To the period of Haller also belongs the discovery of oxygen, in 1774, by Priestley, a discovery which was destined to have profound influence upon the subsequent development of physiology, so that even now physiology consists largely in tracing the way in which oxygen enters the body, the manner in which it is distributed to the tissues, and the various phases of vital activity that it brings about within the living tissues. Charles Bell.--The period of Haller may be considered as extending beyond his lifetime and as terminating when the influence of Müller began to be felt. Another discovery coming in the closing years of Haller's period marks a capital advance in physiology. I refer to the discovery of Charles Bell (1774-1842) showing that the nerve fibers of the anterior roots of the spinal cord belong to the motor type, while those of the posterior roots belong to the sensory type. This great truth was arrived at theoretically, rather than as the result of experimental demonstration. It was first expounded by Bell in 1811 in a small essay entitled _Idea of a New Anatomy of the Brain_, which was printed for private distribution. It was expanded in his papers, beginning in 1821, and published in the Philosophical Transactions of the Royal Society of London, and finally embodied in his work on the nervous system, published in 1830. At this latter date Johannes Müller had reached the age of twenty-nine, and had already entered upon his career as the leading physiologist of Germany. What Bell had divined he demonstrated by experiments. Charles Bell (Fig. 55) was a surgeon of eminence; in private life he was distinguished by "unpretending amenity, and simplicity of manners and deportment." [Illustration: Fig. 55.--Charles Bell, 1774-1842.]