JOHANNES MULLER 517 



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 under- 

 stand how Miiller was led to adopt the view of subjective idealism. 



During this period at Bonn, however, the duties which, as a 

 teacher, Miiller imposed upon himself, together with the unremitting 

 employment in the lines of his original investigations with all its con- 

 comitant 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 univer- 

 sity and with this a recompense of two hundred thalers which made 

 possible for him a journey up the Ehine and through southern Ger- 

 many. 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 Miiller's fiery 

 subjective period, and the beginning of his great objective physiologico- 

 anatomical period, which covered the years of his most brilliant achieve- 

 ment. 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 him- 

 self worthy to be ranked among the first zoologists of his time. In 

 his work, " On the Development of the Beproductive Organs," which 

 appeared a few years later, Miiller 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 phylo- 

 genetic and ontogenetic development is considered in both man and 

 the lower animals. 



In the latter part of Miiller's life at Bonn occurred two significant 

 physiological discoveries: First, he definitely proved, through a con- 

 vincing 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 Miiller 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 Betziiis. Hardly a year later, Miiller announced his discovery of 



