It is not a little surprising how many of the investigations that afterward were to give fame to Virchow were initiated {239} by his great teacher, Müller. It was Müller whose study of tumors led Virchow to devote himself to this subject and give us the best pathological work on it that has ever been written. Virchow himself notes with regret that Müller turned aside from pathology and never finished the promised work which was to have contained his theory of the origin of tumors. Another work in which Virchow followed in Müller's footsteps was the development of craniometry and, in general, the scientific investigations of skulls. Müller had interested himself very much in microcephalic skulls and Virchow assisted him in the investigations of them. Many years afterward Virchow established the science of craniology in the department of anthropology, and succeeded in throwing not a little light on the origins of races by his discoveries in this matter. After Schoenlien's discovery of the parasite of favus, Müller became interested in the parasitology of human beings, and with Retzius, the famous Swedish anatomist, investigated certain molds which occur in the respiratory passages of birds. They succeeded in demonstrating that these vegetable parasitic growths were a form of Aspergillus. Their studies in the white owl particularly called general attention to the possibility of such molds occurring as parasites of animals. Later on, Virchow showed that these same molds occur occasionally in the respiratory passages of men. Virchow found them in three bodies at autopsy, all of them being run down individuals, two of them old subjects, and all sufferers from chronic bronchitis. Usually, when the parasites were found, there was a distinct tendency to very low resistive vitality in the tissues, sometimes proceeding even to the extent of beginning pulmonary gangrene. In reviewing the subject Virchow [Footnote 7] said that the light thrown {240} upon it by the investigations of Müller and Retzius was of the greatest possible assistance in enabling him to identify the parasite when he found it in human subjects. [Footnote 7: Virchow's Archiv, Bd. ix.]