Until the middle of the last century very little had been done in the way of experimental study of physiology and pathology. Physicians depended almost entirely on bedside observations. Some of these physicians were wonderful men, and often their observations were remarkably shrewd. But the human body is a complex machine, the organs are so interdependent, that in the presence of any given set of symptoms and signs of disease, it was almost impossible to be sure just what caused them, and, consequently, what was best to do for the patient. When the experimental method was adopted disease could be observed systematically, conditions could be controlled, and the phenomena that resulted could be studied intelligently because the experimenter knew exactly what had produced them. In such experiments mammals are the animals chiefly used, because in most respects they most nearly resemble man, himself a mammal. Practically all the domestic mammals have been used, horses, cattle, sheep, goats, swine, dogs, cats, rabbits, guinea-pigs, and rats and mice; monkeys are also used. And all have made wonderful contributions to medicine or surgery or both. TYPES OF EXPERIMENTS ON ANIMALS I There are several classes of experiments. Some are in the field of pure research, not having for their object any immediate benefit to man or animals. Experiments of this nature were carried on some years ago in work on bubonic plague among rodents in California. It was discovered that ground squirrels have a disease similar to plague and yet distinctly different. By a long series of experiments it was found that monkeys are susceptible to this disease, and it was predicted that eventually cases would be found in man. As a result of this work a bacteriologist in Cincinnati was able to identify the disease in persons in his own vicinity. Another investigator found it among persons in Utah, and showed that it is carried from infected rabbits and ground squirrels by biting insects. It also was shown that the disease is widespread over the United States. With this knowledge of the means of transmission of the disease it is comparatively easy to prevent the infection of man. II Another class of experiments is carried on by surgeons to develop dexterity before they attempt operations on man. Such experiments are usually carried out on dogs. The animals are invariably under complete anesthesia and usually they are killed by added ether at the end of the experiment. [Illustration: _Does this dog look unhappy? Ten years ago Buster had an operation performed on the stomach; the results have been of aid in the study of digestion. Buster has not suffered thereby, and she has saved much suffering to others. She is receiving a visit from the author._]