THE SENSATION OF SIGHT. 231 



of our nerves, but only the changed condition of the 

 nervous fibres which we call the state of excitation or 

 functional activity. 



Now all the nerves of the body, so far as we at present 

 know, have the same structure, and the change which we 

 call excitation is in each of them a process of precisely 

 the same kind, whatever be the function it subserves. For 

 while the task of some nerves is that already mentioned, 

 of carrying sensitive impressions from the external organs 

 to the brain, others convey voluntary impulses in the 

 opposite direction, from the brain to the muscles, caus- 

 ing them to contract, and so moving the limbs. Other 

 nerves, again, carry an impression from the brain to 

 certain glands, and call forth their secretion, or to the 

 heart and to the blood-vessels, and regulate the circula- 

 tion. But the fibres of all these nerves are the same 

 clear, cylindrical threads of microscopic minuteness, con- 

 taining the same oily and albuminous material. It is 

 true that there is a difference in the diameter of the 

 fibres, but this, so far as we know, depends only upon 

 minor causes, such as the necessity of a certain strength 

 and of getting room for a certain number of independent 

 conducting fibres. It appears to have no relation to their 

 peculiarities of function. 



Moreover, all nerves have the same electro-motor 

 actions, as the researches of Du Bois Reymond ^ prove. 

 In all of them the condition of excitation is called forth 

 by the same mechanical, electrical, chemical, or thermo- 

 metric changes. It is propagated with the same rapidity, 

 of about one hundred feet in the second, to each end of 

 the fibres, and produces the same changes in their electro- 

 motor properties. Lastly, all nerves die when sub- 

 mitted to like conditions, and, with a slight apparent dif- 



* Professor of Physiology in the University of Berlin. 



