It is not surprising that a young man, engaged too exclusively at this sort of investigation, should have impaired his nervous equilibrium to some degree, and made symptoms, otherwise unimportant, appear to him as the index of serious illness. For a time Müller despaired of ever being himself again. When he had regained his health, however, he realized what had been the essential cause of his nervous condition; and so he never went back to his introspective observations, considering their results somewhat in the nature of a series of illusions. After this, Müller devoted himself for ten years strictly to his physiological investigations. The best knowledge of what Müller accomplished for scientific medicine, during these early years, can be obtained from Virchow's summation of the discoveries of this period made shortly after his great teacher's death. Virchow says: "It was Müller who introduced to the knowledge of physiologists and physicians the doctrine of reflex actions, which had been already indicated by Prochaska, and simultaneously discovered by Marshall Hall and himself. Just before this Müller succeeded in showing an {233} easy mode of performing experiments on the anterior and posterior roots of the spinal nerve in corroboration of Bell's teaching of their diverse functions. Thus he had the privilege of establishing for all time two of the greatest practical discoveries of the physiology of the nervous system. "Next to the nerves the blood became the subject of his researches and he not only naturalized in German medicine the accurate knowledge of the fibrin and blood-corpuscles, which Hewson had cultivated with such fertility in English literature, but he also managed by simple experiment to demonstrate the peculiar composition of the vital fluid. The discernment of right methods of investigation lay ever open to his clear and cultivated intellect, and he knew well that there were cases in which the scalpel and experiments could not determine a question, and where the truth was only to be elicited by means of chemical agents and physical instruments. It was thus he discovered the peculiar gelatinous substance found in cartilage, called chondrin; thus he proved the existence of lymphatic hearts in the amphibia, and thus that he determined not only the organs but all the laws which are concerned in the production of the human voice.