THE NERVOUS TISSUES. 



159 



mass, the fibrillae of which are no longer recognizable. Such pic- 

 tures, which formerly were supposed to represent the normal condi- 

 tion of the nerve-fibers, gave rise to the conception of an axis-cyl- 

 inder (vid. Technic). That which is known as an axis -cylinder is 

 therefore, in reality, the changed contents of the axial space. It may 

 be stated, however, that the term axis-cylinder is still much used, 

 since the methods commonly employed in the investigation of the 



nervous system do not 

 preserve the axial cord 

 in its integrity, but nearly 

 always result in the for- 

 mation of an axis-cylin- 

 der. Consequently, al- 

 though we shall make 

 use of the term, its limit- 

 ations are to be kept in 

 mind. 



Medullated nerve - 

 fibers vary greatly in di- 



Ranvier's 

 node. 



Fig. I2O. Medullated nerve-fibers from a rabbit, 

 varying in thickness and showing internodal segments 

 of different lengths. In the fiber at the left the neuri- 

 lemma has become slightly separated from the under- 

 lying structures in the region of the nucleus ; X I 4- 



- Nucleus. 



Fig. 121. Remak's fibers 

 (nonmedullated fibers) from the 

 pneumogastric nerve of a rabbit ; 

 X3 6 - 



ameter, but whether this points to a corresponding variation in 

 function has not been fully decided. Fine fibers possess a diameter 

 of 2 4/*, those of medium size 49 /^, and large fibers 920 fj. 

 (Kolliker, 93). A division of medullated fibers during their course 

 through a nerve is relatively rare. The greater number of fibers pass 

 unbranched from their central origin to the periphery, and only when 

 in the neighborhood of their terminal arborization do they begin to 

 divide. A point of division is always marked by a node of Ranvier. 



