THE NERVOUS TISSUES. 



75 



Owing to the absence of the middle coat in these positions, the 

 outer sheath, or neurilemma, is brought into contact with the 

 continuous axis-cylinder. The portions of the fibre included be- 

 tween two constrictions the internodes, or 

 internodal segments vary in length with 

 the size of the fibre, being longer (about 

 i mm.) in large and much shorter in thin 

 fibres. Each internode pos- 

 sesses a single nerve-cor- FlG 

 puscle, usually about its * 

 middle, and probably elon- 

 gates during the growth of 

 the nerve. The neurilemma 

 is not broken by the nodes 

 into segments, but forms a 

 continuous sheath. When 

 a medullated nerve - fibre 

 branches, the bifurcation cor- 

 responds in position to a node 

 of Ranvier. 



After treatment with silver 

 nitrate the positions of the 

 nodes of Ranvier are rendered 

 conspicuous by the appear- 

 ance of minute dark-brown 

 crosses : the transverse arm 



Medullated nerve- 

 fibres after treatment 

 with osmic acid, from 

 frog : A , fibre displays 

 the incisions of the me- 

 dulla, or Schmidt-Lan- 

 termann segments ; B, 

 the medullary substance 

 exhibits a reticulated 



Silvered nerve-fibres: A, 

 small bundle of medullated 

 fibres displaying the silver 

 crosses at several nodes ; B, 

 node of Ranvier under high 

 power : the horizontal limb 

 of the cross is produced by 

 the stained intersegmental 

 cement-substance ; the ver- 

 tical limb is formed by the 

 colored axis-cylinder ; C, 

 silvered axis-cylinder show- 

 ing a bi-conical enlarge- 

 ment and the transverse 

 markings or lines of From- 



is formed by the stained, appearance, 

 internodal albuminous sub- 

 stances, forming an annular disk, sometimes called the constricting 

 band (Ranvier), while a stained portion of the axis-cylinder con- 

 tributes the less distinctly marked vertical lines of the cross. 

 Closely-placed transverse markings, known as Frommann's lines, 

 as well as bi-conical swellings, occasionally are noted along the axis- 

 cylinder after treatment with silver ; their significance, however, is 

 still undetermined. 



The medullary substance is very prone to post-mortem change, 

 the coagulated or partly disintegrated myelin producing various 

 grotesque distortions in the contour of the nerve-fibre. After treat- 

 ment with osmic acid and other reagents, the white substance of 

 Schwann displays oblique markings which are, apparently, clefts or 

 incisions involving the middle coat ; relying upon these appearances, 

 many regard the medullary substance as made up of elongated pieces, 

 the Schmidt-Lantermann segments, several of which are in- 

 cluded within each internode. 



