NERVE TISSUE. 



195 



fibres, as it does other fatty substances, here acts much less 

 thoroughly and more slowly ; there must certainly, therefore, 

 be a difference in the constitution of these two different 

 fibrous substances. 



Our fine nerve tubes present an additional peculiarity. 

 Every mistreatment, pressure, pulling or reagent to which it 

 is subjected causes a certain displace- 

 ment of the medulla, so that unnaturally 

 thinned spaces interchange with rounded 

 bulgings (e). The latter have been desig- 

 nated as varicosities, and varicose nerve 

 fibres are spoken of. Nothing of the 

 kind exists during life. 



We here touch upon another unsettled 

 question. Ranvier, at present the first 

 histologist of France, called attention to 

 a familiar phenomenon, to constrictions 

 which occur in the course of broad 

 medullated (peripheral, but not central) 

 fibres. Formerly, however, these con- 

 strictions were always regarded as a 

 product of the methods of preparation. 

 Now, these constricted places (Fig. 171) 

 are pretty regularly situated, and be- 

 tween every two, very nearly at half 

 the distance, one meets with a nucleus 

 of the sheath of Svvann (a). It is thus 

 in mammals, birds, and amphibia ; but 

 in fishes the number of nuclei is greater 

 between every two of these constric- 

 tions. 



These Ranvier's " constriction rings," 

 as the Germans have christened them, 

 deserve — although we are at present far removed from an 

 accurate knowledge of them — every consideration. The 

 medullary sheath certainly isolates the axis cylinder ; but 

 this medullary space permits the penetration of nutrient 



Fig. 171. — Nerve fibres of 

 the frog; a, after treatment 

 with picro carmine : b, c, d, with 

 osmic acid ; e, with nitrate of 

 silver. 



