At Müller's departure from Berlin he was presented by Rudolphi with an English microscope, as a testimonial of the old professor's appreciation of the young man's labors while under his observation. As Müller's pecuniary resources were very limited, this must have been an especially acceptable gift, since it enabled him to continue his researches in embryology, and it was not long before these began to bear fruit. At Bonn, to which Müller returned, he set up as a Privat-Docent in the University, and for several years eked out by teaching the allowance his mother could give him, and even by the practice of medicine.
Bonn, at this time, had a population of perhaps 30,000, [{231}] and had some eighteen regular practitioners of medicine. It is easy to understand, then, that Müller's practice did not add materially to his pecuniary resources. It was not long before he gave up the practice of medicine entirely, led to the step by the sad death of a friend, who, while under his care, suffered from perforation of the intestines, followed by peritonitis. Notwithstanding the rather precarious state of his finances, at the age of twenty-six, Müller married Anna Zeiler, the daughter of a landholder in the Rhineland, not far from Bonn. He had previously dedicated to her a poem, in which he promised her, in lieu of more material advantages as a marriage settlement, an immortal name. The young man seems to have felt something of the genius that was in him, but, then, so have others, and their presages have not always been confirmed by the issue. Shortly before and after his marriage, he applied himself so hard to his investigations of many kinds that within a few months he broke down. The government allowed him a furlough, and for several months he wandered with his bride along the Rhine, in what has been described by a biographer as a "one-horse shay," and came back to his work renewed in mind and body.
As a matter of fact, Müller's breakdown was what would be called at the present time a neurasthenic attack, induced by overwork and too great introspection. He had been experimenting upon himself in many apparently harmless ways, but by methods which often cause serious trouble. It was not an unusual thing for him to fast, in order to note the physiological effect on his mind and senses of the absence of proper nutrition. He would often lie awake for hours at night in the darkness, experimenting upon himself and noting the phenomena induced, especially in his sight, by the total absence of light. He devoted himself, too, to the investigation of the curiosities of second sight; those interesting [{232}] reminders of things seen long ago, though without producing much impression, and which recur at unexpected moments, to make us think that we are seeing again when we are really only unconsciously remembering. He used to exercise a good deal the faculty of bringing up objects into his vision with all the physical peculiarities of actual sight. In this his master was Goethe, who had written extensively on this subject in treating of the phenomena of vision, and who was able himself to recall to his imagination with great vividness the many shades of colors of objects with the sensory satisfaction of actual vision. Müller had this imaginative power only for the reds.
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.
"The special researches of the Bonn epoch are those of the minute structure and anatomy of the glands. They put an end to the controversy which had existed so long between adherents of Malpighi and Ruysch, concerning the sacculated extremities of the glandular follicles, and obtained for us a correct knowledge of these important organs throughout the whole animal kingdom. Perhaps his most important work is that of the Ducts of Müller, the structures (named after him) which form so important a part of the genito-urinary system in the embryo."
Practically all this had been accomplished before he was [{234}] quite thirty-two years of age. In the autumn of 1832, Rudolphi, the professor of physiology at Berlin, died. As Virchow says, candidates sprung up on every side, and some who were the least qualified considered themselves best fitted for the position. Müller took an unusual step which illustrated his decision in character, though in any other it would have seemed an evidence of conceit. He declared, in an open letter, laid before the Minister of Prussia, that his claims were superior to those of any other living physiologist, except John Frederick Meckel. So powerful was the impression produced upon the minister by this letter that he immediately appointed Müller to the vacant chair.
Not long after his appointment to the chair of physiology at the University of Berlin Müller completed the well-known "Hand-book of Physiology," which established his reputation. The book is sometimes spoken of as an experimental physiology, but this is not correct. Müller was no more a mere experimentalist than Haller, and he, himself, heartily detested the tendency which experimental physiology had assumed in France, especially under the influence of Magendie. Part of Müller's aversion to experimental physiology was aesthetic. He could not bear the idea of inflicting so much pain as many of his colleagues inflicted without a thought. In his panegyric of Rudolphi, Müller says: "Rudolphi looked upon physiological experiments as having no relation to anatomical accuracy, and it is no wonder that this admirable man, who had at every opportunity expressed his abhorrence of vivisection, took up a hostile position against all hypotheses and conclusions insufficiently established upon physiological experiments." Müller adds: "We could not have failed to share his righteous indignation, had we seen how many physiologists were using every effort to reduce physiology to an experimental science by the live dissection and agonies [{235}] of innumerable animals, undertaken without any definite plan, and yielding often only insignificant and imperfect results."
Müller shared these views of Rudolphi with regard to vivisection. The uncertainty of the conclusions, the amount of suffering inflicted, and the indefiniteness of the conditions of experiment, so that the conclusions could not have any very great weight, or any special accuracy of information, made him consider such experiments, unless very carefully conducted by trained investigators, as largely a waste of time and infliction of unnecessary pain and a leading astray of physiological advance because of the uncertainty involved.