712 ADDRESS TO THE DEPARTMENT OF ANATOMY 



Local differentiations of the nervous network, which was no 

 doubt distributed over the whole body, took place on the forma- 

 tion of organs of special sense, and such differentiations gave 

 rise to the formation of a central nervous system. The central 

 nervous system was at first continuous with the epidermis, but 

 became separated from it and travelled inwards. Ganglion-cells 

 took their origin from sensory epithelial cells, provided with 

 prolongations, continuous with the nervous network. Such 

 epithelial cells gradually lost their epithelial character, and finally 

 became completely detached from the epidermis. 



Nerves, such as we find them in the higher types, originated 

 from special differentiations of the nervous network, radiating 

 from the parts of the central nervous system. 



Such, briefly, is the present state of our knowledge as to the 

 genesis of the nervous system. I ought not, however, to leave 

 this subject without saying a few words as to the hypothetical 

 views which the distinguished evolutionist Mr Herbert Spencer 

 has put forward on this subject in his work on Psychology. 



For Herbert Spencer nerves have originated, not as pro- 

 cesses of epithelial cells, but from the passage of motion along 

 the lines of least resistance. The nerves would seem, according 

 to this view, to have been formed in any tissue from the con- 

 tinuous passage of nervous impulses through it. "A wave of 

 molecular disturbance," he says, " passing along a tract of 

 mingled colloids closely allied in composition, and isomerically 

 transforming the molecules of one of them, will be apt at the 

 same time to form some new molecules of the same type," and 

 thus a nerve becomes established. 



A nervous centre is formed, according to Herbert Spencer, at 

 the point in the colloid in which nerves are generated, where 

 a single nervous wave breaks up, and its parts diverge along 

 various lines of least resistance. At such points some of the 

 nerve-colloid will remain in an amorphous state, and as the wave 

 of molecular motion will there be checked, it will tend to cause 

 decompositions amongst the unarranged molecules. The de- 

 compositions must, he says, cause " additional molecular motion 

 to be disengaged ; so that along the outgoing lines there will be 

 discharged an augmented wave. Thus there will arise at this 

 point something having the character of a ganglion corpuscle." 



