NERVE TRUNKS 



219 



The axis cylinder is composed of a large number of primitive 

 fibrillae. This band occupies about one- 

 fourth the diameter of the tube and is 

 the true conducting element, as is shown 

 by its invariable presence, its continuity 

 and other considerations equally con- 

 clusive. It is demonstrated under the 

 microscope with difficulty in fresh speci- 

 mens. It is directly connected with a 

 nerve cell, and is the essential part of 

 the fiber. The process of the cell which 

 becomes the axis cylinder is not, as was 

 once thought unbranched, but itself 

 sends off " collaterals " in the gray sub- 

 stance. These collaterals, however, do 

 not actually join any other nerve cells 

 or fiber. 



The average diameter of medullated 

 fiber is about -pfc-y in., though all are 

 said not to preserve the same diameter 

 throughout their course. 



(B) The non-medullated fibers (fib- 

 ers of Remak) seem to be simple axis 

 cylinders without the other atomical 

 elements peculiar to medullated fibers. 

 They make up a large part of the trunks 

 and branches of the sympathetic sys- 

 tem, and represent the filaments of 

 origin and distribution of all nerves. 

 They are thought by some to possess 

 a neurilemma. They are pale gray 

 in color. 



Nerve Trunks. The above remarks 

 apply to a single nerve fiber. These 



FIG. 63. Non-medullated 

 nerve fiber. 



Vagus of dog. b, fibrils; u, 

 nucleus; p, protoplasm sur- 

 rounding it. (Stirling.) 



fibers seldom run an 



