76 Can Science Explain Life? 



products which take no active part in the genera- 

 tion of impulses but which contain the delicate 

 fibril-endings hidden within them. 



Whenever a nerve-fiber traverses a consider- 

 able distance it is covered by a protecting neuri- 

 lemma, and in some cases also by an inner medul- 

 lary sheath which prevents the diffusion of ions 

 through the side-walls of the axis-cylinder. Near 

 the end-organs these investments terminate ab- 

 ruptly or leave the axis-cylinder, which imme- 

 diately arborizes into a number of delicate ter- 

 minals surrounded by irregular accumulations of 

 coagulation-products, from which it may be as- 

 sumed that vigorous metabolism is taking place in 

 these regions. 



The transmission of nerve-impulses is peculiar 

 in that it always takes place in only one direction, 

 although the electrical conductivity of nerve-fibers 

 is the same in both directions. It appears, there- 

 fore, that the same end-organs which generate an 

 impulse must also determine the direction thereof 

 and that the direction is always the same for the 

 same end-organ, unless we make the highly im- 

 probable assumption that the end-organs generate 

 alternating impulses which are rectified while 

 passing through the synapses. Although it is 

 quite possible that the synapses may possess only 

 unidirectional conductivity, yet without a differ- 



