The Layered Brain

As a student, I was introduced to the architecture of the brain through a specific phylogenetic lens, using the terms archicortex, paleocortex, and neocortex. While the term archaeocortex was still in use at the time, it has since largely been superseded by archicortex. These designations were more than mere labels; they reflected a chronological narrative of evolutionary appearance, categorised from oldest to newest.

1. Archicortex: The phylogenetically oldest component of the cerebral cortex, primarily represented by the hippocampus and the dentate gyrus.

2. Paleocortex: Considered the intermediate or "old" cortex, it is fundamentally associated with the olfactory system.

3. Neocortex: The newest and most sophisticated addition, appearing first in early mammals and now accounting for approximately 90% of the human cerebral cortex.

While this three-part schema remains a useful pedagogical starting point, it is, in truth, a gross simplification. Each section consists of distinct neuronal layers, and the phylogenetic order itself is a matter of debate. While many sources cite the archicortex as the most ancient, some neuroanatomists argue that the olfactory system appeared first, or at least concurrently. I suspect the professor who taught us this order belonged to the latter school of thought. In a lecture on the cranial nerves, he adopted a similarly phylogenetic approach: he argued that the olfactory nerves were the oldest not just anatomically, but in terms of how life first "contended" with its environment. The first interaction was chemical; what we call "smell" is, in effect, the detection of airborne molecules. It requires little imagination to see this as a primary evolutionary necessity.

He extended this logic to the remaining cranial nerves. If the first challenge was chemistry, the second was electromagnetic radiation—specifically, the spectrum we call light. Thus, the optic nerve followed. Whether this specific evolutionary sequence is entirely accurate is perhaps a moot point; its value lies in providing an entry into the brain’s daunting complexity. It suggests an underlying logic to biological structure, reinforcing the idea that the brain is not a single, unitary object, but a record of evolutionary history.

This perspective aligned with the Triune Brain hypothesis, a theory that was particularly popular during my student years. Originated in the 1960s by Paul MacLean and popularised by Carl Sagan in his 1977 book The Dragons of Eden, the hypothesis used the archicortex, paleocortex, and neocortex as the biological foundation for a theory of three distinct evolutionary "brains" residing within the human skull. MacLean’s model was an expansion of these cortical categories; he integrated them with deeper structures, such as the basal ganglia (or "R-complex"), to suggest a functional hierarchy of behaviour that moved from primal survival to complex reasoning.

The enduring popularity of the Triune Brain in the public imagination is a testament to the power of compelling narrative in popular science. Through Sagan’s prose, a hypothesis was "frozen" in the public consciousness, persisting long after the scientific community had moved toward more integrated models. Today, neuroscience has largely moved past these "additive" layers in favour of network neuroscience and neuroplasticity, which view the brain as a highly interconnected system rather than a series of autonomous chronological strata.

In the decades since, the Triune Brain hypothesis has fallen out of favour. However, I wonder if the theory still holds value as a pedagogical "thinking tool" or an object for critical analysis. It could serve as a rigorous exercise for students—not merely as a historical footnote, but as a challenge. It is a rare and perhaps necessary experience in scientific education to be handed a once-revered hypothesis with the instruction: "Explain exactly why this is now considered incorrect."

Penultimate Circulation Figure

My notes say that this figure depicting the human circulation is from a textbook entitled The Human Body - An Introduction to Structure and Function by Faller and Schuenke.

This is the penultimate such illustration in this series. I have only one more. Please be sure to click the label marked 'The Circulation' to access all such posts. My idea was to illustrate how the same topic can be illustrated in very different ways.

On the Difference Between ‘Complex’ and ‘Complicated’

I have sought a way to present this distinction as simply as possible, for the concept is not inherently difficult. We often assume a subject is impenetrable simply because the terms ‘complex’ and ‘complicated’ are involved. Yet, systems described by these words are not necessarily beyond our understanding. Consider the word ‘intricate’; it elicits curiosity rather than trepidation. We should view ‘complex’ and ‘complicated’ with the same equanimity.

In common parlance, these words are used interchangeably. This is particularly prevalent in our attempts to understand biological organisms: one observer might describe a physiological process as ‘complicated,’ while another labels it ‘complex.’ Strictly speaking, however, they are not exact synonyms. In a technical sense—one with profound implications for biology—they carry subtly distinct meanings.

A system is not defined as ‘complicated’ merely because it is difficult to grasp. While it may comprise a vast multitude of components, these parts operate in a predictable, linear fashion. Given sufficient time, one can map every piece and understand how they interconnect. A grandfather clock serves as a perfect illustration. It possesses numerous gears and springs, yet each has a discernible, specific function. If one understands the mechanics of each gear, the operation of the whole becomes entirely predictable. Similarly, a jumbo jet may consist of some six million parts, but the interactions between them are clear, designed into the system, and governed by known physical laws. Such systems are built for robustness—the ability to resist change and maintain a fixed state of operation.

A complex system, conversely, is characterized by a large number of components that interact in dynamic, unpredictable, and non-linear ways. These systems exhibit emergent properties—behaviors or patterns that arise from the collective interactions of the parts which cannot be predicted by studying those parts in isolation. To borrow the classic adage, the whole is truly greater than the sum of its parts. The defining feature here is emergence; the system’s global behavior remains hidden if one only examines its dissected components. Where complicated systems seek robustness, complex systems rely on resilience—the capacity to adapt, self-organize, and evolve in response to environmental shifts.

The fundamental difference, then, lies in predictability and emergence. A complicated system is a sum of its parts; its behavior as a whole is a known quantity. If a gear in the grandfather clock breaks, it can be identified, replaced, and the system will resume its expected performance. This reductionist approach—fixing the part to fix the whole—is simply not possible with a complex system.

Biological examples are particularly pertinent here, and the distinction is perhaps most stark when considering the difference between anatomy and physiology. A cadaver is a complicated structure; it can be meticulously dissected, its parts mapped and named with absolute certainty. However, the moment life is introduced, we transition from the complicated to the complex. The living organism swarms with activity much like a flock of birds or a school of fish. Such patterns arise from simple, local interactions between individuals, yet the shifting geometry of the flock cannot be predicted by observing a single bird. While every individual remains subject to physical laws, those laws are not the sole arbiters of the system’s behavior.

The danger arises when we apply the logic of ‘complicated’ systems to ‘complex’ ones. This is a critique frequently leveled at modern medicine. While the human organism is undeniably a complex system, the clinical approach often mirrors the repair of a complicated machine. This is not to suggest that such an approach is never successful; however, treating complexity with a "complicated" mindset often results in ineffective interventions or unintended consequences.

This category error explains why the same treatment can yield radically different results in two different patients. In a complicated system, input A always leads to output B. In a complex, resilient system, the organism may adapt to input A in ways the "mechanic" never anticipated. When a practitioner finds it impossible to help a patient despite following the manual, they may feel a sense of professional failure—forgetting that they are navigating the unpredictable waters of complexity, not the fixed gears of a clock.

Skipping Guts

This anecdote from the dissecting room concerns a first-year medical student I once observed holding a length of intestine between outstretched hands. As he began to step over it, the temptation in his mind was palpable: he was contemplating whether he could, or indeed should, use the organ as a skipping rope. His hesitancy appeared to stem less from a sense of moral propriety or professional decorum than from a practical concern regarding the material’s structural integrity. He seemed to fear that the specimen might snap, sending the severed ends flying uncontrollably. His quandary was swiftly resolved; before he could proceed with the ‘experiment’, a brief word from me brought the matter to an end.

Beyond the theater of the macabre, I have never found the standardized lengths assigned to the various sections of the human alimentary canal entirely convincing. Textbooks typically offer a prescriptive set of figures: approximately six meters (twenty feet) for the small intestine and one and a half meters (five feet) for the large intestine. Such neat, rounded numbers suggest a uniformity that rarely survives the reality of individual variation.

In simpler organisms, such as the worm, gut length essentially mirrors body length, presenting little in the way of a morphological puzzle. Even so, measurement is fraught with difficulty; in life, the worm is a constant flux of contraction and relaxation. Physically, the creature is little more than a length of gut encased in a sheath of nerves and glands—reproductive organs notwithstanding. In contrast, the human digestive tract is significantly longer relative to total body length, a necessity driven by the immense metabolic demands of our physiology, particularly that of the brain. This trade-off between encephalization and digestive architecture—often discussed as the Expensive Tissue Hypothesis—results in a complex series of convolutions wound tightly within the abdominal cavity. When the gut is removed, the resulting void is a stark reminder of the space required to sustain our biology.

The utility of numerical measurements depends largely on one’s subjective perception of length. If asked to cut a piece of string to a specific length in meters or yards without a rule, few could do so with any degree of accuracy. When applied to the internal architecture of the body, these measurements lose even more of their meaning. Given the inevitable variations dictated by an individual’s stature, it might be more instructive to express gut length as a proportion or percentage of the person’s height, rather than as an abstract static value.

Such attempts at standardization have a historical precedent in physiology. For the purposes of comparison, the "normative" 70 kg man (and a 60 kg woman) has long served as the benchmark against which biological data is measured. While this model persists as a useful shorthand, it is increasingly criticized as an outdated construct—a "Reference Man" that fails to account for the true diversity of the human form.

A Gibbon's Finger

Many years ago, while conducting research into the osteology of the hand, I spent time examining bony specimens and radiographs across various primate species. This work took me to the British Museum (Natural History) in London to study skeletal remains, and subsequently to London Zoo, where I was permitted to search through their extensive radiographic archives. One particular radiograph remains etched in my memory: the hand of a lar gibbon.

The image revealed an old trauma. At some point, the gibbon had suffered a severe fracture of a middle phalanx—I cannot recall if it was the index or middle finger, though both are notably prominent in this species. In the course of the injury, the distal segment of the finger had rotated ninety degrees, leaving the broken ends entirely unaligned. The resulting configuration was T-shaped: the proximal phalanx and the proximal half of the middle phalanx formed the vertical upright, while the distal half of the middle phalanx and the distal phalanx itself formed the crosspiece. Most striking was the presence of a robust fibrocartilage callus at the junction, firmly bridging the two disparate ends of the bone.

From a biomechanical perspective, such a deformity is significant. Given that lar gibbons rely on a specialized "hook grip" for brachiation, a T-shaped finger would have been a substantial mechanical hindrance. Yet, the animal clearly survived long enough for complete ossification to occur, suggesting a remarkable level of adaptation to its impaired mobility.

This specimen serves as a vivid illustration of how the biological healing process proceeds with total indifference to alignment. While the healing of displaced fractures is well-documented—frequently appearing in the archaeological record—most cases retain at least a rudimentary longitudinal orientation. Humans have recognized the necessity of orthopedic reduction since antiquity; the Edwin Smith Papyrus, a medical treatise dating to approximately 1600 BC (and likely reflecting practices from centuries earlier), describes the manual manipulation of fractures.

The gibbon’s finger, however, achieved a state of complete union despite being as misaligned as is physically possible. It prompts a reflection on the nature of medical "success." In modern orthopedics, success is defined by the restoration of anatomical alignment and function. Biologically, however, success is simply union and survival. The callus does not care for the aesthetic or the mechanical ideal; it merely seeks to close the gap.

I neglected, perhaps foolishly, to check the date on that radiograph. During a gap year after leaving school I worked at London Zoo, albeit in the modest confines of the gift shop. Each morning and evening, my commute took me past the gibbon enclosure. I often wonder if the very individual with the T-shaped finger was among those swinging beside me as I walked to and from work.

The Circulation Again

 This figure is taken from Tortora's Principles of Anatomy and Physiology. (I cannot remember which edition.)

At this point I am beginning to run out of things to say about figures depicting the human circulation. Fortunately the reason I am posting these figures is to illustrate how illustrations (pun intended) can differ even when depicting the same thing. One must consider the intentions of the person in charge of producing the book and what their motivating factors were. It is not just a matter of 'presenting the facts'. Facts are always interpreted.

A Comment by Seneca

“For this reason I hold that there is nothing of eminence in all such men as these, who never create anything themselves, but always lurk in the shadows of others, playing the role of interpreters, never daring to put once into practice what they have been so long in learning. They have exercised their memories on other men’s materials. But it is one thing to remember, another to know.”

— Seneca (4 BC–AD 65)

I debated whether to place my commentary before or after this passage. Ultimately, I have chosen to conclude with my own reflections, allowing the reader the opportunity to ponder Seneca’s words in isolation before I offer my interpretation.

This excerpt is drawn from Seneca’s Epistulae Morales ad Lucilium—specifically Letter 33, which addresses the futility of learning solely through maxims. In the Roman context, these maxims (sententiae) were pithy, self-contained moral truths intended for rote memorization. While useful for the novice, Seneca argues that a persistent reliance on them prevents the student from ever reaching intellectual maturity.

Upon encountering this letter, I was immediately reminded of my own experiences as a student. I recall instructors who taught strictly from the textbook, possessing little to no independent insight into their subjects. Under such guidance, outdated and erroneous content often persisted; with the benefit of hindsight, these errors now seem glaring. Such teachers acted merely as conduits for "somebody else’s facts," filling the minds of their students with borrowed information rather than cultivated knowledge.

Conversely, I also encountered educators who looked beyond the constraints of the syllabus. They enriched their lessons with personal experience and anecdote, imbuing dry facts with deeper meaning. They did not merely transmit information; they provided new prisms through which to view familiar concepts, transforming the academic into the lived.

In this passage, Seneca offers a critique that remains strikingly relevant to modern education. He speaks to those who teach without interpretation or the desire to provoke original questions. For Seneca, there is a profound difference between being a mere "interpreter"—one who echoes the shadows of others—and being an "author." The former remains a perpetual student, never daring to step out of the secondary literature to claim their own voice or test their learning in the crucible of practice. This failure to lead is often a failure of engagement rather than ability.

That this cycle can be broken is something I have seen in my own practice. I was once tasked with teaching a subject I had never previously studied. By first laboring to make the material meaningful and interesting to myself, I discovered the necessary pathways to make it equally engaging for my students.

Seneca’s Letter 33 is a brief but essential read. Indeed, the entirety of his correspondence with Lucilius offers much to the modern reader concerned with the transition from remembering to knowing.