Experimental Physiology in the 1700s

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Experimental Physiology in the 1700s


Physiology is the study of the function of living organisms and their components, including the physical and chemical processes involved. The eighteenth century has been referred to as the Age of Enlightenment, or the Age of Reason, when knowledge in general advanced, and especially that relating to science and medicine. During this time, discoveries in physiology expanded due to a group of investigators known as experimental physiologists.

The study of human anatomy had been advanced by the daring work of Leonardo da Vinci (1452-1519) and Andreas Vesalius (1514-1564) in the previous two centuries, yet the processes by which the internal systems worked were still an enigma. Eighteenth-century scientists built upon this work. Unlike modern scientists who have distinct specialties, these early investigators would master several diverse fields. Scholars such as René de Réaumur (1683-1757) studied many disciplines in addition to physiology. Italian physiologist Lazzaro Spallanzani (1729-1799) contributed not only to human biology but made unusual discoveries concerning animal reproduction. In addition, Giovanni Borelli (1608-1679) studied movement, and Francis Glisson (1597-677) and Albrecht von Haller (1708-1777) studied irritability (muscle contraction).

The eighteenth century was rife with conflicts between science and religion. It saw the advent of mechanism, which envisioned living things as simple machines. This rivaled the ancient philosophy of vitalism, which proposed that all living things have an internal or "vital force" that is not accessible and cannot be measured. This force activates living things. Georg Ernst Stahl (1660-1734), German physician and chemist, supported vitalism, which held sway over biology until the early twentieth century. Stahl even lost his best friend, Friedrich Hoffmann (1660-1742), over differences of philosophical opinion. This quarrel between friends is just one example of how the optimism of their time was tempered by heated emotional debates over theory.


The Age of Enlightenment had its roots in the brilliant breakthroughs of the previous centuries, including the work of Vesalius, a Flemish anatomist and physician who is often called the founder of anatomy. The Greek physician Galen a.d. 129-216?) had earlier established many of the basic principles of medicine accepted for centuries. Galen's beliefs about physiology were a composite of the ideas of Plato (427?-347 b.c.), Aristotle (384-322 b.c.), and Hippocrates (460?-377? b.c.). He believed that human health was the balance of four major humors, or bodily fluids. The humors were blood, yellow bile, black bile, and phlegm. Each humor contained two of the four elements (fire, earth, air, and water) and displayed two of the four of the primary qualities of hot, cold, wet, and dry. Galen proposed that humor imbalances could be in specific organs and the body as a whole. He prescribed certain remedies to put the body back in balance. Galen's ideas held sway in all areas of medicine until the mid-1600s

In 1543 Vesalius dared to point out errors in the Galen's assumptions and proceeded to conduct dissections and experimental research. He showed that Galen had more information about animals than about humans, and also that translators throughout the ages had introduced many errors into Galen's texts. The ideas of physiology lasted until William Harvey (1578-1657) correctly explained the circulation of the blood.

Some scholars mark the beginning of the scientific revolution at 1543, the year that Versalius published On the Fabric of the Human Body and Nicholaus Copernicus (1473-1543) published On the Revolutions of the Heavenly Spheres, in which he challenged the old Ptolemaic belief that the Earth was the center of the universe. Both books did much to diminish the hold of classical Greek science, making possible the development of new ideas.

English physician William Harvey discovered how blood circulates in mammals, a finding that conflicted with the accepted ideas of Galen. Many of these achievements were impressive on paper, but did not impact the daily work of medicine. Major problems with disease existed in the beginning of the 1700s. War, plagues, and crowding in cities brought health hazards and high mortality rates.

Sir Isaac Newton (1642-1727) became an idol of the Enlightenment. His experiments and natural philosophy established a model. He led Herman Boerhaave (1668-1738), a Dutch professor, to apply physics to the expression of disease. According to Boerhaave, health and sickness were explained in terms of forces, weights, and hydrostatic pressure.

These views definitely encouraged experimentation. While the Church stymied new science and encouraged the acceptance of ancient truths, scientists began to observe, dissect, and challenge, leading to many new discoveries—as well as controversies. Physiology was only one of the fields advanced.


The impact of eighteenth-century science was not limited to changes in research methodology, but effected a fundamental new concept of reality with its foundation in observation and experiment. This basic change in perspective is called a paradigm shift, where one model is changed for another.

The eighteenth century as also an age of optimism. Even though war and disease took a toll, "enlightened" intellectuals in the Netherlands, Britain, France, and Germany proclaimed better times were on the way. Science and technology would give man control over nature, social progress, and disease. In the 1790s the marquis de Condorcet (1743-1794) declared that future medical advances, along with civil reason, would extend longevity even to the point of immortality.

Today, scientists typically focus their efforts in one field, or even a subspecialty of a particular field. For example, a molecular biologist may concentrate on one particular structure within the cells, such as the DNA of ribosomes. It is now difficult to imagine how diversely knowledgeable many of the early investigators were.

René de Réaumur was an eighteenth-century French scientist who researched and worked in a multitude of fields. In 1710 King Louis XIV put him in charge of compiling lists of all the industrial and natural resources in France. He devised a thermometer that bears his name, improved techniques for making iron and steel, and worked to uncover the secret of Chinese porcelain. He was one of the first to describe the regeneration of lost appendages by lobsters and crayfish. His work with animals led him to the study of digestion.

Scientists were beginning to ask key questions about how things worked. For example: How does digestion happen? By some internal vital force? By chemical action of gastric juice? By mechanical churning and pulverizing? Pondering these problems, Reaumur conceived of brilliant studies of digestion in birds. Since birds do not have teeth to grind food, he was convinced the structure called the gizzard acts to masticate and make food pieces smaller. He forced seed- or grain-eating birds to swallow tubes of glass and tin. The animals were dissected two days later. He was amazed to find the glass tubes shattered, but the action of the gizzard had smoothed and polished the pieces. The tin tubes were crushed and flattened.

Turning his attention to carnivorous or meat-eating birds, he trained a pet kite to swallow small food-filled tubes. He enclosed pieces of meat in a tube, closed the ends, then encouraged the bird to swallow. Later the bird would regurgitate the tube with the food in it. Analysis of the food showed that it had been partially digested and that digestion was more chemical than physical. He also attacked the prevalent idea of the day that putrefaction or rotting was the way food is digested. Réaumur had a great impact on the debate over digestion. He was one of the most esteemed members of the Academy of Sciences during the first part of the century, who wrote widely, and became known throughout the European community.

Lazzaro Spallanzani was an Italian physiologist who also contributed to many areas of science. Spallanzani received a sound classical education and became a professor of logic, metaphysics, Greek, and physics at the University of Modena in 1760. Although he worked during the day teaching, he devoted all of his leisure time to scientific investigation. Two scientists, Georges Buffon and John Turbeville Needham (1713-1781), had published a biological theory that all living things contain not only inanimate matter but "vital atoms" that go out into the soil and are recycled into plants. They claimed that the small moving objects found in pond water were the vital atoms from organic material. However, studying samples of pond water, Spallanzani was led to agree with Anton van Leeuwenhoek (1632-1722), the Dutch scientist who observed microscopic organisms. Spallanzani concluded that these moving objects were indeed small "beasties" or animals, and not just vital atoms.

Spallanzani was fascinated by the mechanism of how tails of salamanders, snails, and tadpoles regenerate. He devised several experiments to disprove the idea of spontaneous generation. This idea held that things like maggots came from nonliving decaying meat. Francesco Redi (1626-98) conducted a famous experiment with decaying meat that disproved this notion, but the idea of spontaneous generation persisted in many circles.

In 1773 Spallanzani did an important series of experiments on digestion. Using himself as the subject, he swallowed small linen bags that had different kinds of food. He would then regurgitate the bags and study the content. This enabled him to determine that digestive juice has special chemicals that target different kinds of foods. He also determined the solvent powers of saliva.

Spallanzani received much recognition for his study of reproduction. He also revealed that oxidation occurred in the blood. By 1800 it was accepted that oxygen combined with carbon in food to generate animal heat. Spallanzani took every opportunity to travel, exchange information with other scientists, and study new phenomena. This was also within the true spirit of the age of Enlightenment.

A tradition was developing among physiologists called "iatrochemistry," the chemistry of healing living things. This term might today be related to biochemistry. The iatrochemists explored links between respiration and combustion. Joseph Black (1728-1799), a Scottish professor, developed the idea of latent heat, which is given off in breathing. Later, Antoine Lavoisier (1743-1794) identified this as carbon dioxide. Black noted that this breathed-out air was not toxic, but could not be breathed to sustain life.

Giovanni Borelli was an Italian physiologist who first explained muscular movement. He studied muscular action, gland secretion, respiration, heart rhythm, and neural responses. He attempted to analyze these body movements in terms of physics. In his book The Movement of Animals he used the principles of mechanics to analyze movement. Looking at the contraction of muscles, he proposed that their operation was triggered by processes similar to chemical fermentation. He assumed respiration was a mechanical process, with the lungs driving air into the bloodstream. He described the bone structure as a series of levers, where muscles worked by pushing and pulling. He also believed air had something in it that sustained life. Air was a medium for elastic-like particles that went into the blood, enabling motion. According to Borelli, what could not be measured or weighed was mysticism and not scientific.

Borelli used the term iatrophysics, or iatrochemistry, to relate physics and chemistry to medicine. The word "iatro" is a Greek term that means to heal. One can see the word in "psychiatry," meaning to heal the mind, or in the word "pediatrics," meaning to heal children.

Francis Glisson was the first researcher to grapple with the idea of irritability. Glisson was a professor of physics at Cambridge and did a pioneering study on a "new" disease—rickets. As he studied the excretion of bile into the intestines, he noted that the reaction implied nerves were present. He became convinced that irritability (contraction) was a property of tissue and that nerves were independent structures. The idea of irritability was forgotten until Swiss biologist Albrecht von Haller (1708-1777) revived it in the next century. Glissons's observations were radical for his time, and prompted by Harvey's discoveries and mechanical philosophy.

Experimental physiology reached new heights through the work of Haller, a prolific writer who has been dubbed the father of experimental physiology. Born in Bern, Switzerland, he was a child prodigy who came under the influence of Hermann Boerhaave. One of his first labors of love was to revise an edition of Boerhaave's Institutes of Medicine, which showed that experimentation would produce new accounts of vitality and the relationship between the body and soul. Boerhaave's model of balance focused on the nervous system. He also became a strong advocate of the mechanistic philosophy.

Haller went to the University of Göttingen and served as professor of medicine, anatomy, surgery, and botany, He became very well known in the scientific community because of impressive experimental work at the newly formed university. He wrote an eight-volume encyclopedia called Physiological Elements of the Human Body (1757), regarded as a landmark in medical history. A devout Christian, Haller's writings evince the view that human beings consist of a physical body, which may be analyzed in terms of forces and matter, and an eternal soul.

Haller shocked the scientific community in 1753 when he suddenly resigned his prestigious position at Göttingen to return to Bern to continue his experiments, write, and develop a private practice. Experiments into breathing and circulation led Haller to be the first to recognize how respiration works and how the heart functions with it. He found that bile digests fats and identified many of the stages of embryonic development. He conducted anatomical studies of the brain and reproductive and circulatory systems.

However, his outstanding contribution was in the study of muscle contractions. Building on the results of 567 experiments—with 190 performed on himself—he showed that irritability is a special property of muscle. Irritability in medicine means capable of reacting to a stimulus or is sensitive to stimuli. He found a slight stimulus applied directly to a muscle causes a sharp contraction. He found that sensibility is the specific property of nerves. Sensibility is defined medically as the capacity to receive and respond to stimuli. In his book On the Sensible and Irritable Parts of the Human Body, he clearly established that irritability or contracting was a property of all muscular fibers, and that sensibility or carrying the message was the domain of nerves. His studies and writings became the foundation of neurology.

In his later years Haller spent much time cataloging scientific literature. His four volume Bibliography of the Practice of Medicine lists 52,000 publications on anatomy, botany, surgery, and medicine.

The idea of applying mathematics and measurements to science was also spurred by Boerhaave, who called the studies "iatromathematics." An Anglican clergyman, Stephen Hales (1677-1761), devised a unique experiment to measure the force of blood. He inserted a goose trachea, attached to a 11-ft (3.4 m) glass tube, into the jugular vein and carotid artery of a horse. This enabled him to measure how far the blood would extend into the tube.

Hales was fascinated with experiments with animals and used many different animals in inhumane ways. For example, interested in nerve action, he cut off the head of frogs and pricked the skin in different places to study nerve reflexes. His callous use of animals incited the strong disapproval of animal rights people of his day. These were called anti-vivisectionists, and they developed a strong voice in opposing physiology experimentation on animals. Led by Samuel Johnson, a great literary figure in Britain, the group proclaimed that such torture of animals had never healed anyone. This debate would continue into later centuries.

In the Age of Reason two philosophies of the biological nature of life were debated: vitalism and mechanism. The mechanistic theory viewed living organisms as machines. Thus, the whole of the body is the sum of its parts, programmed to run and operate on its own. This is called a reductionist position because it proposed that biology is simply reduced to physical and chemical laws. Vitalism, on the other hand, is a school of thought dating back to Aristotle. In this view the nature of life is seen as a result of a vital force peculiar to living organisms. This force is different from all other forces and controls the development and activities of the organism. One proponent of vitalism was Georg Ernst Stahl.

Stahl studied medicine at Jena and as a student became friends with Frederich Hoffman. Both went to the newly created medical school at Halle, where Stahl became a professor of practical medicine, anatomy, physics, and chemistry. When Hoffmann became an ardent convert to iatromechanics—a staunch mechanistic belief that living things were machines—he and Stahl parted ways after 20 years of friendship. Their philosophical differences had made the former friends bitter rivals.

Stahl was influenced by the German Pietists, a Protestant group that stressed devotion and the tolerant aspects of Christianity, similar to the Quakers or Friends. He insisted that mechanical and chemical laws were not enough to explain the mysteries of living beings. He determined that life must have direction by a special force and described this force using the Latin word anima. This gave rise to the name animism for the theory. Vitalism and anti-materialism are later words for this same view.

Stahl developed the constructs of the phlogiston theory. According to him, phlogiston was a substance that escaped from or was exchanged between materials involved in combustion. Phlogiston was from the Greek phlogiston, which means "to burn." Stahl built a practical theory that phlogiston was also a part of body physics and energy.

When Lavoisier discovered the role of oxygen in combustion, the phlogiston theory was proved wrong. However, modern researchers have praised Stahl for pioneering the use of molecular research in chemical analysis, and recognizing the difference between physical and chemical reactions.

Seeking to understand the workings of physiology, Enlightenment researchers made enormous contributions to the human capacity to master the environment. In searching for the answer to questions concerning the nature and internal processes of living things, they were able to advance knowledge of both the structure and the function of living systems.

Though historians note that the Age of Reason was marked by enthusiasm and hope, such great expectations were often tempered by disappointment, particularly when an overemphasis on theory and philosophical speculation actually inhibited scientific reason. However, the work of these eighteenth-century scientists laid the foundations for nineteenth-century advances in biology.


Further Reading

King, Lester F. The Philosophy of Medicine: The Early 18th Century. Cambridge, MA: Harvard University Press, 1978.

Koyre, Alexandre. The Astronomical Revolution: Copernicus-Kepler-Borelli. New York: Dover, 1992.

Lindeman, Mary. Health and Healing in Eighteenth Century Germany. Baltimore: Johns Hopkins University Press, 1996.

Porter, Roy. The Greatest Benefit to Mankind A Medical History of Humanity. New York: W. W. Norton, 1997.

Porter, Roy, ed. Medicine in the Enlightenment. Amsterdam: Rodolfe, 1995.

Spallane, J. The Doctrine of Nerves. London: Oxford Press, 1981.

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