THE NERVE FIBER 



133 



structure. The nature of these fibrils is not well understood. They are 

 apparently continuous with the neurofibrillar network of the cell body. 

 In certain nerve fibers of the lower animals these fibrils have a ten- 

 dency to collect into the center of the axis cylinder, leaving a peripheral 

 clear zone; this distribution is especially characteristic of those fibers 



FIG. 145. ISOLATED NERVE FIBERS FROM A FROG. 



The axis cylinders, the enveloping myelin sheaths, and the nodes of Ranvier are 

 clearly shown. Intra-vitam methylene blue stain. (Barker, after von Kollikcr.) 



which are not supplied with a medullary sheath. In mammals., however, 

 the fibrillae occupy a larger portion of the axis cylinder, the clear peri- 

 pheral area being correspondingly diminished until in man it can scarcely 

 be recognized. The fibrils of the lower animals are also coarser. 



Apathy, studying chiefly the lower animals, has considered these 'ulti- 

 mate fibrillce' to be the conducting element of the nerve fiber. Others, 

 however, lay greater stress upon the intervening clear portion, the 



