From Psychic to Cephalic

In the first edition of Ann B. McNaught and Robin Callander’s Illustrated Physiology, published in the 1960s, the authors refer to a "psychic phase" in their description of gastric digestion. This phase describes the release of gastric secretions triggered simply by seeing, smelling, or thinking about food prior to eating—a reflex famously demonstrated in Ivan Pavlov’s conditioning experiments. While the specific phrase "psychic phase" has largely vanished from contemporary anatomy and physiology textbooks, the underlying physiological concept remains intact. Modern medical literature overwhelmingly employs the term "cephalic phase" of gastric secretion. This shift in nomenclature, occurring over a relatively brief period, reflects a broader evolution in scientific framing. The legacy designation "psychic phase" (or "psychic secretion") was favoured by early twentieth-century scientists because the trigger was fundamentally mental, originating from the psyche. By contrast, the contemporary preference for "cephalic"—derived from the Greek kephalē, meaning head—privileges anatomical precision over psychological terminology. It underscores that the reflex is governed by the brain via the vagus nerve before food is ever consumed. Notably, the subsequent digestive phases described by McNaught and Callander—the gastric phase and the intestinal phase—have retained their original names.

The central question is whether this terminological replacement represents a genuine improvement. There is a strong case to be made that "cephalic" is less conceptually meaningful than "psychic". While "cephalic" refers merely to the anatomical head, "psychic" pertains directly to the mind. The phase in question operates not merely within the head as an physical location, but within conscious mental experience. Furthermore, the term "psychic" implicitly acknowledges that organisms possessing the necessary neurological apparatus are also subjects endowed with minds. In capturing these cognitive and mental dimensions, "psychic" provides a far more evocative and accurate representation of the phenomenon than the purely localized descriptor "cephalic".

This historical shift in terminology was largely a product of how science evolved throughout the latter half of the twentieth century. When the term "psychic secretion" was coined around the turn of the twentieth century, psychology and physiology remained deeply intertwined disciplines. As the century progressed, however, they diverged into separate, highly specialized academic fields. Concurrently, the word "psychic" underwent a semantic drift in everyday English, becoming increasingly associated with paranormal, supernatural, or telepathic phenomena. Authors of medical textbooks deliberately abandoned the term to insulate medical discourse from any unintended associations with mysticism.

Additionally, modern physiology operates within a strictly physicalist and materialist framework. Empirical science encounters inherent difficulty in isolating, measuring, or physically locating a "thought" or a "mind" in a standardized laboratory setting. Whereas the mind is subjective, the head is physical. Adopting the term "cephalic" grounds the digestive reflex securely within physical anatomy. It delineates a clear structural pathway: tracing the signal from physical sensory receptors on the head (the eyes and nose) to a physical processing centre in the brain, and down a physical nerve pathway via the vagus nerve. Moreover, the cephalic phase encompasses physical chemical and mechanical inputs, such as tasting and chewing. If food is placed in the mouth of a completely distracted individual who is not consciously anticipating a meal, physical contact with the taste buds still triggers this identical gastric response. Because tasting and chewing represent physical, cranial sensory inputs rather than purely mental concepts, "cephalic" emerged as a more biologically inclusive umbrella term.

Nevertheless, in this pursuit of rigid physical precision, modern textbooks risk stripping away the elegant, holistic relationship between human consciousness and bodily function. The original designation "psychic" served as a subtle acknowledgment that neurological pathways belong to sentient beings possessing minds. While avoiding confusion with occult phenomena is an understandable objective, completely excluding reference to "the mind" risks throwing the baby out with the bathwater.

The Radial Pulse

At one time or another, most people have felt their own pulse, or perhaps that of another. Although pulsations can be detected at various anatomical landmarks across the human body, colloquial references to ‘the pulse’ almost invariably denote the one palpated at the wrist. This is the radial pulse, named for the pulsation of the radial artery against the underlying bone.

While veins are categorised into deep and superficial systems, arteries are conventionally classified as strictly deep. In the limbs, arterial vessels track bones closely, cushioned by surrounding muscle and connective tissue. This configuration conveys a clear evolutionary advantage, keeping high-pressure conduits out of harm’s way—barring instances where a fractured bone yields sharp fragments capable of lacerating an adjacent vessel. Whereas a severed peripheral vein, possessing a thinner and more compliant wall, will tend to collapse and allow blood loss to be staunched with external pressure, a severed artery presents a far graver emergency. The thicker, muscular walls of an artery hold the lumen patent even when transected, while the elevated hydrostatic pressure within compounds the haemorrhage.

If arteries are consistently deep structures, how then can the radial pulse be palpated so readily near the skin’s surface? Does this proximity not render the radial artery ‘superficial’ in the same sense as the veins often visible alongside it? The resolution to this apparent paradox lies in a subtle anatomical distinction: although the radial artery is undeniably close to the body’s surface at the wrist, it remains, fundamentally, a deep structure.

This distinction is far from mere semantic wordplay; rather, it reflects a foundational principle of anatomical architecture. The radial artery strictly maintains its deep course along the radius from which it takes its name. As the arm tapers towards the wrist, the overlying muscular tissue transitions into slender tendons, and the superficial fascia thins significantly. Consequently, there are simply fewer anatomical layers intervening between the skin and the vessel. The surface has, in effect, drawn closer to the artery; the artery itself has not migrated towards the surface.

Darwinian Imperatives

In biology and evolutionary theory, the "Darwinian imperatives" broadly denote the fundamental biological requirements for individual survival and species propagation—or, more succinctly, survival and reproduction. Frequently, these imperatives are conceptualised at the population level, or reduced to fluctuating gene frequencies over time, abstracted entirely from the organism in which they reside. The primary concern here, however, is the central place of the individual animal, whether human or non-human.

Of these imperatives, individual survival appears straightforward, encompassing the acquisition of food, water, and shelter. Yet the concept expands naturally to include broader self-care routines, extending even to medical and pharmaceutical intervention—what clinical healthcare designates as activities of daily living. Ultimately, all such actions collapse into the single, essential category of personal survival.

Individual reproduction, by contrast, is far more complex. For higher animals, reproduction is never strictly an individual affair; it mandates a sexual partner, even if contact between the pair is fleeting or non-existent. Because the resulting offspring are genetically distinct from both parents, reproduction is accurately understood not as the replication of an individual, but as the transmission and copying of a recombined subset of genetic information.

Crucially, no animal—other than humankind—consciously sets out to reproduce or foresees the creation of offspring. The outcome is neither anticipated nor intended. Instead, what is biologically ingrained—or "programmed"—within the individual is the execution of a "reproductive act". A clear distinction must therefore be drawn between the evolutionary outcome of reproduction and the behavioural system driving the reproductive act.

This programmed impulse can express itself across diverse orientations. It may be directed towards members of the complementary sex, the same sex, or oneself, and occasionally towards other species or inanimate objects. While only the first orientation yields biological offspring, all these manifestations stem from the selfsame underlying neurological and hormonal drive.

This brings us to the status of the "reproductive system" within anatomical and physiological frameworks. Modern medical textbooks frequently isolate this system, often placing it at the very end as if it were an afterthought. This separation is instructive: unlike every other organ system, the reproductive system contributes nothing to individual homeostatic survival. Furthermore, it functions for only a fraction of the organism's lifespan—dormant throughout prepubescence, non-functional post-menopause in females, and subject to senescent decline in males. Thus, one of the two foundational Darwinian imperatives rests upon a physiological system active for only a limited segment of the individual's life.

Two Quotes

Here are two quotes that I consider worth mulling over:

“The greatest of all the accomplishments of twentieth-century science has been the discovery of human ignorance.”

    Lewis Thomas, physician and essayist, quoted in The Boston Globe, May 18, 1998

“For this, indeed, is the main source of our ignorance—the fact that our knowledge can only be finite, while our ignorance must necessarily be infinite.”

    Karl Popper, philosopher of science, in Conjectures and Refutations, 1960

A Growing Level of Respect

Numerous superlatives can be applied to the act of dissecting a human body. When one remains mindful that a cadaver represents the physical legacy of a living person, concepts such as honour, privilege, and responsibility immediately come to mind. In contemporary medical education, students are actively encouraged to humanise the donor to a far greater extent than was customary during my own formative career between the 1970s and 1990s. This transformation is entirely proper and represents the standard that ought to have prevailed historically. Because early anatomical dissections relied almost exclusively upon executed criminals—performed without consent as an extension of judicial punishment—reverence for the deceased was virtually non-existent. Consequently, when voluntary body donation eventually superseded execution, an ethos of formal respect was slow to take root in the dissecting room.

In my own experience, it was not until the early 1990s that tangible manifestations of deep respect towards the dissected dead began to emerge systematically. Initially, a university chaplain would visit the dissecting room—the place where staff, students and cadavers had met—to deliver a few carefully chosen remarks before work commenced. Within a few years, however, this role was transferred to a retired professor who had returned as an anatomy demonstrator. This transition was likely intended to establish a secular, non-religious atmosphere, accommodating a student body—and a donor cohort—comprising diverse faiths and secular worldviews that permit anatomical donation.

Today, the level of respect demonstrated within my former department is profound. Observing contemporary practices reveals an institutional ethic strikingly distinct from the environment of my earlier years. The institutional attitudes of my generation now feel exceedingly distant, leaving me with the distinct sense of being a product of a bygone era—a conclusion that is, in all likelihood, entirely accurate.

Desmond Morris (1928-2026)

Desmond Morris (1928–2026) died earlier this year. I have always admired his ability to view human beings through a purely biological and evolutionary lens. Such an undertaking required a degree of detachment from one’s own species that is by no means easy to achieve. It brings to mind a rather risqué anecdote regarding a Martian visiting Earth whose interests, without divulging too much, were not dissimilar to those of Masters and Johnson—who were themselves, perhaps, less scientifically detached than is commonly assumed. It is frequently the mythical Martian who is invoked in jokes and literary conceits whenever a profound sense of detachment from ourselves is called for; the Martian’s perspective forces us to pause and look afresh at human nature.

Morris, however, could not rely upon the device of a Martian perspective, but instead employed the rigorous framework of a serious scientist. When his landmark work, The Naked Ape: A Zoologist's Study of the Human Animal, was published in 1967, it met with predictable criticism from within the scientific establishment. Yet it was the reaction of the general public that proved particularly revealing—especially when viewed from a "Manwatching" perspective (to borrow the title of one of his subsequent works). Religious and moral commentators viewed The Naked Ape as a direct challenge to humanity’s position within a divinely instituted order. They judged its descriptions of reproductive physiology to be excessively explicit, even pornographic, prompting widespread demands for the book to be banned from school libraries. Naturally, such condemnation served only to fuel sales.

A distinct yet complementary methodology had previously been adopted by the American cultural anthropologist and sociologist Horace Miner (1912–1993). In 1956, he published an article in American Anthropologist entitled "Body Ritual among the Nacirema". The essay remains one of the most widely assigned texts in introductory sociology and anthropology courses, serving as a foundational instrument for introducing students to cultural relativism and the necessity of objective observation prior to undertaking fieldwork. Beyond this, however, I must refrain from further exposition, offering not even a spoiler alert.

What Morris achieved was a systematic interrogation of ordinary human behavior from a specific ethological perspective. This prompts consideration of a contemporary phenomenon that remains equally perplexing: how has the act of self-display—standing motionless in various artificial poses to be scrutinised or photographed—come to be regarded as a virtue? Why has it evolved into a lucrative profession, commanding extraordinary remuneration? What drives so many individuals to pursue the status of a model? Stated in more fundamental terms, why do people aspire to make a living by standing still, or to be photographed without clothing? These questions apply equally to male and female subjects, though male and female observers may well arrive at divergent conclusions. While one might propose answers along classic ethological lines, the questions are worth posing precisely because they are so rarely asked. Indeed, one wonders what Morris would have made of modern social media, saturated as it is with individuals uploading repetitive photographs of themselves. Why is a single image deemed insufficient? What impels the uploading of multiple, virtually indistinguishable iterations of the same pose? We routinely overlook these peculiarities despite—or perhaps because of—their omnipresence on our digital screens.

Consider, for instance, how many Instagram images are captured in public toilets. Such locations are frequently selected simply because they offer readily accessible full-length mirrors. Yet this raises a prior question: why the overwhelming impulse to capture and display full-length representations of oneself in the first place?

This impulse towards visibility exists in tension with the nature of modern digital observation. Cyberstalking is formally defined as the persistent use of digital communications to monitor, harass, or intimidate an individual, constituting a fixated and illegal pattern of behavior intended to induce fear or distress. But what of instances where harassment and intimidation are absent? What if the interaction consists solely of "following" or repeatedly visiting a specific profile in complete silence? Does this constitute a form of "silent cyberstalking"? And conversely, what psychological and behavioral effect does it have on an individual to know that they are being observed by thousands of unseen eyes?

One wonders what reaction would meet these "Desmond Morris-like" inquiries today. What impact might an updated version of The Naked Ape achieve if directed at the digital, social-media-saturated era?

The Antiquity of ‘Neo-’

I recently shared some reflections on the concept of the "layered brain." That piece began as a set of preliminary notes for this post, but my musings on the broader cerebral architecture took precedence. Here, I wish to return to my original subject: the neocortex. This specific focus was prompted by a compelling segment in the BBC series Secrets of the Brain, presented last year (2025) by Jim Al-Khalili. As Al-Khalili is a professor of physics, I found the choice of presenter for a neurological survey rather curious at first; however, I now realise that his extensive work in quantum biology provides a natural bridge to the study of complex biological systems. Indeed, viewing the brain through the lens of information theory and thermodynamics offers a perspective that a purely clinical approach might overlook.

As an identifiable structure within the fossil record, we learn that the neocortex is of a staggering age, likely emerging at least 200 million years ago. There is a distinct irony in this antiquity when one considers the term’s etymology. ‘Neocortex’ is a hybridism, grafting the Greek prefix neo- (‘new’) onto the Latin cortex (‘bark’ or ‘rind’). Despite its ancient roots, the name reflects its status as the phylogenetically "newest" or most recently evolved layer of the cerebral cortex. This creates a fascinating temporal paradox: while 200 million years is a vast expanse in the context of human history, it is relatively recent when measured against the three-and-a-half-billion-year odyssey of life on Earth.

The term entered the lexicographical record between 1905 and 1910. Although coined as a neutral descriptive label for the evolutionary age of the tissue, it arrived during a formative period in neuroscience. At the time, the marked expansion and complexity of this structure in humans were frequently interpreted as the biological mandate for human superiority. It was regarded not merely as a mammalian trait, but as the "crowning achievement of evolution," providing the physical basis for our unique capacity for cognition, language, and abstract thought.

However, Secrets of the Brain introduced a complication to this narrative in the form of Morganucodon. This mammaliaform creature first appeared approximately 205 million years ago. Its name, meaning "Glamorgan tooth," pays homage to the area of South Wales where its remains were first discovered. While the soft tissue of the cortex does not fossilise, the impressions left upon the interior of the cranium suggest the presence of a neocortex.

Significantly, Morganucodon preceded the "true" mammals of the modern sense, yet it seemingly possessed this defining feature. This raises a difficult question: is the neocortex truly unique to mammals? The standard academic response remains a hesitant "yes," yet the area is far more contested than the BBC series might suggest. The evidence for neocortical structures in early creatures, combined with the presence of functionally similar regions in birds and reptiles, necessitates a more nuanced evolutionary history. While non-mammals lack the specific six-layered architecture of the neocortex, many possess homologous structures—such as the dorsal ventricular ridge—that perform analogous higher-order functions.

A neutral path through this debate suggests that various ancestral lineages possessed the "mammalian blueprint" for a neocortex, regardless of the divergent evolutionary paths they subsequently followed. What remains certain is that any claims regarding human exceptionalism based solely on this physical structure should be approached with the utmost caution. As we peer into the fossilised echoes of Morganucodon, we find that our "new" brain is, in fact, an ancient inheritance.

Skipping Guts - Addendum 2

Beginning with the idea of expressing gut lengths as a proportion of height (Skipping Guts), I wandered into the idea of a standard (or reference) male and female (Skipping Guts - Addendum). While, to my knowledge, gut length is not mentioned in the reference tables to which I referred, the weights of different organs are.

Based on the International Commission on Radiological Protection (ICRP) standards, the physiological differences between the 70kg "Reference Man" and the 60kg "Reference Woman" are precisely defined to ensure consistency in medical and scientific research. Thus, they supply standardised organ weights (or perhaps I ought to say 'mass', as these values are no doubt applied to astronauts).

Organ Mass Comparisons
The Reference Man (170 cm, 70 kg) generally has larger organ masses than the Reference Woman (163 cm, 60 kg) due to overall body size, though the proportions relative to body weight vary.

Source: ICRP Publication 89

…and one could go on. There are other standards. For example, basal metabolic rate (BMR) also mentioned in ICRP Publication 89 but, having added two addenda to my original post, I will stop here.

A Thin Blue Thread

Human dissection can hardly be described as a dull or tedious pursuit. However, for many students, the encounter with the cadaver can prove onerous, even dispiriting. This is rarely a reflection of the subject matter itself, but rather the pedantry of its presentation. The requirement to memorise an exhaustive catalogue of nomenclature—frequently in an ancient, calcified tongue—can rapidly extinguish any innate curiosity. In contrast, a true study of anatomy should be synonymous with the fascinating and the intriguing.

Within this category of the intriguing, one must include the discovery of the unexpected. This requires a certain intellectual plasticity; one must be sufficiently open-minded to appreciate a find that deviates from the textbook norm. I recall a senior anatomist who, upon being shown a structure resembling a vein coursing along the internal aspect of the left abdominal wall—likely an aberrant epigastric vessel—simply dismissed it with a cursory flick of the forceps. It was a moment where dogma overrode discovery.

Perhaps the most compelling enigma I encountered was a fine, blue synthetic thread embedded within the wall of the right ventricle in the heart of an elderly male cadaver. The mystery lay in the absence of any thoracic intervention; the skin of the chest was entirely devoid of scarring. We were left at a loss. Utilising modern AI tools to review the possibilities, I have since identified a plausible explanation, even if the opportunity for physical verification has long since passed.

The filament was likely nylon or perhaps polypropylene—the latter notably introduced as the surgical monofilament Prolene by Ethicon in 1969. Designed to be inert and non-reactive, these materials can remain sequestered within bodily tissues for decades without eliciting symptoms. As for its presence in the myocardium, venous migration appears the most probable culprit. It seems likely that a fragment of suture material became liberated elsewhere in the body and entered the "highway" of the venous return. Carried through the vena cava to the right side of the heart, it eventually became permanently lodged in the ventricular wall.

Our failure to trace the thread to its source was ultimately a consequence of the rigid boundaries of the dental student syllabus. As their curriculum was strictly confined to the head, neck, and thorax, we never ventured into the abdomen or lower limbs where the point of origin—perhaps a forgotten inguinal or femoral repair—lay hidden. While we will never know for certain, a reasoned hypothesis for such a survival feels infinitely more satisfying than an unresolved mystery. There is, after all, a quiet and haunting poignancy in the find: a thin blue thread of plastic, indifferent to the passage of time, outlasting the very biological life it was designed to support.

...and finally, my last circulation figure

 

This is from the same source as a previous post, being taken from Tortora's Principles of Anatomy and Physiology.

I conclude this series with a figure that differs from those I have posted previously. The only reference to anything anatomical here is to the lungs and capillaries. In so doing this figure raises the question of how to view the human circulation. Do we put the heart at the centre and follow the course of the blood it pumps out? If so, do we first follow it around the systemic circulation or the pulmonary? Or vice versa?

Or do we follow this approach? This figure follows the blood from its place of oxygenation to where that oxygen is delivered and back to where it gets oxygenated again. The heart takes on a more secondary role. Emphasis here is on the oxygen the circulation carries. (Not forgetting the place of carbon dioxide in all this.) The approach I used to take was very much akin to this. The emphasis here is on physiology rather than anatomy.

Coincidentally, this approach returns one to the idea of the simple closed circulation, not least that of fish from whence all forms of vertebrate circulation evolved. One might suggest that the human (mammalian) circulation is really just a single, simple loop that has been twisted into a figure of 8 (or ∞) with the heart at the crossing point.

Please be sure to click the label marked 'The Circulation' to access all related posts. My idea was to illustrate how the same topic can be illustrated in very different ways. View the others to see what I mean.

The Perineum and the Saint

The Christian doctrine of the Fall owes much of its modern architectural rigour to Saint Augustine of Hippo (354–430). While one might avoid a full theological discourse here, it is undeniable that Augustine’s interpretation of the Garden of Eden narrative has profoundly shaped various—though not all—strands of Western Christian thought.

It is perhaps Augustine's specific attitude towards humanity that manifests most memorably in a somewhat unholy reference to human (and particularly female) anatomy. His assessment of human nature post-Lapsarian was fundamentally pessimistic, viewing humanity as a massa peccati (a mass of sin), naturally inclined toward evil due to an inherited corruption of the will. This dim view may have its roots in his early adherence to Manichaeism—a dualistic philosophy that regarded the material world and the flesh as inherently corrupt. Consequently, a phrase frequently ascribed to him describes mankind as being born "between urine and faeces"—or, in the Latin, inter faeces et urinam nascimur. By this, he makes a direct, if crude, reference to the perineum.

The term perineum entered the English lexicon in the early 15th century via Late Latin, having been borrowed from the Ancient Greek perinaion. Its roots are a combination of peri- (meaning "around" or "near") and inan or inein ("to empty" or "evacuate"). It describes that specific anatomical site between the anal and urethral openings where, in the female, the vaginal opening is situated.

However, a scholarly convenience is at play here: while Augustine is the perennial source for this quote, the phrase does not appear in any of his surviving works. Hence, one often finds the phrase "attributed to Saint Augustine" used. Its earliest known appearance in print dates to an 1875 edition of Arnobius of Sicca’s Arnobii Adversus Nationes Libri 7, edited by August Reifferscheid. A variant, inter stercus et urinam nascimur ("we are born between dung and urine"), appears in 17th-century texts, such as Joannes Christophorus Avemann's 1695 Disputatio medica inauguralis, where it was actually attributed to the Roman philosopher Seneca. To further muddy the hagiographic waters, the phrase has also been linked to Bernard of Clairvaux.

Despite the lack of primary evidence, the phrase persists in modern theological and literary circles to illustrate the "messy" physical reality of birth in contrast to spiritual ideals. This juxtaposition is especially striking when compared to traditional religious iconography. Where medieval depictions of the Nativity or the Birth of Mary are typically sanitised and ethereal, this "Augustinian" sentiment forces the reader to confront the visceral proximity of the sacramental and the excremental. Without the convenience of this attribution, writers seeking to highlight this tension would find themselves without their most potent rhetorical weapon.

Moving beyond the polemics of the saints, the pelvic floor remains a critical anatomical region. A vast array of clinical issues can arise from the natural process of childbirth alone, quite independent of patristic commentary. While the term pelvic floor dysfunction has faced academic criticism for its lack of medical specificity, the very breadth of the problems it encompasses—from urinary incontinence to pelvic organ prolapse—makes the term particularly telling. We indeed enter the world via a complex anatomical threshold, a reality that persists whatever theological interpretations are put on it.

Skipping Guts - Addendum

 In a previous post, I suggested that it might be better to equate gut length to height rather than textbooks giving them absolute lengths. In so doing, I referred to a physiological 70 kg man (and 60 kg woman) that formed the standard against which comparisons could be made. While height was not proposed for a standard man or woman, specific heights were proposed to accompany these physiological standards.

The 70 kg man (and 50 kg woman) reference was primarily established by the International Commission on Radiological Protection (ICRP) to create a uniform model for radiation dose calculations and anatomical comparisons.

• Reference Man (70 kg): The standard height is 170 cm (approximately 5'7"). This "Reference Man" was defined in ICRP Publication 23 (1975) as a Caucasian male between 20 and 30 years of age living in a temperate climate.
• Reference Woman (60 kg): The ICRP's updated standard for a "Reference Woman" (often used alongside the 70 kg man) is typically 60 kg with a height of 163 cm (5'4"). In some older or different physiological contexts, a 50 kg woman is often paired with a height of 152–158 cm (5'0"–5'2") to maintain a similar Body Mass Index (BMI) of approximately 20–22.

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.

Femur Fumeur

Those unacquainted with the dissecting room are often struck by its instrumentation. A standard kit is modest enough—scalpels, forceps, and various probes—but one notices that a kit’s volume tends to expand in direct proportion to the time one spends in these environments. While some individuals possess a talent for losing tools, others, like myself, seem to attract them. I hasten to clarify that this is not a matter of larceny; I have never deliberately stolen an instrument, yet I invariably possess more than I began with.

Most of these tools would not be out of place in a modern operating theatre. However, the dissecting room frequently employs an implement seldom seen in surgery: the tenon saw. While orthopedic surgeons certainly use saws for amputations or osteotomies, theirs are specialized instruments—designed to be entirely sterilisable, with smooth surfaces that offer no refuge for organic debris. In the dissecting room, such clinical necessities are absent. Here, the saws are identical to those one would find in a carpenter’s workshop; they are, in every sense, tools for woodwork.

I recall passing a group of students, one afternoon, who were attempting to saw through the left femur of a male cadaver. The cut was positioned just above the mid-thigh, roughly a third of the way down from the hip. Despite their exertions, they were making no progress. When I was called over, the students were visibly baffled; they reported that they had been sawing with such vigor that smoke had begun to rise from the site of the cut.

Taking up the saw myself, it was not the sensation of a blade against bone that I felt. It was that of saw blade against surgical steel. We soon discovered that the individual had undergone a total hip replacement. Upon eventually removing the distal portion of the limb, we exposed the lower extremity of the prosthetic femoral stem.

Two details were particularly striking. First, the metal remained as pristine and reflective as the day it was implanted. Second, despite the formidable density of the alloy, the students’ persistence had left a mark: the tenon saw—a tool designed for timber—had cut a groove some three or four millimeters into the metal.

The students had indeed been trying very hard. One cannot wonder at the smoke.