528 



STRUCTURE OF NERVE-FIBRES. 



that the fibres have a double contour, while the substance itself breaks up into smaller or larger 



droplets, due not to coagulation (Pertik), but, according to Toldt, to a process like emulsifica- 



tion the drops pressing against each other. Thus, the fibre is broken up into masses so that 



it has a characteristic appearance (fig. 368, 6). It contains a large amount of cerebnn and 



lecithin, which swell up to form mvelin-like forms in warm 



water. It also contains fatty matter, so that these fibres are 



blackened by osmic acid, [while boiling ether extracts cholestenn 



from them]. Chloroform, ether, and benzin, by dissolving the 



fatty and some other constituents of the fibres, make them very 



transparent. [Some observers describe a fluid lying between the 



medulla and the axis-cylinder.] 



C. The Sheath of Schwann, or the neurilemma, lies imme- 

 diately outside of and invests the white sheath (fig. 368, 6, c), 

 and is a delicate structureless membrane, comparable to the 

 sarcolemnia of a muscular fibre. 



D. Nerve-Corpuscles. At fairly wide intervals under the 

 neurilemma, and lying in depressions between it and the 

 medullary sheath, are the nucleated ncrvc-coi-pusclcs, which are 

 readily stained by pigments (fig. 371). [They may be compared 

 to the muscle-corpuscles, the nuclei being surrounded by a small 

 amount of protoplasm which sometimes contains pigment. They 

 are not so numerous as in muscle.] [Adamkiewicz describes 

 nerve-corpuscles, or ' ' demilunes " under the neurilemma, quite 

 distinct from the ordinary nerve-corpuscles. They are stained 

 yellow by safranin, while the ordinary nerve-corpuscles are 



stained by methylanilin.] 



Banvier's Nodes or Constrictions. The neuri- 

 lemma forms in broad fibres at longer, and 

 in narrower ones at shorter intervals, the nodes 

 or constrictions of Ranvier (fig. 368, 6, t, t; 

 fig. 371 ; fig. 372, fs). They are constrictions 

 which occur at regular intervals along a nerve- 

 fibre ; at them the white substance of Schwann 

 is interrupted, so that the sheath of Schwann 

 lies upon the axis-cylinder [or its elastic sheath] 

 at the nodes. The part of the nerve lying 

 between any two nodes is called an inter- 

 annular or inter-nodal segment, and each such 

 segment contains one or more nuclei, so that 

 some observers look upon the whole segment 

 as equivalent to one cell. 



The function of the nodes seems to be to 

 permit the diffusion of plasma through the outer 

 sheath into the axis-cylinder, while the decom- 

 position-products are similarly given off. ^ [A 

 colouring-matter like picro-carmine diffuses into 

 the fibre only at the nodes, and stains the 

 axis-cylinder red, although it does not diffuse 

 through the white substance of Schwann. 



Incisures (of Schmidt and Lantermann). 



Fig. 372 



Fit* S7S 

 Each interannular segment in a stretched nerve Medullated genre-fibres 

 shows, running across the white substance, a 

 number of oblique lines, which are called 



M c d u 'l 1 a t e d * ?wl ' ,,Mm ^ s * ^^' ^3). They indicate that 

 nerve - fibres tne 8e 8 ,nent i s built up of a series of conical 

 blackened bv sect i ns eacn * which * s bevelled at its ends, 



with 

 osmic acid, a, axis-cylinder; 

 s, sheath of Schwann; n, 

 nucleus ; p, p, granular sub- 

 stance at the poles of the 

 nucleus; r, r, Kanvier's 

 osmic acid, fs, and the ^ evels are arranged in an imbricate n ^ w ' here the medullary 



Kanvier's node ! nanner > *J one v , er the other > whlle the *?* ht sheath is interrupted and 



sch, Schwann's l " te val ^ etw ^ n th f em . a W ears as an 1 . ncisu , re ; the axis-cylinder appears ; 



sheath. ^S c iL L? ; n ** matteF * h h incisures of Schmidt, 



a cylinder cone (kuhnt). ' ' 



Neuro-Keratin Sheath. According to Ewald and Ki'thne, the axis-cylinder, as well as the 

 white substance of Schwann, is covered with an excessively delicate sheath, consisting of neuro- 

 keratin, and the two sheaths are connected by numerous transverse and oblique fibrils, which 

 permeate the white substance. [The myelin seems to lie in the interstices of this mesh-work.] 



[Bod-like Structures in Myelin. If a nerve be hardened in ammonium chromate (or picric 

 acid), M'Carthy has shown that the myelin exhibits rod-like structures, radiating from the 



