STRUCTURE OF THE NERVOUS TISSUE. 



509 



(Ranvier). 

 A. Intercostal nerve of the mouse, treated with sil- 



B. Nerve-fibre from the sciatic nerve of a full-grown 

 rabbit. A, node of Ranvier ; M, medullary sub- 

 stance rendered transparent by the action of 

 glycerine ; CY, axis-cylinder presenting the lines 

 of Fromann, which are very distinct near the 

 node. The lines are less marked at a distance 

 from the node. 



situations, break up into fibrillae. A 

 fibrillated appearance, indeed, is often 

 observed in nerves in their course, and 

 it is now the general opinion that the 

 axis-cylinders are composed of fibrillae ~ 

 held closely together by connective 

 substance. This fibrillated structure 

 of the nerves is quite prominent in 

 some of the lower orders of animals. 



The various appearances which 

 the nerve-fibres present under differ- 

 ent conditions are represented in 

 Figs. 172 and 173. 



Non-nwdullated Nerve - Fibres. 

 These fibres, which are largely dis- 

 tributed in the nerVOUS System, ap- FlG . m.-Nodes of Ranvier and lines of Fromann 



pear to be simple prolongations, 



without alteration, of the axis-cylin- ver nitrate. 



ders of the medullated fibres. They 



are found chiefly in the peripheral 



terminations of the nerves and in the 



filaments of origin of the fibres from 



the nerve-cells. Some anatomists think that they have a 

 delicate investing membrane, but this has not been satis- 

 factorily demonstrated. 



Gelatinous Nerve -Fibres (Fibres of Remak). There 

 has been some difference of opinion with regard to the 

 physiology of the so-called gelatinous nerve-fibres. Some 

 anatomists have regarded them simply as elements of con- 

 nective tissue, and others have described them as axis-cyl- 

 inders surrounded with a nucleated sheath ; but the fibres 

 do not present the lines of Fromann when treated with sil- 

 ver nitrate. While elements of connective tissue may have 

 been mistaken for true nerve-fibres, there are in the nerves, 

 particularly in those belonging to the sympathetic system, 

 fibres resembling the nerve-fibres of the embryon. These 

 are the true, gelatinous nerve-fibres, or fibres of Remak- 

 All the nerves have this structure until about the fifth 

 month of intraiiterine life, and in the regeneration of 

 nerves after division or injury, the new elements usually 

 assume this form before they arrive at their full develop^ 



FIG. 174. Fibres of rn pr) t 

 Remak ; magnified L 



300 diameters (Rob- The true, gelatinous nerve-fibres present the following 



With the gelatinous characters : They are flattened, with regular and sharp bor- 



seeiTtwo oTthe^ ders, grayish, pale and always fibrillated, with very fine 



deredner d ve r -fibres r " granulations, and a number of oval, longitudinal nuclei, a 



34 



