Electedby the Assembly of Professors meeting in November 1855, Claude Bernard gave his first lecture at the Collège de France on Friday February 29 1856, and continued teaching every Wednesday and Friday of the 1856 academic year on the theme of the effects of toxic and medicinal substances. In 1858, he devoted his lectures to the physiological properties and pathological alterations of the body's various fluids, then, in 1859, to recent discoveries in physiology and their application to medicine, while from 1863 onwards, he dealt with experimental pathology and then experimental medicine.

The title of his courses perfectly sums up his work, as he experimented with the effects of curare using poisoned arrowheads he had obtained from South America, as he said : " physiology, that's me " and as the founder of the experimental method, in opposition to dogmatism and false science.

Fiercely opposed to experiments on humans, and in particular to drug tests on patients and surgical operations with random results, but convinced of the need to test on the living, Claude Bernard regularly resorted to animal vivisection, at a time when modeling and stem cell reproduction did not exist.

As early as 1850, he established the physiological basis of anesthesia, applying the principles of his experimental method, and gave his Leçons sur les anesthésiques et l'asphyxie at the Collège de France from 1871 to 1875.

Claude Bernard, a forerunner in the application of exact scientific measures to the living world, was also responsible for fundamental discoveries concerning glycemia. He studied the role of sugar in the animal and human organism, discovered the control of glycemia by the central nervous system and proved the presence of sugar in cerebrospinal fluid. With the so-called liver wash experiment, he demonstrated the role of this organ in the storage and diffusion of sugar in the blood to maintain a constant glycemic level.

Far removed from the man-machine analysis to which some of his contemporaries confined themselves, Claude Bernard conceptualized the relationships between the body's organs and created the notion of an internal milieu, whose stability depends on regulation by internal secretions. According to his analysis, " the fixity of the internal environment is the condition for a free and independent life : the mechanism that allows this is the one that ensures the maintenance in the internal environment of all the conditions necessary for the life of the elements ".

An excellent pedagogue, he conceived his courses as an introduction to the scientific discoveries of the day, and regularly published the results in magazines or didactic works that attracted a wide readership. Reading his Introduction à l'étude de la médecine expérimentale inspired ÉmileZola to write Le Roman expérimental, a manifesto of his naturalist doctrine, in 1880. Several of Bernard's works were translated into English during his lifetime.

Claude Bernard held the chair of medicine until his death in 1878, assisted by Auguste Tripier, Paul Bert and, from 1873, Arsène d'Arsonval. His work had a major impact on generations of physicians and biologists, and the concept of the internal environment is still taught today.