Blood: From Humor to Hematology
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Related TopicsAlchemy Blood circulation Chemistry Hematology Humoral theory Life as concept Medicine Phlebotomy Physiology
Today blood can be defined simply as the red liquid flowing through the arteries and veins of humans and other animals. But in early modern philosophy and science, blood acquired many different meanings: it transformed from being one of the four cardinal humors – believed to determine a person’s physical and mental qualities – into a raw material which was treated, studied, measured, and investigated. With the development of new methods and instruments, this vital fluid also came in a variety of shapes and sizes: it came to be understood as globules, as consisting of chemical elements, as having various temperatures and weights. Although medical treatments like bloodletting were slow to change, blood itself underwent a major revolution as the result of long and uncertain endeavors of study and experimentation. And considering the discovery of blood circulation as well as the development of hematology and blood chemistry, blood serves as a significant case study for the history of the scientific revolution. This article will discuss these major developments in the early modern study of blood.
Histories of Blood
The Humor of Blood
Historians of medicine have traced back the history of rational ideas about blood to the learned physicians of ancient Greece (Conrad et al. 1995; Arikha 2007). Although the notions of blood would change significantly during the early modern period, ancient ideas about blood continued to have a profound impact on early modern medicine.
The Hippocratic Corpus and works of Greek physician Galen (129–200 CE) formed the core of medical theory and practice in premodern Europe. According to Galen’s humoral theory, health and disease depended on the balance or imbalance of the four essential humors: phlegm, yellow bile, black bile, and, of course, blood. In this system, the humors were in harmony with the four Aristotelian elements (air, water, fire, earth) and the four qualities (hot, cold, dry, wet). The humor of blood, for example, was considered hot and wet, while black bile was believed to be dry and cold. Over time, the humors were also linked to the four temperaments: sanguine, phlegmatic, choleric, and melancholic. Blood literally was the most sanguine of all four humors, allowing humans to be characterized by optimism, patience, and thoughtfulness – as opposed to being irritable or melancholic in the case of yellow and black bile, respectively. But although the humors were related to the bodily fluids, they were not identical with them. What, for example, were black and yellow bile? Humoral theory was above all a rational and logical theory, which was not necessarily grounded in observation and experience.
Early modern physicians and surgeons were well aware that patients should not lose too much blood, because they feared the loss of the innate heat, moisture, and vital forces of the body. Still, most European doctors believed in the advantages of a regular bloodletting, because the removal of excessive heat and accumulation of bad substances would prevent cramps, inflammations, and other disorders.
The practice of bloodletting was far from limited to European medicine. Phlebotomy spread along the migration routes of Eurasia to become an integral part of Arabic, Ayurvedic, Unani, and Chinese medical practices (Kuriyama 1995). The nomadic Scythians (8th–2nd centuries BCE) in particular practiced bloodletting and had considerable contact with both Greek and Chinese cultures. Chinese theoretical principles underpinning bloodletting were strikingly similar, as traditional Chinese medical philosophy revolved around the qi, a balance between yin and yang, and the five elements. But as the centuries passed, European and Chinese perceptions of bloodletting sharply diverged. While in Europe bloodletting remained a popular practice until the turn of the nineteenth century, Chinese physicians instead developed acupuncture as the “bloodless surgery.”
The Circulation of Blood
The seventeenth century saw a major shift in the way physicians understood the flow of blood in humans and animals. Historians of science attribute this shift predominantly to the work of the English physician William Harvey (1578–1637) (Cohen 2015). Up until 1600, medical students across Europe learned about the movement of blood according to Galen’s descriptions of two systems: first, that blood was made from food, endowed with natural spirits, flowing from the liver through the veins to all parts of the body to nourish it; and second, that it was imbued with “vital spirit” drawn in from the air in the lungs, flowing from the heart via the arteries to all parts of the body to vivify it. As the body used up some of the arterial blood, only a little of the venous blood had to spill over into the arterial vessels. The notion that the movement of blood was one-directional – from the liver and the heart to all parts of the body – made perfect sense. Venous blood was darker because of the nutriments. Arterial blood, on the other hand, pulsated and spurted out when the vessel was punctured, because of the liveliness of the vital spirit (Beauchamp 2000).
But if Galen’s system of blood was so widely accepted, why did Harvey feel the need to research this subject and did he eventually discover a completely different system of blood flow? Looking at his education answers this question. After his studies at Cambridge, Harvey went to the greatest medical school, Padua University in Italy. There he attended anatomy lectures by Hieronymus Fabricius (1537–1619), who had been investigating the generation (i.e., procreation), respiration, and local motion of animals – reviving Aristotle’s ancient practice of anatomical investigation (Cunningham 1997). Indeed, from the sixteenth century onward, natural philosophers were not simply reiterating ancient ideas about blood and the body. Instead, they became interested in complementing Galen’s theories with knowledge based on observation and experiment. New discoveries supported this view. Andreas Vesalius (1514–1564) in particular dissected many corpses in order to understand human anatomy. In comparison to Galen’s, Vesalius’ De humani corporis fabrica (“On the Fabric of the Human Body,” 1543) presented the most accurate description and the human body to date. Fabricius discovered the so-called valves in the veins, hypothesizing they slowed down the flow of blood.
It is important to stress, however, that Harvey’s notion of the circulation of blood was not immediately embraced by physicians and philosophers. Accepting his ideas meant that Galen’s theory of the two blood systems was flawed, and the centuries-old practice of bloodletting utterly useless. In fact, Harvey was shocked by his own discovery. As an Aristotelian, he also remained fairly conservative in his philosophical thinking and continued to espouse the doctrine of the four bodily humors. It was only after a few decades that his theory of circulation was accepted and taught at universities. One of the most outspoken adopters of blood circulation was the French mathematician and philosopher René Descartes (1596–1650), who used it to support his theory of the body acting as a machine. Descartes turned Harvey’s discovery into a classic example of mechanical philosophy applied to animated bodies. Known as iatromechanism, this body of thought viewed living bodies as the result of very complex, hydraulic machinery (Riskin 2016).
Transfusion did not become part of mainstream therapeutic practice. When in February 1668, a man in France died from the effects of a blood transfusion; the experiments were stopped. But by that time, Harvey’s revolutionary notion of blood circulation had been widely accepted. The transfusion experiments also demonstrate that in the minds of physicians, blood continued to inhere vital properties and a person’s characteristics.
The Monthly Evacuation of Blood
Blood gained particular meaning in female health. Early modern women and physicians debated the complex phenomenon of menstruation, because many uncertainties surrounded the menstrual blood. What was the relationship between menstruation and health? What role did menses play in conception and gestation? And was menstrual blood more similar to venous or arterial blood? Mirroring the shift in perceptions concerning the motion of blood, the medical understanding of menses also underwent significantly changes.
In ancient and medieval medicine, the female body was generally regarded as inferior to the male body. The menses, in combination with humoralism, conclusively proved this point. Men’s bodies were thought to be warm, grew large, produced much hair, and effortlessly urinated or sweated out excess humors that had built up within them. Women’s bodies, on the other hand, were seen as colder and hence smaller, smoother, and physically more fragile. And because of the coldness and frigidity of their bodies, they needed to regularly purge accumulated excess by means of a monthly evacuation. These views added to a general taboo against the menses, or “the curse of women” (Shail and Howie 2005; Gueneau-Jalabert 2002).
The menstrual blood itself was considered toxic. This had its advantages. According to Galen, menstruation eliminated plethora or excess blood in women’s bodies, thereby sparing them from multiple diseases believed to be caused by superfluous blood. Similarly to the removal of bad blood during venesection, then, the normal menstrual blood was secreting harmful matter.
The discovery of the circulation of the blood in the human body had a profound impact on medical thinking about menstruation. No longer did a separate system of blood flow exist. French physicians such as François Mauriceau (1637–1709) and Jean Astruc (1684–1766) therefore argued that menses resembled normal blood. Stressing that the menses was “quite red and quite lively,” they stressed its resemblance to the general mass of blood, opposing its supposed corruptness. Pregnancy provided yet another argument for the benign quality of blood: the absence of menstrual blood during pregnancy was explained in reference to the menstrual blood as fetal nourishment. Physicians calculated that 9 months’ worth of menstruation was of sufficient quantity to nourish a fetus during that period. Similarly to venous, arterial, or hemorrhoidal blood, menstrual merely came to denote the bodily location where blood originated, and, vice versa, language employed to describe menstruation was sometimes used to understand loss of blood in men (Pomata 2001; McClive 2015).
Cathy McClive (2015) has shown that menstruation became the focal point in people’s thinking about female fertility and health. In the seventeenth century, the regularity of a woman’s evacuations increasingly became an important signifier. Letters and diaries show that women increasingly self-monitored their menstrual habits. In learned and popular books, physicians defined the average quantity and quality of menses women were expected to evacuate each month. Regular menstruation pointed to a well-disciplined, orderly, and moral body. Whenever a woman was ill or unable to get pregnant, medical practitioners pressed men and women to restore and maintain regularity.
Significant shifts in medical understandings of menstruation occurred in the late seventeenth century. No longer toxic or corrupt, the menstrual blood was seen in the context of blood in general. Humoralism nevertheless continued to influence people’s thinking about menstruation, because balance in quantity and regularity in time largely defined female health and fertility.
The Color of Blood
Although blood appears to be homogeneously red, early modern barbers and surgeons were intimately familiar with the color changes that could occur in the fluid. For example, when blood was slowly coagulating in a bowl, it appeared to be darker at the bottom and lighter at the top. A major question was whether the color of blood said anything about the fluid? While some natural philosophers and physicians stressed the importance of color in anatomical and physiological studies, others claimed that it was not a useful property to explore substances at all (Bertoloni Meli 2011).
Why is blood red? In his Traité de l’homme (1648), René Descartes (1596–1650) considered this question, wondering how a white fluidlike chyle could transform into a deep red fluid that was blood. Borrowing an analogy from Galen, he explained that just as the white juice of black grapes turns into red wine, in the liver the chyle passes through the pores and “takes on the color and acquires the form, of blood.” His theory on color was supported by a mechanical account. Roughly, Descartes reasoned the spin of particles to correspond to a particular color: the smallest rotational speed would make us see blue, the greatest spin red. Thomas Willis, in contrast to Descartes, provided a chemical reason for the change of color of blood. In Diatribae duae medico-philosophicae (1659), he argued that this phenomenon resulted from the combination of the sulfurous particles of blood with those of salt and spirit.
Other physicians, however, regarded the question of how blood obtained its colors irrelevant. In 1650s Pisa, the Italian anatomists and physicians Giovanni Alfonso Borelli (1608–1679) and Marcello Malpighi (1628–1694) performed numerous anatomical observations and in the process observed the blood. Whereas Galenist physicians had claimed that the four humoral fluids could be easily recognized in blood, these Italian professors disagreed. Galenists believed that the red portion of blood was black bile, whereas the lighter, watery part was yellow bile. Malpighi falsified this claim by turning clotted blood upside down: the dark part at the bottom turned bright at the top, and vice versa. He therefore stated that the dark or bright colors in blood were “accidents” unrelated to the change of substance or its “mixing.” The sensory qualities could be changed, without altering the substance. Borelli and Malpighi were probably inspired by Galileo Galilei’s Assayer (1623), in which he distinguished between “objective” or primary qualities on the one hand and “subjective” or secondary qualities on the other. Those primary qualities included size, motion, spatial relation to other bodies, and number, because these were inseparable from the corporeal substance.
In 1660s England researchers Willis and Richard Lower attributed a significant role to the change of color in blood (Frank 1980). By bloodletting a dog using different vessels, they observed the difference in color between the bright red arterial blood and the darker venous blood. Willis surmised that the color change occurred in the heart. But during another vivisection, Lower and Robert Hooke (1635–1703) observed that the blood coming out of the right ventricle of the heart was venous. This meant that the change of color of blood did not occur in the heart. Numerous other vivisections investigated the difference between venous and arterial blood, which eventually demonstrated that it was not the motion of the lungs, a ferment in the heart, or body heat that caused the colors change, but exposure to air.
Domenico Bertoloni Meli (2011) has shown how Malpighi, too, had observed that blood in the lungs changed in color, but thought that the substance of blood remained the same. Blood appeared to flow inside the vessels, and color, after all, was no valid indicator of change. His English colleagues decided to look at the color of blood afresh. Their reflections on the nature of blood and the location of its change in color were of great importance to medical and scientific understandings of blood. For even though the color of blood had returned as important property, it had undergone a major reconceptualization: from proof of the humors of blood, it had become evidence of the role of air in respiration.
The Shape of Blood
For centuries, blood flowing through the arteries and veins of the living body remained firmly fluid and homogeneous. Only once released from the body did it start to clot and divide into a hard, red part and a watery part (serum). This notion of a homogenous blood changed in the second half of the seventeenth century, when red globules were discovered in blood thanks to the visual evidence provided by the microscope. It provided further support for and even introduced new ideas of corpuscularianism to notions of the body.
The microscope was invented around 1610, but it was not until the 1660s that microscopic observations became part of systematic studies. Robert Hooke (1635–1703) and Nehemiah Grew were the most prominent microscopists at the time, both investigating a wide range of living creatures, in particular plants and insects. This was no easy feat, considering they had to make use of sunshine or candles as light source and relied on drawings as the only permanent representation of what they could see. Nevertheless, providing visual microscopic evidence in the form of wonderful plates, Hooke’s and Grew’s works revealed the previously invisible world of minute beings and cellular structures of plants and insects.
Descriptions of blood at the microscopic level had an enormous impact on the medical and learned community. The idea that blood consisted of globules would fascinate researchers for decades to come. In the early eighteenth century, for example, Dutch physician Thomas Schwencke (1694–1767) argued that one red globule consisted of a cluster of six serum globules, together gaining weight and darkness, but preserved the same size. Furthermore, he believed that every serum globule was composed of six chyle globules. One red blood globule therefore consisted of 36 chyle globules, stuck together by an inherent, elastic force emanating from each individual globule.
This trituration or fine division into particles would not last. Once microscopes with a larger magnification were constructed in the second half of the century, Englishman William Hewson (1739–1774) found that the red cells were not globular, but as “flat as a guinea.” Hewson made many other observations. Reporting in the Philosophical Transactions of the Royal Society, he found the red cells in children to be larger than in adults. Using serum rather than water to dilute the blood under examination, he also discovered “colorless cells” or “central particles” (probably the cells we now call white cells). Similarly to Schwencke, Hewson presumed these colorless cells would develop into red corpuscles.
Descartes’ original understanding of blood had already presupposed certain particles. But the shape of blood only became apparent by means of observations with the microscope. It further spurred the early microscopists and physicians to explain the composition of bodily materials in terms of corpuscles.
The Elements of Blood
For all their importance, the microscopic discoveries of the later seventeenth century did not answer questions about the nature and production of blood. What exactly was blood made of? Natural philosophers and physicians therefore increasingly turned to chemistry as a method of investigation, hoping that it would help them to resolve the physiological problem of the nature of blood (Verwaal 2017).
Robert Boyle grew to become one of the most influential early modern natural philosophers. His studies led him to propose a theory of matter based on elements and corpuscles. Although today he is best known for his experiments with the air pump, for many years he also investigated blood. In his Memoirs for the History of Human Blood (1684), he collected all kinds of observations on blood over a time span of 20 years. Boyle also reported that distilling blood resulted in an oily and phlegmatic part and a liquor or spirit. This spirit of human blood was made of a volatile salt and phlegm and “endowed with divers qualities, which are active and medicinal.” Clearly, Boyle intended to use chemistry as a method to investigate animated bodies and aimed to encourage others to carry out comparable work.
Chemistry continued to evolve in the course of the eighteenth century. At universities, medical professors and students increasingly approached blood with chemical methods, experimenting on blood, handling it, and determining its properties using instruments as well as their own senses. The Dutch professor Herman Boerhaave was particularly influential in incorporating chemistry in the medical curriculum at Leiden University, because “[n]o-body but a chemist could say what kind of liquor the blood is” (A New Method of Chemistry, 1727). He argued that only chemistry could reveal the active principles of blood, which was essential for distinguishing between signs of health and disease.
Physicians started to envision blood as a complex blend of rudimentary substances and principles. Boerhaave’s successor at Leiden, Hieronymus Gaubius (1705–1780), performed many chemical experiments on blood to uncover its “constituent parts.” He found that blood drawn from a healthy person coagulated and split into three parts: the white and watery part called serum, a hard and red part called crassamentum, and a membranous substance called fiber. Distilling a quantity of blood at different temperatures revealed four basic elements: water, bitter oil, volatile salt, and black earth. These were the basic elements believed to constitute the entire human body. The proportion of the principle of water in relation to the other elements imparted fluidity to bodily liquids and flexibility to solid parts. Similarly, the principle of earth assigned the quality of thickness to the fluids and of firmness to the solids.
Understanding blood in terms of these principles helped Gaubius explain various pathologies. If someone had consumed too much glutinous foods, he reasoned, the level of crassamentum would rise in disproportion to the serum and even cohere to earth elements. The blood would circulate more slowly, stagnate, obstruct, and potentially cause a tumor. By contrast, if a person drank too much, the resulting superfluous water in the serum could lead to hemorrhages (Institutiones pathologiae medicinalis, 1758).
Early modern chemistry thoroughly transformed perceptions of the nature of blood. Thinking about its constituent parts and principles not only allowed physicians to investigate blood more closely, it also enabled them to theorize about blood in its varying physiological and pathological states.
The Temperature of Blood
In 1743, Thomas Schwencke, a city physician from The Hague, coined the word “hematology” in his systematic study of blood, titled Haematology, or the History of Blood. Hematology referred not only to the treatment of blood, including phlebotomy and the means to stem bleeding, but also the acquisition of new knowledge about blood (McCann 2016; Verwaal 2017). What had brought about this specialization in medical thought? And how did it impact the way blood was perceived?
Schwencke also gained a new understanding of blood and coagulation by measuring temperature. Thermometry had caused heated debates among natural philosophers, because how were the fixed points in thermometry to be established? And what was the use and applicability of thermometers? (Chang 2004). Yet Schwencke and other physicians were quickly convinced of the use of the thermometer, arguing that the only way to properly measure degrees of heat was with a quicksilver-filled thermometer. He thus took the temperature of the blood in numerous oxen and cows, measuring simultaneously in the carotid artery and jugular vein, the blood vessels in the neck. His thermometers measured 97 and 94 degrees Fahrenheit, respectively. Schwencke concluded that the body’s heat was dependent upon the movement and friction of blood. Excitement, for example, caused the pulse to throb more rapidly as the heat increased.
Schwencke established that a healthy person had a temperature of 96°, reasoning there existed a correlation between body temperature, mental state, and disease. Future physicians, Schwencke argued, ought to systematically measure the temperature of patients in all diseases, which would greatly improve diagnostics. Physician and professor of medicine in Vienna, Antonius de Haen (1704–1776), continued hematological research. Having access to hundreds of patients in the Citizen’s Hospital in Vienna – where orphans, the poor, and the elderly were accommodated – de Haen made the first attempt to test body temperatures in health and disease by thermometer. De Haen’s numerous observations and systematic investigations of blood temperature in patients falsified some ancient notions of blood. Whereas Hippocrates had famously stated that young children had a higher body temperature than adults, and the elderly even colder body temperatures, de Haen provided the evidence disproving that claim, i.e., that all humans share the same body temperature.
Measuring was crucial in hematology. The invention of new instruments allowed researchers to measure the density, weight, and temperature of blood, in different persons and circumstances. These properties of blood were quickly considered to be useful in future studies of disease. Although temperature as a diagnostic indicator proved to be less useful than initially thought, the very enterprise had placed hematology firmly on the map as a new and scientifically rigorous specialization.
The Pulse of Blood
A property of blood that proved more useful in diagnostics was the pulse. Looking into ways to obtain knowledge about diseases, French physician Théophile de Bordeu (1722–1776) claimed that the essential character of a disease could only be understood when the living body was observed – rather than by dissection of corpses or the chemistry of dead blood. In order to diagnose and treat patients, Bordeu focused his attention on the symptoms and circumstances of living, pulsating blood. But how could the pulse at the wrist inform the doctor about a patient’s disease?
Sphygmology, or pulse diagnosis, was far from new. Galen had already devoted 16 essays on the topic of feeling the pulse. In his On the Pulse, he wrote of the differences of the pulse and how to distinguish them, the causes of different pulses, and their prognoses. Galen described five characteristics – velocity, size, strength, quality, and degree of filling of the artery – and explained how these appeared in various conditions and combinations depending on the state of the patient. By means of an elaborate system of different pulses – and assuming that there existed a unique type of pulse for every organ and every disease – a physician could diagnose. But in the centuries since Galen, the practice of pulse feeling had not gained any permanence in medical diagnoses.
Boym’s collection of works mainly dealt with varieties of the pulse and tongue in health and disease and discussed the correlation between the pulse and meridians and channels throughout the body according to Chinese anatomy. The book forced European philosophers and physicians to rethink their notion of the pulse of blood and the relation between Asian and European classics more widely. Whereas some authors discussed the work and judged it as proof that the Chinese classics were to be appreciated in a similar vein as the Greeks’, others were of the opinion that these works were no more than dry and tired texts. The English critic William Wotton (1666–1727), for example, dismissed the ancient Chinese medical texts as “tedious” and “ridiculous” and its images “whimsical.” One Sir John Floyer (1649–1734), on the other hand, found in them inspiration for further medical development on the pulse.
English physician Floyer gained a fascination for Chinese medicine and diagnostics, possibly through Oxford University’s library reception of Chinese books and the visit of Chinese scholar Michael Shěn Fúzōng, a convert to Catholicism who had become Procurator of the China Jesuits in Rome. Having read the Specimen of Chinese Medicine, Floyer was convinced that the pulse could equip physicians with an exact diagnosis of their patients’ illnesses and thus result in better treatments. No stranger to modern technologies, Floyer developed a special glass watch to count the number of beats and argued that there was a correlation between the number of beats per minute and the physical condition of the body. Having performed several experiments with the pulse-watch, Floyer published his findings in his pioneering work The Physician’s Pulse-Watch (1707). In this book, Floyer compared and contrasted ancient theoretical basis for pulse feeling by Galen with Chinese expertise on palpation. He corrected Galen’s “errors” based on research with his own pulse-watch and presented tables indicating a series of ages, pulses, and habits of the body, directly linking a patient’s pulse to disease. In short, thanks to the combination of Chinese and Greek medicine, and the addition of an innovative mechanical device, the pulse of blood had become an object of experimentation and systematic observation.
The understanding of blood underwent major transformations in early modern philosophy, science, and medicine. As natural philosophers and anatomists studied the body more closely, ancient ideas of humoral blood as well as menstruation were increasingly challenged by these new findings. Ultimately, the circulation and color of blood falsified humoralism, even though it did not immediately change medical practice. The shape, elements, and temperature of blood, along with the invention of new instruments, spurred developments in blood research, in particular the field of hematology, which would eventually become an important medical specialization.
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