NERVE-FIBRES AND NERVE-CELLS 119 



Johannes Miiller (1832), the results of which caused him 

 to draw wide conclusions. He pointed out that when a man 

 receives a blow on the eye, although the bruising of the 

 eyelids causes pain, the only message which the optic nerve 

 transmits is a report to the brain that stars have flashed their 

 light upon the retina ; that when the nerve of hearing is 

 irritated, the patient hears a noise; that when a nerve going 

 to a gland is stimulated, a flow of secretion follows ; or to a 

 muscle, contraction is induced, and so forth. The result of 

 stimulating a nerve is to produce an effect of the same kind 

 as that which the impulses ordinarily travelling along the 

 nerve produce. So far Miiller was right, but he generalised 

 his results in the law of the specific energy of nerves, in 

 which he wrongly attributed the specific effects to the 

 nerves and not, as we do now, to the organs of the brain 

 to which they deliver their messages. "A sensory nerve 

 is not merely a passive conductor, but each nerve from 

 an organ of special sense possesses certain inalienable forces 

 or qualities which the causes of sensations do but excite 

 and render apparent. Sensation is therefore the trans- 

 mission to consciousness, not of a quality or of a state of 

 external bodies, but of a quality or of a state of our nerves, 

 a state to which the external cause gives origin." "This 

 truth, which results from a simple and impartial analysis of 

 facts, not only leads us to recognise that the different sensory 

 nerves are animated by special forces independent of the 

 general difference which distinguishes them from motor 

 nerves, but also points out the means of setting physiology 

 free for ever from a host of errors concerning the alleged 

 possibility of replacing one nerve by another." 



With a view to testing the truth of this law of the specific 

 energy of nerves, many attempts were made to make nerves 

 oin other trunks than their own. When a nerve has been 

 divided, its cut ends, if they are placed in contact, join again. 

 If it be a motor nerve, the fibres on the central (cerebro- 

 spinal) side of the injury, which are the processes of cells in 



