Russian physiologist and experimental psychologist Ivan Petrovic Pavlov (1849-1936, second from right) operating on a dog with three colleagues. Pavlov received the 1904 Nobel Prize for his work on the physiology of the digestive glands. He is best remembered for his work on conditioned reflexes, in which he conditioned dogs to salivate in anticipation of food by ringing a bell each meal time (Pavlovs dog). Eventually, the bell alone provoked salivation. These experiments became the foundation of behaviourist psychology. He died of double pneumonia at the age of 86. Photographed in the Physiology Department, Imperial Institute of Experimental Medicine, St Petersburg, Russia, circa 1902.