STRUCTURE OF NERVE-FIBRES. 



527 





Node of 

 Ranvier. 



Primitive 



Sheath. 



by reagents, they are not only refractive, bnt exhibit a double contour, the margins being dark 

 and well defined.] Each fibre consists of 



[1. Schwann's sheath, neurilemma, or primitive sheath ; 



2. White substance of Schwann, medullary sheath, or myelin ; 



3. Axis-cylinder composed of fibrils and surrounded by a sheath 

 called the axilemma ; 



4. Nerve-corpuscles.] 



A. The axis -cylinder, which occupies | to ^ of the breadth of the 

 fibre, is the essential part of the nerve, and lies in the centre of the 

 fibre, like the wick in the centre of a candle (fig. 368, 6, a). Its 

 usual shape is cylindrical, but sometimes it is flattened or placed 

 eccentrically [this is most probably due to the hardening process 

 employed]. It is composed of fibrils 

 [united by cement or stroma; they 

 become more obvious near the termi- 

 nations of the nerve, or after the action 

 of reagents, which sometimes cause 

 the fibrils to appear beaded. It is 

 quite transparent, and stains deeply 

 with carmine or logwood], while dur- 

 ing life, its consistence is semi-fluid. 

 According to Kupffer, a fluid "neuro- 

 plasma" lies between the fibrils 

 [while, according to other observers, 

 the whole cylinder is enclosed in an 

 elastic sheath peculiar to itself and 

 composed of neuro-keratin. This 

 sheath is called by Kuhne, the axi- 

 lemma. Each axis-cylinder is* an enor- 

 mously long process of a ganglionic 

 cell]. 



Fromann's Lines. Chloroform and 

 collodion render it visible, while it is 

 most easily isolated as a solid rod, by 

 the action of nitric acid with excess 

 of potassium chlorate. When acted 

 on by silver nitrate, Fromann observed 

 transverse markings on it, but their 

 significance is unknown (fig. 368, 8). 



B. The white substance of Schwann, 



medullary sheath or myelin, surrounds \\ jmll Substance 



the axis-cylinder, like an insulating \\ /j| Schwann. 



Node of 

 Ranvier. 



Nerve- 

 Corpuscles. 



Axial 

 Cylinder. 



Fig. 369. Fig. 370. Fig. 371. 



Fig. 369. Transverse section of the nerve-fibres of the spinal cord, the axis-cylinders like dots 

 surrounded by a clear space (myelin). Fig. 370. Remak's fibre from vagus of dog. 

 b, fibrils; n, nucleus; p, protoplasm surrounding it. Fig. 371. Medullated nerve-fibre of 

 a rabbit acted on by osmic acid, x 400. 



medium around an electric wire. In the perfectly fresh condition it is quite homogeneous, 

 highly glistening, bright, and refractive ; its consistence is fluid, so that it oozes out of the 

 cut ends of the fibres in spherical drops (fig. 368, x), [myelin drops, which are always marked 

 by concentric lines, are highly refractive, and best seen when a fresh nerve is teased in salt 

 solution]. After death, or after the action of reagents, it shrinks slightly from the sheath, so 



