Chap, xiv.] NERVE-FIBRES. 107 



fibre stands in a certain relation to the distance of its 

 periphery from the nerve-centre and to its functional 

 activity. 



A medullated nerve-fibre in the fresh condition 

 is ^a bright glistening cylinder, showing a dark 

 double contour. Either spontaneously afcer death, 

 or after re-agents as water, salt solution, dilute 

 acids or after pressure and mechanical injury, the 

 outline of the nerve-fibre becomes irregular ; smaller, 

 or larger, glistening dark -bordered droplets and masses 

 appear and gradually become detached. These 

 droplets and masses are derived from the fatty sub- 

 stance constituting the medullary sheath or white 

 substance of Schwanh (see below). When a nerve- 

 fibre within the bundle undergoes degeneration during 

 life, either after section of the nerve or after other 

 pathological changes, or in the natural course of its 

 existence (S. Mayer), the medullary sheath breaks up 

 into similar smaller or larger globules or particles, 

 which gradually become absorbed. 



140. Each medullated nerve-fibre (Figs. 64A, 66) 

 consists of the following parts : (a) the central axis 

 cylinder. This is the essential part of the fibre, and is 

 a cylindrical or bandlike, pale, transparent structure, 

 which in certain localities (near the terminal distribu- 

 tion, in the olfactory nerves, in the central nervous 

 system), and especially after certain re-agents, shows 

 itself composed of very fine homogeneous or more or 

 less beaded fibrillse the elementary or primitive fibrHIte 

 (Max Schultze) held together by a small amount of a 

 faintly granular interstitial substance. The longitu- 

 dinal striation of the axis cylinder is due to its being 

 composed of primitive fibrillae. The thickness of the 

 axis cylinder is in direct proportion to the thickness 

 of the whole nerve-fibre. The axis cylinder is said 

 to be enveloped in its own hyaline more or less elastic 

 sheath, composed of neurokeratin. 



