200 THE VIBRATORY THEORY OF NERVE ACTION. 



itself. If we examine the axis cylinder, say, of the 

 sciatic nerve of a frog, what do we find ? — a firm, tough, 

 fibrona-like, flattened band, not easily torn, and evi- 

 dently consisting of a tissue of slow growth ; — in fact, 

 the very last characters we should expect to meet with 

 in a tissue prone to rapid chemical cbange. IS'either is 

 a structure surrounded by ten times its thickness of 

 oily matter (myelin) favourably situated for taking 

 U]_) new materials and quickly getting rid of products 

 of decay. One of the least permeable substances in 

 the body is the myelin of the nerve fibre, and yet 

 through this must pass all the materials from the 

 blood to renovate the disintegrated axis cylinder, if 

 nerve action is due to chemical change in the nerve 

 fibre itself. 



249. The vibratory theory of nerve action. — 

 Again, some think that nerve action depends upon 

 the molecules of nerve fibres being thrown into 

 vibration, and the axis cylinder of a nerve has been 

 spoken of as if it had been proved to consist of a 

 chain or chains of very minute spherical particles. 

 But the axis cylinder is not composed of matter 

 which would more readily propagate motor impulses 

 than the matter of ordinary fibrous tissue. Moreover, 

 it varies much in character and in thickness in 

 different parts of its course. The impulses supposed 

 would be much modified during their transmission. 

 This theory leaves one of the most important facts 

 connected with nerve fibres unexplained ; for upon 

 such a supposition what purpose could be served by 

 the very thick layer of the white substajice of 

 Schwann, and in that part of the nerve only which 

 lies between its central and peripheral distribution ? 

 Why do we find, moreover, that this investment is 

 invariably so much thicker where a number of nerves 

 run for a long distance parallel to one another than 

 where they cross one another at considerable angles ? 



250, Experimental investigation inconclusive. — 



