understand truly the things of the world outside of ourselves, but are cognizant only of the changes brought about in the sense-substance by the thing itself. From these considerations we can readily understand how Müller was led to adopt the view of subjective idealism.
During this period at Bonn, however, the duties which, as a teacher, Müller imposed upon himself, together with the unremitting employment in the lines of his original investigations with all its concomitant labor and thought, had induced, soon after his marriage in 1827, a state of mental and physical exhaustion. Upon the eve of a nervous break-down he secured a leave of absence from the university and with this a recompense of two hundred thalers which made possible for him a journey up the Rhine and through southern Germany. On this trip he was accompanied by his newly married wife. Soon, however, with bettered health he returned to Bonn, where in 1830 he was made professor ordinary.
This event marks the end of what we may term Müller's fiery subjective period, and the beginning of his great objective physiologico-anatomical period, which covered the years of his most brilliant achievement. He was now devoting himself to many branches of scientific work, especially to his morphological studies. Through his anatomical and systematic researches on the scorpion and spiders, he showed himself worthy to be ranked among the first zoologists of his time. In his work, "On the Development of the Reproductive Organs," which appeared a few years later, Müller traced the development of these organs in man and in animals. Coincident with this he was pursuing his researches into the development of other organs, and produced his treatise on the secreting glands. In this excellent work the phylogenetic and ontogenetic development is considered in both man and the lower animals.
In the latter part of Müller's life at Bonn occurred two significant physiological discoveries: First, he definitely proved, through a convincing series of experiments on the frog, the view which had been first announced by the Englishman, Charles Bell, in 1811: that the anterior roots of the spinal cord are motor, and that the posterior roots are sensory in function. In reality this experiment was simple enough. In a frog Müller cut on one side the anterior and on the other the posterior nerve roots of the spinal cord. On the side on which the posterior roots were cut the frog was wholly insensible, while the side on which the anterior roots were cut remained quite paralyzed. This experiment awakened in the scientific world of that time a storm of applause. The fortunate experimenter journeyed to Paris in order to demonstrate the fact before Alexander von Humboldt and Cuvier. Versalius in Stockholm had the experiment performed by Retziüs. Hardly a year later, Müller announced his discovery of