Claude Bernard (1813-1878) performing vivisection in his laboratory at the College de France, after a painting by L'Hermitte of 1899. Bernard was the founder of the principle of the internal environment, the ability of animals to regulate their internal temperature, acidity and osmotic pressure. This concept was later expanded by Cannon as 'homeostasis'. Bernard also described the liver's function in regulating blood sugar levels, and the preferential affinity of haemaglobin to oxygen and carbon monoxide. The figure fourth from left is Paul Bert (1833-1886) who first documented respiratory adaptation to altitude in humans.