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.