after his death). (4). In "De Homine" (1662) Descartes proposed a mechanism for automatic reaction in response to external events: the foundations of reflex theory. According to Descartes, "external motions affect the peripheral ends of nerve fibrils, which in turn displace the central ends. As central ends are displaced, interfibrillar space is rearranged and the flow of animal spirits is thereby directed into the appropriate nerves." (4). Descartes believed the rational soul made contact with the body at the pineal gland, as he found the pineal gland to be the only brain structure that did not have a bilateral duplicate and as he thought it to be exclusive to humans. In Descartes' model, the body could receive signals from the external world and establish contact with the pineal gland, which could make for a change in interfibrillar spaces and a different outflow of animal spirits, if there was conscious sensation. In this way, body could affect mind. Likewise, mind could affect body: voluntary actions could somehow cause the soul to bring about a change in the flow of animal spirits. (4).
Not long later, English anatomist Thomas Willis also discussed a hydraulic model of the nervous system. Willis has been credited with producing wonderful pictures of the brain, showing that under the folds of the cerebral cortex were subcortical structures like the striatum, the thalamus and the corpus callosum. He distinguished between cortical gray matter, which he thought was responsible for animal spirits, and white matter, which he thought distributed the spirits to the rest of the organism and bestowed upon it sensation and movement. (5). For Willis, the striatum was responsible for linking the mind to the brain, and did experiments to show that if blood was kept from reaching the brain, nerve function would stop because the vital spirits couldn't be converted to animal spirits. Willis believed that the animal spirits were created in the brain and that from there it filled the medullar trunk (which he described in "The Anatomy of the Brain" as being like the chest of a musical organ, receiving "wind to be blown to all the pipes"). From there the spirits were carried to the nerves and their many fibers, interwoven in muscle and membrane, imparting to them a "motive and sensitive or feeling force." So, "by pressures outwardly placed on the flood motion is effected and by inward pressure sensation is received." (5). Willis did sense that the fluid model might not be accurate because it was not consistent with the speed with which he believed sensations and muscular actions to occur. Keeping that inconsistency in mind, Willis "likens the animal spirit to rays of light filling the passages of nerves and muscles." (5). Willis was among the last of the animal spirit theorists, and it was probably the discovery of electricity that brought a real change in the concept on the 'animal spirit'.
Many, many developments were to follow and it is futile on my part to attempt to brush by them all in a paper of this scope. With technological advances like the galvanic cell, increasingly more effective microscopes, and nerve galvanometers, development of techniques like artificial nerve stimulation and ablation, and concepts like evolution that were to follow, came a heightened understanding of the intricacy and immensity of the nervous system. It was from these models of hearts, livers, waterworks and clocks that we emerged from the 17th century. It wouldn't be until late in the 19th century that the terms neuron and synapse would even be coined, and early in the 20th century that the blood-brain barrier and the all-or-none principle would come into light. (1).
Clearly, the timeline for neuroscience is studded with brilliant moments, each struggling, refuting, building on the others. We have come a long way, but the most exciting part remains in seeing how much further we might progress.
2)Consciousness: Classical Greek view of the Mind
3) Consciousness: 17th Century view of the Mind
5) Consciousness: Early Neurophysiology