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 Physiologic 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 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 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 Muller 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 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 cf experimental demonstration. It was first ex- 

 pounded by Bell in 1811 in a small essay entitled Idea 0} 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 Muller had reached the age of twenty- 



