Human Digestion Studied by William Beaumont, Theodor Schwann, Claude Bernard, and William Prout
Human Digestion Studied by William Beaumont, Theodor Schwann, Claude Bernard, and William Prout
At the beginning of the 1800s the process of digestion was a mystery wrapped in conjecture and debate. During the nineteenth century, however, four men contributed important pieces toward solving the puzzle of digestion. American William Beaumont (1785-1853) first observed the workings of a living person's stomach in a patient with a gunshot wound that did not heal. Englishman William Prout (1785-1850) showed that hydrochloric acid was in digestive juice. German physiologist Theodor Schwann (1810-1882) discovered pepsin, the enzyme responsible for digestion in the stomach. French investigator Claude Bernard (1813-1878) uncovered the roles of the pancreas and liver in digestion and showed that the major organ of digestion was not the stomach but the small intestine. By the end of the century the work of these four had established that digestion does not happen in the stomach alone but is a complex process beginning with saliva in the mouth and involving the entire digestive tract.
The nineteenth century began with controversy about the nature of body physiology and the meaning and nature of life and disease. Many physiologists believed in vitalism, a doctrine based on the view that a "vital" or spiritual force causes life, prompting them to argue that a process like digestion could not be described in chemical or mechanical terms. Vitalism was in vogue during this time and would remain popular for several decades.
Interest in digestion had roots in classical Greece. Explanations of digestion included stomach heat, putrefaction, grinding, and fermentation. The seventeenth and eighteenth centuries spawned a great debate between those who believed it was a chemical process and those who insisted it was a grinding, mechanical process.
Seventeenth-century researcher Jan Baptiste van Helmont (1579-1644) had proposed that chemical action digested food by fermentation. Frenchman Rene de Reaumur (1683-1757) and Italian Lazzaro Spallanzani (1720-1799) experimented not only on animals and birds but also on themselves and argued that digestion was chemical. But the vitalists ridiculed their ideas, contending that in no way could human processes be described in such unspiritual terms.
At the turn of the nineteenth century European researchers were very interested in digestion and bitter controversy raged, especially in France. A well-known textbook on physiology by French professor Francois Magendie (1783-1855) argued that digestive juice was not a solvent and that any presence of acid in the stomach was caused by the breaking down of food or by saliva. The argument was so intense during the 1820s that the French Academy of Science sponsored a contest on the process of digestion in animals. At this time American medicine lagged behind and, consequently, no major American researchers contributed to this debate in Europe.
Such was the background in the early years of the nineteenth century. The vitalists were in command. Controversy over chemical versus mechanical explanations was burning in Europe. Monistic theories of disease—the idea that all diseases have one cause and should be treated by bleeding and purging—were the major influences on the practice of medicine. Doctors had little formal training and became licensed by serving an apprenticeship with another doctor. Into this atmosphere came the four men whose research challenged these beliefs and set the stage for our understanding of the digestive system today.
On June 6, 1822, William Beaumont, a frontier army doctor, was called to a fur company store in Mackinac Island, Michigan Territory, to treat a Canadian trapper who had been shot at close range. Little did Beaumont realize that the call would give him the opportunity to change the course of knowledge about digestion, as well as lead to the development of experimental medicine.
The shot had created a wound the size of a man's hand in the abdominal area and, in spite of great effort to close it, left a hole in the stomach, called a gastric fistula. Beaumont thought surely his patient, Alexis St. Martin, would die, but within the year he was recovering and in good health—with the open hole still in his stomach. Beaumont could look directly into his stomach and observe its motion. He could pour food and drink in and siphon out the contents. Beaumont hired St. Martin to work for him so that he could continue his experiments.
In England William Prout, a physician turned chemist, investigated the gastric juices of animals with brilliant experiments. In 1824 he extracted stomach juices and demonstrated it contained hydrochloric acid. When he published his work, his contemporaries could hardly believe that such a strong acid could exist in the stomach of organisms and not cause harm. However, such was Prout's credibility that in 1827 they accepted his research into the digestion of food nutrients.
Prout divided food into categories: water, carbohydrates, fats, and proteins. While many of his ideas were based on speculation, he did publish an analysis of the saccharinous or carbohydrate class. Nutritionists today still use his classifications.
Beaumont, in America, had no knowledge of the controversy and interest in digestion in Europe. However, he did have access to a living human stomach and could go beyond what they could do. Although he had no research experience or training, Beaumont was a careful observer, wrote in his journals, and proceeded in an orderly manner.
On August 1, 1825, he began his controlled experiments by suspending cooked beef, salted beef, salt pork, raw beef, corned beef, stale bread, and cabbage in the stomach on silk strings and closing the hole with a bandage. St. Martin continued his work around the house. At one, two, and three o'clock Beaumont observed the way each item was digesting and carefully recorded his observations. When St. Martin complained of sickness, Beaumont observed a number of white spots in the stomach. He later realized he was looking face to face at indigestion. As might be expected, St. Martin tired of his role as a human guinea pig and needed some persuasion to continue.
In 1833 Beaumont published his findings in the book The Experiments and Observations of the Gastric Juice and Physiology of Digestion. After performing some 200 experiments over a period of 10 years, Beaumont listed 51 conclusions regarding the chemical nature of digestion. Primary among these was that the stomach secreted gastric juice from folds in the lining and that gastric juice was the agent of chemical breakdown. He described the inner coat of the stomach as pale pink covered with a mucous coat that changes appearance when diseased. The stomach moves sideways and up and down to churn its contents. He also described how alcohol causes gastritis or inflammation to the stomach lining. He found that vegetables were less digestible than other foods and that milk coagulates early in digestion.
Beaumont had revealed more about the stomach than had been known before. He earned recognition both in Europe and the United States. He also noted an important factor that he did not quite understand. An unknown substance was present that researchers in the United States could not identify because of the lack of organic chemistry analysis. That discovery would be the next piece of the puzzle.
German physiologist Theodor Schwann, working with the famous physiologist Johannes Mueller (1801-1858) in Berlin, became very interested in investigating digestive processes. He isolated a substance from the stomach that was separate from hydrochloric acid and called that substance pepsin. This turned out to be Beaumont's unknown factor. Pepsin, the first enzyme to be prepared from animal tissue, works with the hydrochloric acid to break down protein.
Schwann later discovered the muscular nature of the esophagus, noting that it contained striated muscle and acted as a pipe to move food from the mouth to the stomach. He also was the first to use the term metabolism to describe chemical changes in living tissue and applied the idea of cell theory to animals.
Claude Bernard, a French physiologist, developed an early interest in digestion while working as an assistant to François Magendie. Fascinated by Beaumont's research, Bernard replicated the gastric fistulas in animals. His wife and daughter, along with other antivivisectionists, strongly opposed his experiments on live animals .
One day Bernard noted that laboratory rabbits were passing clear urine like meat-eating animals. He assumed the animals had not been fed and were digesting their own tissues. He fed meat to the animals and studied the pancreas in autopsy. He found that pancreatic juice breaks down fat molecules into fatty acids and glycerol. While most of the research previously assumed all digestion took place the stomach, he showed the small intestine to be the major organ of digestion. Later, he found the nerves that control the digestive process.
His work on the pancreas led to a second great discovery—the role of the liver in digestion. He isolated glycogen, a white starchy substance, and determined the complex substance was made by the liver, stored as a reserve of carbohydrates, then released to keep a constant blood sugar level. In 1865 he wrote a textbook called An Introduction of the Study of Experimental Medicine, in which he argued that the precepts of vitalism do not explain life and urged that all medicine be based on methodical and experimental processes.
The work of these four men laid the foundation for the understanding of digestion and the treatment of its many complex diseases. By the end of the century, vitalism was waning, living systems were explained by physical and chemical processes, and experimental medicine using the scientific method was advancing.
EVELYN B. KELLY
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