Monro Distinguishes Between Lymphatic and Blood Systems

Date 1757

Alexander Monro observed a system of fluid absorption associated with lymphatics that appeared to possess its own valvular system. As a result, he argued the system is unique and separate from that of the circulatory system for blood.

Locale Berlin, Germany

Key Figures

• Alexander Monro II (1733-1817), a Scottish physician and anatomist
• William Hunter (1718-1783), a Scottish physician and surgeon
• William Hewson (1739-1774), a British anatomist
• Gasparo Aselli (1581-1626), an Italian anatomist who first described lymphatic vessels
• Thomas Bartholin (1616-1680), a Danish physician who observed the thoracic duct in humans, and who coined the term “lymphatic”

Summary of Event

The existence of a system of vessels separate from the blood system was suspected for two thousand years. The first actual description of such a unique system was that by the Italian anatomist Gasparo Aselli, who in 1622 reported the release of a milky fluid from the lacteal vessels observed in the intestine of a dog. Aselli referred to these vessels as “milk veins,” reflecting the prevailing (but erroneous) viewpoint that the function of these vessels was the transport of intestinal material to the liver, where it was to be used for the synthesis of blood. Aselli’s illustrations were later published posthumously.

Other anatomists and physicians made similar observations of a system of vessels, including Danish physician Thomas Bartholin in his description of the human thoracic duct, now known as the vessel through which lymph is returned to the blood. In 1653, Bartholin coined the term “lymphatic” to describe the vessels of the system. Bartholin, however, did not know the function of the lymphatics.

Another physician, Alexander Monro, was one of a line of significant figures in anatomy during this era. He was the youngest son of Alexander Monro (Primus), anatomist at the University of Edinburgh. Alexander enrolled in his father’s anatomy class at the age of eleven and by age twenty-one had been appointed to the position of professor of anatomy at the university (1754). In October, 1755, Monro was awarded his medical degree. His medical thesis was noteworthy in that it included original research, unusual at a time when most medical theses were basically literature reviews.

Monro’s thesis described his investigations into the structure of the testis, including circulation of the blood as well as a secondary system later determined to be the lymphatics. Monro observed that if he injected mercury into the vas deferens, the resultant pressure would cause the bursting of the epididymis. The mercury, suddenly released, entered the lymphatics. The results indicated to Monro that the lymphatic vessels were not a portion of the arterial system.

Following graduation, Monro went to London, where he attended lectures delivered by William Hunter, a noted anatomist and former student of Monro’s father. Hunter’s research also included that of the blood and lymphatic circulations. One of Hunter’s lectures, dealing with the absorbent mechanisms of the lymphatics, led Monro to believe that Hunter had read his medical thesis. There is no evidence that Hunter’s observations were anything but completely independent of Monro’s earlier work, however.

In 1757, Monro traveled to Berlin, where he continued his study of anatomy under Johann Friedrich Meckel (the elder), who was considered one of the outstanding physicians in that field. Monro’s experimental work in Berlin was a continuation of the work in his medical thesis—the origin and function of the lymphatic system.

Monro’s published work of his research in Berlin included much of what modern science would consider a literature review. He included references to previous studies and interpretations on the subject. Ironically, he did not include the work by Hunter. By injecting mercury into various tissue sites, as well as injections directly into lymphatic vessels, Monro demonstrated that there is no direct communication between the arteries and veins that carry blood and the lymphatic vessels; that is, they are separate systems. Furthermore, the movement of fluids through the lymphatics is directional—lymph flows in one direction and enters the blood stream near the heart at the subclavian vein.

Monro also observed that the swelling of lymphatic glands (buboes) occurs in the vicinity of infections; more specifically, he observed that venereal disease (sexually transmitted diseases) resulted in swellings in the region of the genitals. Infections in various places in the body would result in the movement of the (then unknown) agent through the nearest lymphatics toward the thoracic duct, from which the infection would enter the blood stream. Monro’s conclusion was that the lymphatic vessels are “absorbents,” draining fluid from body tissues and returning it to the bloodstream.

Monro was not the only individual to claim priority in discovering the lymphatic system. The strongest claim in the dispute was that by Hunter. Although Hunter had not published his observations of the lymphatic system earlier in his career, notes originating with Charles White, a student of Hunter, included references to the lymphatics as early as 1752. Monro eventually attacked Hunter over the issue of priority, arguing, without substantiation, that Hunter had obtained his ideas only after reading Monro’s medical thesis.

Hunter was not the only one who claimed the discovery. William Hewson had studied with Hunter during the 1750’s, attended medical school, and, in 1762, became Hunter’s partner. In the late 1760’s, Hewson published several papers in which he demonstrated a role for the lymphatic system in a number of vertebrates, including humans. As a result of this work, Hewson was elected as a fellow to the Royal Society. In his papers, Hewson stated he had observed the presence of lymphatics in organisms such as fish and turtles as early as 1762. Monro quickly disputed Hewson’s claims of priority, arguing instead that he, Monro, had observed such structures in these animals some five years earlier. The issue was never completely settled during Monro’s lifetime and was eventually dropped.

The matter of who should be accorded the honor for discovering the lymphatic system is one of figuring out who initially provided the information to his peers. The greater issue for the period, however, is represented by the expansion of scientific investigation beyond the classical views of early Greek or Roman theory. Indeed, the period represented one of enlightened thought in all fields of human endeavor, including the arts and sciences as depicted in writings from persons as diverse as Voltaire, Adam Smith, and Thomas Jefferson. The Industrial Revolution would shortly supersede the agricultural economy of most of Europe.

Whereas actual applications to scientific practice were limited during this period, the introduction of the smallpox vaccine being the most notable exception, scientific theory had begun a significant period of expansion. For example, rather than relying on outmoded views of anatomy dating to Galen nearly eighteen hundred years earlier, eighteenth century anatomists carried out direct observations and experimentation. The discoveries by William Harvey of the circulation of the blood, and Monro (or Hunter) of the lymphatic system, are such examples.

Significance

Issues of priority aside, the significance of the work carried out by Alexander Monro II and William Hunter lies less in proving the existence of the lymphatic system then in demonstrating it was a system separate from that of the blood. Furthermore, Monro and Hunter discovered its specific role in the body, as an absorbent that collects fluid from tissues throughout the body and then returns those fluids to the bloodstream. The existence of the lymphatic system within a wide variety of organisms was also demonstrated after later researchers combined the research carried out by the three claimants for priority; therefore, all the work of all three was important.

Monro also demonstrated that as a collecting system for fluid originating at sites of infection, the lymphatic system was capable of disseminating that infection throughout the body. Such work would later become a major factor in understanding the role of the lymphatic system in immune response.

Bibliography

Eales, Nellie. “The History of the Lymphatic System, with Special Reference to the Hunter-Monro Controversy.” Journal of the History of Medicine (July 29, 1974): 280-294. A history of the subject, with emphasis on the arguments about who made the first discovery of the system.

Finlayson, C. P. “Alexander Monro (Secundus).” In Dictionary of Scientific Biography, edited by Charles Gillispie. Vol. 9. New York: Charles Scribner’s Sons, 1974. A brief biography that examines Monro’s major scientific achievements.

McDowell, Julie, and Michael Windelspecht. The Lymphatic System. Westport, Conn.: Greenwood Press, 2004. A popular account of the history and makeup of the lymphatic system.

Porter, Roy, and Maurice Kirby. Disease, Medicine, and Society in England, 1550-1860. New York: Cambridge University Press, 1995. A relatively brief account of the impact of disease on English society during this period, as well as the effects of medical discoveries on the evolution of public health.