Ivan Pavlov
In the 1890s, Pavlov was investigating the gastric function of dogs by externalizing a salivary gland so he could collect, measure, and analyze the saliva and what response it had to food under different conditions. He noticed that the dogs tended to salivate before food was actually delivered to their mouths, and set out to investigate this "psychic secretion", as he called it. Pavlov performed and directed experiments on digestion, eventually publishing The work of the digestive glands in 1897, after 12 years of research. His experiments earned him the 1904 Nobel Prize in Physiology and Medicine.