"I considered any attempt to save his life entirely useless," Beaumont wrote. However, not only did St. Martin survive the shotgun wound, he and Beaumont ended up conducting a series of medical experiments that made Beaumont's reputation as a pioneering physiologist.

Before then, two ideas had dominated debates over the stomach's role in human digestion. One theory pictured the stomach as a kind of fermenting vat, a cauldron of chemicals stoked by heat. A second theory suggested that digestion occurred mechanically 3/4 by grinding. Neither school had produced direct evidence to prove its claims, though not for lack of effort.

To test their beliefs, proponents of the chemical view experimented on buzzards, sheep, dogs, cats and even themselves. One Italian researcher, Lazzaro Spallanzani, fished for his own gastric juice by swallowing sponges tied to strings. Spallanzani was able to demonstrate that gastric juice would dissolve food outside his stomach. The chance to see if Spallanzani's findings held true within the stomach fell to Beaumont, an ambitious country doctor serving as a U.S. Army post surgeon on a remote island bustling with fur trade.

According to Reginald Horsman in Frontier Doctor: William Beaumont, America's First Great Medical Scientist, St. Martin's external wound healed. But despite repeated attempts to close it, a hole remained in his stomach wall. The experiments began at noon on August 1, 1825. Beaumont baited a string with food and lowered it into St. Martin's stomach cavity. Lunch for St. Martin that day consisted of seasoned a la mode beef, raw salted lean beef, raw salted fat pork, raw lean fresh beef, boiled corn beef, stale bread and raw cabbage. Beaumont removed the food at one, two and three o'clock to observe the rate of digestion.

The experiment ended at five o'clock. St. Martin felt weak, his stomach and head ached; the next day, he suffered indigestion. On August 7, a week after his first try, Beaumont siphoned an ounce of gastric juice from St. Martin's stomach. He placed the juice in a vial with a piece of corned beef as large as his pinky, corked the top, submerged the vial in a saucepan of water, heated it to 100 F and parked the vial in a sand bath to keep its temperature constant. At the same time, he slid a similar slab of corned beef into St. Martin's stomach, regularly removing it for observation. The experiment, repeated several times with different foods, provided the first physical evidence that simulated, test tube digestion closely resembled internal digestion, the only difference being that it was slower and less complete. Beaumont's experiments provided the first physical evidence that simulated, test tube digestion closely resembles internal digestion and did more to shed light on the role of gastric juice in human digestion than the years of theorizing that preceded it.

In late 1825 Beaumont grudgingly suspended his experiments after St. Martin left Mackinac for Canada. Beaumont had hired the vulnerable, poverty-stricken trapper as a live-in handyman, an arrangement that provided St. Martin with an income and enabled Beaumont to experiment while closely monitoring his patient's condition. Understandably, St. Martin wished to resume his normal existence.

The experiments tested St. Martin's patience. Frequently, Beaumont removed food from the trapper's stomach during digestion to observe changes. St. Martin's suffering underwrote Beaumont's progress. The doctor's experiments at Prairie Du Chien helped confirm that digestion was a chemical process, in which gastric juice acted as a solvent.

Four years and many experiments later, Beaumont published the results of his 283 tests on St. Martin in The Physiology of Digestion with Experiments on the Gastric Juice, published in 1833. In it, Beaumont concluded that digestion is a chemical process and that gastric juice acts as a solvent.

His work won wide acceptance. Generally ignorant of the debates raging in Europe over the physiology of human digestion and aligned with no school or ideology, Beaumont was viewed as an objective observer and an honest reporter. His book - self-published and dedicated to his mentor, Surgeon General Lovell - eventually sold more than 3,000 copies in its first edition. It became a trusted source for medical students and opened new avenues of research.

Beaumont's peripatetic military career kept him moving. From 1825 through 1828 he served at Fort Niagara, Fort Howard and Fort Crawford. To continue his digestive experiments on St. Martin, Beaumont signed a contract with his subject and paid him an advance. But St. Martin disappeared into Canada, a foreign jurisdiction. Beaumont, writing Lovell, expressed the belief that St. Martin would eventually spend his advance and be "willing to recant his villainous obstinacy and ugliness, and then I shall be able to regain possession of him again, I have no doubt."

Meanwhile, Beaumont also attempted to persuade Congress to reimburse him the cost of his past experiments on St. Martin and to finance his future research. His proposal was shelved. In 1834, the army assigned Beaumont to Jefferson Barracks, ten miles from St. Louis, and then granted Beaumont's request to be stationed at the arsenal in St. Louis, which allowed him to establish a private practice. St. Louis was growing rapidly (its population doubled then quadrupled in the 1830s and 1840s) kept Beaumont so busy in his lucrative private practice that he attempted to reject an offer in 1836 to become chair of surgery at St. Louis University's medical department. Though Beaumont made some desultory attempts to bring St. Martin to St. Louis to renew his experiments, his plans fell through.

Beaumont's thriving St. Louis practice was interrupted by ugly controversies. In 1840, Beaumont's medical treatment of Andrew Jackson Davis, owner of the Missouri Argus, brought him grief. Davis, was attacked and caned on the head by William P. Darnes, a prominent local politician who had received unflattering coverage in the Argus. The assault fractured Davis's skull. A week after Beaumont operated, the patient died and Darnes was charged with third-degree manslaughter.

In Darnes's defense, his lawyer argued that it was not his client's caning but Beaumont's operation that had caused Davis's death. The defense called as expert witnesses several St. Louis physicians who criticized Beaumont's medical judgment, and the jury convicted Darnes on a lesser charge-fourth-degree manslaughter. Soon after the trial, Beaumont was elected president of the St. Louis Medical Society. After his term expired, he took no further part in the society's activities.

Embittered and increasingly deaf, Beaumont withdrew into the company of family and friends. He bought a big white frame house on 40 acres of land about a mile from the center of St. Louis, now bounded by Jefferson Avenue and Beaumont Street. After falling and hitting his head on ice-covered steps in March 1853, he died the following month from complications and was buried in Bellefontaine Cemetery in St. Louis.