The Scale of Blood

By Darren N. Wagner [1]

“A” is for “Blood”

Examining blood has been instrumental to Western medical practice since antiquity, but methods and technologies for observing blood have changed dramatically, and so too have the social meanings of blood. I was newly reminded of this change while viewing the anatomical collection at the University of Glasgow’s Hunterian Museum this spring. One particular case explains that the expansive 18th-century museum of William Hunter (1718–83) in London at 16 Great Windmill Street had a simple cataloguing system organized by letter to sort its many anatomical and pathological specimens.[2] As shown in figure 1, Hunter catalogued his collection by body part, organ, condition and pathology. To get a sense of this system, RR is for Gravid Uterus, MM for Monster, EE for Brain, DD for Breasts, U for Liver, T for Lungs, C for Veins and B for Arteries. Blood claims the alpha spot. Vital, ubiquitous and essential, blood seems fitted for top billing. Blood was, after all, centrally important to gathering empirical knowledge about the body, and was crucial to Hunter’s provision of medical education. Yet actual blood is a notable absence in these displays.

Fig. 1: Case label at the Hunterian Museum, Glasgow showing the cataloguing system used by William Hunter. Courtesy of the Hunterian Museum, University of Glasgow.

Fig. 1: Case label at the Hunterian Museum, Glasgow showing the cataloguing system used by William Hunter. Courtesy of the Hunterian Museum, University of Glasgow.

Blood presented distinct challenges for display in 18th-century anatomy museums. Once ex vivo, coagulation changes blood’s color, consistency and structure. Harden coagulum could be preserved to show pathologies like aneurysms but for most anatomical preparations blood was removed to stave off decomposition and avoid adulterating preservation fluids. In lieu of blood, anatomists often filled vascular spaces with substances such as metals (like mercury, tin or lead), alcohol, various waxes, resins, or animal extracts (like tallow or spermaceti). When perusing the gallery of Hunter’s museum, observations about blood are often made without actual blood being on display. Encounters with fresh blood could be witnessed via the activities in Hunter’s dissection and preparation rooms, or attending a demonstration in the anatomy theatre.[3] But observing blood through an intermediate, whether a graphic representation, a physical model or a magnifying lens, alters the available meanings and knowledge to the viewer. For instance, preparations of vasculature injected with colored wax or mercury facilitate a specialist medical understanding about blood and its role in the body. Such anatomical structures that result reveal where blood goes in the body, but those demonstrations strip away firsthand experiential knowledge of blood’s substantive qualities. The rich color, distinctive scent and dynamic consistency of fresh blood are lost. The injected wax in figure 2 looks remarkably bloodlike and evokes blood’s integral role in sex. But it has lost that quintessential fluidity and its resonances. When studying the glans, a medical student would have to disregard the sheer ornament of that quicksilver cap and focus on the tissue’s detailed vasculature.


Fig. 2: An injected human penis showing the erectile bodies (dyed burgundy), the dorsal veins (dyed crimson) and the glans (injected with mercury). Courtesy of the Hunterian Museum, University of Glasgow.

The disembodied penis appears to be in a fixed state of arousal—swollen with blood that never bleeds or coagulates. For the Georgian medical student or practitioner, such anatomical models complemented regular encounters in which blood was directly observed. Yet new techniques and technologies—such as injecting vasculature—became increasingly common intermediates for seeing and knowing blood.[4]


The Naked Eye

Medics no longer regularly observe blood with the naked eye. Blood pressure is read, the pulse is taken, cells are counted and chemical profiles analyzed. Yet the unmediated scrutiny of blood was standard diagnostic practice in much of Western history. In humoral medicine during the middle ages and early modern period, increasing significance was ascribed to blood over the other humors.[5] Indirect observational methods were used too, such as the medieval diagnostic practice of examining urine to determine the qualities of blood (fig. 3).[6]

Johannes de Ketham’s medical guide, The Fasciculus Medicinae (Venice, 1491), is mostly about phlebotomy but also includes this chart for diagnostic interpretation of urine collected in glasses. Courtesy of the Osler Library, McGill University.

Johannes de Ketham’s medical guide, The Fasciculus Medicinae (Venice, 1491), is mostly about phlebotomy but also includes this chart for diagnostic interpretation of urine collected in glasses. Courtesy of the Osler Library, McGill University.

The special attention given to blood as a key indicator of health is reflected in the abundance of blood-letting manuals published in the early modern period. By the 17th century, barber-surgery was a widespread and established trade that used bloodletting as a primary medical intervention to right imbalances in the body and prevent or treat disorders ranging from headaches to consumptions.[7] Attending barber-surgeons—equipped with a lancet and bowl—made incisions along vasculature according to the problem at hand, drew off a specific quantity of blood and observed what was collected for further diagnosis (fig. 4). However, the 18th century arrived with mounting criticisms from practitioners and patients concerning the validity of diagnoses made from observations of blood via the naked eye. Such a shift in attitude reflected the declining currency of humoral theory in medicine, as well as a new scale of observation made possible by emergent optic technologies such as microscopes.

Fig. 4: Two engraved plates about bloodletting from Pietro Paolo Magni’s Discorsi di Pietro Paolo Magni (Rome, 1626). The left plate illustrates the anatomical sites for bloodletting. The right foldout plate portrays a barber-surgeon drawing blood from the scalp of a bedridden man with an attending physician closely watching the operation. Courtesy of the Osler Library, McGill University.

Fig. 4: Two engraved plates about bloodletting from Pietro Paolo Magni’s Discorsi di Pietro Paolo Magni (Rome, 1626). The left plate illustrates the anatomical sites for bloodletting. The right foldout plate portrays a barber-surgeon drawing blood from the scalp of a bedridden man with an attending physician closely watching the operation. Courtesy of the Osler Library, McGill University.

In the 1660s, the first blood cells—commonly referred to as blood globules—were seen under magnifying lenses. One of the earliest accounts, and the earliest-known graphic illustration, was given by Antoni van Leeuwenhoek, who described the “ovale bloed-deeltjens” [oval blood particles] (fig. 5).[8] Microscopic observations inspired a reimagining of all manner of subjects from fleas to the cosmos.


Fig. 5: Small engraving from Antoni van Leeuwenhoek’s Ontledingen en Ontdekkingen (Leiden, 1686). Leeuwenhoek was a prolific microscopist and natural philosopher. Among his many correspondences with the Royal Society were the first published images of red blood cells, which Leeuwenhoek referred to as red blood globules. Courtesy of the Osler Library, McGill University.

The body was a special site of reconceptualization. As Barbara Maria Stafford argues, magnifying catalyzed the collapse of “the model of the body as an integral whole.”[9] The uniformity of blood, for instance, was suddenly splintered into constituent parts. The scope of new optics also forced empiricists to face their “congenital sensory frailty.”[10] This crisis in observation spurred Thomas Morgan, an 18th-century physician who wrote about medicine and philosophy, to precisely delineate the utmost limit of what can be seen in blood without such an instrument:

It is certain in Fact from Microscopick Observations, that the Diameter of the largest Blood Globules do not exceed of an Inch; ’tis likewise well known, that the Diameter of a Particle visible to the naked Eye, must not be less than of an Inch.[11]

What was smaller than a hundredth of an inch—beyond the reach of the naked eye—required different explanations which might call established paradigms and properties, such as the blood’s vitalist qualities, into question. Like many other 18th-century medics, Morgan subscribed to iatromechanism—the theory that the body acts as a machine. Morgan used this mechanical theory to explain how body heat resulted from friction created by blood globules moving past one another.[12] Such physiological re-conceptualizations based on microscopic observations prompted a growing skepticism towards conventional diagnoses made by the naked eye.


Inflammatory Blood

The derisive comments of the early 16th-century Flemish physician Jean Bapiste van Helmont proved visionary. “Truly I am wont to compare the Lookers into the blood” he writes, “unto those who give their judgement concerning Spanish Wine, and who give their thoughts in beholding of the urine.”[13] For van Helmont, the common diagnostic practice of observing blood was a blatant imposition on patients, where great pronouncements about a person’s health were divined from the murky depths of a barber-surgeon’s bowl. Although his censures did little to stem the torrent of bloodletting at that time, such skepticism grew significantly in the 18th century.

One such critique was leveled by Henry Fielding in his 1749 picaresque novel Tom Jones. The eponymous hero is attended by a surgeon who confesses the following medical misadventure:

I presently made a large orifice in the vein of the left arm, whence I drew twenty ounces of blood; which I expected to have found extremely sizy and glutinous, or indeed coagulated, as it is in pleuretic complaints; but, to my surprize, it appeared rosy and florid, and its consistency differed little from the blood of those in perfect health.[14]

The surgeon is depicted as laughably and worryingly confident in his misguided knowledge. The term sizy, also spelled sizey meant thick or viscous, and it gained traction in late 17th-century medicine. According to Richard Davies, a physician and fellow of the Royal Society, “inflammatory Blood” was often sizy.[15] The term was intended to expand the diagnostic repertoire of blood-gazing medics, but for critics like Fielding such terms were mere medical affectation: jargon smokescreens that obfuscated lay-comprehension and covered up the dearth of accurate knowledge available from such crude diagnostic practices. Fielding’s parody was a criticism of superficial medical diagnoses in an era when technologies like microscopes offered greater insight into blood.

One response to these kinds of criticisms came from the Yorkshire surgeon William Hey, who attempted to standardize the diagnostic interpretation of blood from naked-eye observation.

So frequent are the opportunities of examining blood drawn from the human body, and of observing the relation which its condition bears to the state of the patient from who it is taken; that one might expect to find an uniformity of sentiment, in those who are engaged in the practice of physic, respecting the various appearances of the blood, and their relation to diseases. But so far is this branch of medical knowledge from being fixed, that some of the latest authors seem to have been ignorant of, or inattentive to, the plainest facts, which daily opportunities of observation might have made them acquainted with.[16]

To ameliorate this shortcoming and disunity in medical practice, Hey offered a set of rules to guide diagnoses to be made from “the appearance of the blood.”[17] Such efforts to standardize naked-eye interpretations of blood continued into the 19th century. But by the close of that century, a standard haematology text such as Dr. Richard C. Cabot’s A Guide to the Clinical Examination of the Blood for Diagnostic Purposes would consider the examination of fresh blood almost entirely in terms of microscopy.[18]

The priority given to microscopic knowledge of blood only increased as new optic and instrument-based diagnostic techniques and technologies became more available. Patients and physicians sought deeper and more detailed knowledge of blood’s constituent cells and chemical profiles. Diagnostic innovations—including cytometers, stains, centrifuges and many generations of microscopes—shored up much of the knowledge authority once assumed by barber-surgeons staring at a freshly drawn pint of blood. This trend has proven to be prevailing in modern medicine, as the array and power of diagnostic technologies continues to expand, diminishing the authority of unmediated observation. The health of a patient’s blood is now determined by cell counts, chemistry workups, enzyme analyses, cholesterol levels and an abundance of other assays. The crimson flood coursing through our veins is a dynamic tissue comprised of metabolites, antigens, pathogens, hormones and other minuscule components. We know our blood on a microscopic scale.



Darren N. Wagner is the Postdoctoral Fellow of the Chair for the Social History of Medicine at McGill University, and is affiliated with the departments of Social Studies of Medicine and History and Classical Studies. He researches the cultural history of sexuality, reproduction and neurology in Western medicine from the 17th to the 19th century. He is co-editor with Raymond Stephanson of the collection The Secrets of Generation: Reproduction in the Long Eighteenth Century (University of Toronto Press, 2015).


[1] This research was generously supported by the James McGill Travel Award to the University of Glasgow, the Social Science and Humanities Research Council of Canada, and the McGill Medical Faculty. Special thanks goes to the wonderful staff at the Hunterian Museum and at the Osler Library for their always friendly and proficient assistance.

[2] Hunter’s London museum opened in 1768. Following his death, and in keeping with his will, the museum was moved to Glasgow, where it reopened in 1807.

[3] For the layout of Hunter’s London property, see Helen McCormack, “Housing the Collection: The Great Windmill Street Anatomy Theatre and Museum,” in My Highest Pleasure”: William Hunter’s Art Collection, edited by Peter Black (Glasgow: Paul Holberton Publishing, 2007): 101–16.

[4] Seventeenth- and eighteenth-century anatomists developed syringes and techniques for injecting body parts. See F. J. Cole, “The History of Anatomical Injections,” Studies in History and Method of Science, vol. 2, edited by C. J. Singer, 285–343 (Oxford: Clarendon Press, 1921).

[5] For hemocentrism in physiological doctrines of antiquity, see Valentina Conticelli, “Sanguis Suavis: Blood between Microcosm and Macrocosm,” in Blood: Art, Power, Politics and Pathology, edited by James M. Bradburne (Prestel: Munich, 2001): 54–63. For the greater significance ascribed to blood over the other humors in medieval medicine, see Bettina Bildhauer, Medieval Blood (Cardiff: University of Wales Press, 2006), 24–5.

[6] Bildhauer, Medieval Blood, 23.

[7] In the English context, the Company of Barbers and the Fellowship of Surgeons were united as the Company of Barber-Surgeons of London in 1540. This union split in 1745 with the formation of the Company of Surgeons, which later led to the Royal College of Surgeons in 1800.

[8] Antoni van Leeuwenhoek, Ontledingen en Ontdekkingen (Leiden: Cornelis Boutesteyn, 1686), 26.

[9] Barbara Maria Stafford, Body Criticism: Imaging the Unseen in Enlightenment Art and Medicine (Cambridge, MA: MIT Press, 1993), 340.

[10] Stafford, Body Criticism, 2.

[11] Thomas Morgan, The Mechanical Practice of Physick (London: T. Woodward, 1735), 6.

[12] This theory was seconded in Robert Douglas, An Essay concerning the Generation of Heat in Animals (London: R. Dodsley, 1747), 133.

[13] Jean Baptiste van Helmont, Van Helmont’s Works, translated by J.C. (London: Lodowick Lloyd, 1664), 942. Van Helmont also posed a hyperbolic challenge to bloodletters: “come down to the contest ye Humorists: Let us take out of the Hospitals, out of the Camps, or from elsewhere, 200, or 500 poor People, that have Fevers, Pleurisies, &c. Let us divide them in halfes, let us cast lots, that one halfe of them may fall to my share, and the other to yours; I will cure them without blood-letting and sensible evacuation; but do you do, as ye know (for neither do I tye you up to the boasting, or of Phlebotomy, or the abstinence from a solutive Medicine) we shall see how many Funerals both of us shall have” (526). One wonders whether such a large and exploitative trial of bloodletting ever took place.

[14] Henry Fielding, The History of Tom Jones, edited by R.P.C. Mutter (London: Penguin, 1966 [1749]), 347.

[15] Richard Davies, To Promote the Experimental Analysis of the Human Blood (Bath, 1760), 25.

[16] William Hey, Observations on the Blood (London: J. Wallis, 1779), 3.

[17] Hey, Observations, 44.

[18] Richard C. Cabot, A Guide to the Clinical Examination of the Blood for Diagnostic Purposes (New York: William Wood and Co., 1897).




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  1. Pingback: Whewell’s Gazette: Year 3, Vol. #11 | Whewell's Ghost

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