THE RISE OF PHYSIOLOGY 183 
His chief service of permanent value was that he brought 
into one work all the facts and the chief theories of physiology 
carefully arranged and digested. This, as has been said, 
made physiology an independent branch of science, to be 
pursued for itself and not merely as an adjunct to the study 
of medicine. The work referred to is his Elements of Physi- 
ology (Elementa Physiclogie Corporis Humani, 1758), one 
of the noteworthy books marking a distinct epoch in the 
progress of science. 
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 physiolo y consists largely 
in tracing the way in which oxygen enters the body, the 
manner in which it is distributed to the tissues, and the vari- 
ous 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 Miiller began to be felt. Another discovery com- 
ing 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 fbers 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 ex- 
pounded 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 Miiller had reached the age of twenty- 
