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.