NERVE TRUNKS 



2IQ 



numerous where the physiological activity is supposed to be 

 most active. 



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 contin- 

 uity and other considerations equally 

 conclusive. It is demonstrated under 

 the microscope with difficulty in fresh 

 specimens. 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 unbranch- 

 ed, but itself sends off "collaterals" 

 in the gray substance. These collat- 

 erals, however, do not actually join 

 any other nerve cells or fiber. 



The average diameter of medullated 

 fiber is about ^ooo in., though all are 

 said not to preserve the same diam- 

 eter throughout their course. 



(B) The non-medullated fibers 

 (fibers of Remak) seem to be simple 

 axis cylinders without the other atom- 

 ical elements peculiar to medullated 

 fibers. They make up a large part of 

 the trunks and branches of the sym- 

 pathetic system, and represent the fil- 

 aments of origin and distribution of FIG. 63. Non-medullated 

 all nerves. They are thought by some nerve fiber - 



to possess a neurilemma. They are u> V nucfeu S f ; d ^ 

 pale gray in color. surrounding it. 



Nerve Trunks. The above remarks apply to a single 



