History
What has recently been considered its own subdiscipline?
Physiology
The study of human physiology dates back to at least 420  BC to the time of Hippocrates, also known as the father of medicine. The critical thinking of Aristotle and his emphasis on the relationship between structure and function marked the beginning of physiology in Ancient Greece, while Claudius Galenus (c. 126–199 AD), known as Galen, was the first to use experiments to probe the functions of the body. Galen was the founder of experimental physiology. Jean Fernel, a French physician, introduced the term "physiology" in 1525. In the 19th century, physiological knowledge began to accumulate at a rapid rate, in particular with the 1838 appearance of the Cell theory of Matthias Schleiden and Theodor Schwann. It radically stated that organisms are made up of units called cells. Claude Bernard's (1813–1878) further discoveries ultimately led to his concept of "milieu interieur" (internal environment), which would later be taken up and championed as "homeostasis" by American physiologist Walter Cannon. In the 20th century, biologists also became interested in how organisms other than human beings function, eventually spawning the fields of comparative physiology and ecophysiology. Major figures in these fields include Knut Schmidt-Nielsen and George Bartholomew. Most recently, evolutionary physiology has become a distinct subdiscipline.