204 RECENT PROGRESS OF THE THEORY OF VISION. 



What, therefore, we directly apprehend is not the immediate 

 action of the external exciting cause upon the ends 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 sen- 

 sitive impressions from the external organs to the brain, others 

 convey voluntary impulses in the opposite direction, from the 

 brain to the muscles, causing 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 circulation. 

 But the fibres of all these nerves are the same clear cylindrical 

 threads of microscopic minuteness, containing 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 con- 

 ducting fibres. It appears to have no relation to their pe- 

 culiarities of function. 



Moreover, all nerves have the same electro-motor actions, as 

 the researches of Du Bois Reymond l prove. In all of them the 

 condition of excitation is called forth by the same mechanical, 

 electrical, chemical, or thermometric 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 

 submitted to like conditions, and, with a slight apparent differ- 

 ence according to their thickness, undergo the same coagulation 

 of their contents. In short, all that we can ascertain of ner- 

 vous structure and function, apart from the action of the other 

 organs with which they are united and in which during life we 

 see the proofs of their activity, is precisely the same for all the 

 1 Professor of Physiology in the University of Berlin. 



