and exposition of the subject. In your medical studies you have been accustomed to use your eyes. You see the anatomical specimen, the precipitate of the chemical reaction, the contraction of the muscle as the result of the stimulation of its nerves. Later, you come into contact with the patients; you learn the symptoms of disease by the evidence of your senses; the results of pathological processes can be demonstrated to you, and in many cases even the exciting cause of them in an isolated form.