202 PRACTICAL HISTOLOGY. [XVII. 



four hours in dilute methyl-violet-sB. Wash and mount in glycerine. The 

 elastic fibres are stained of a bright blue, and they seem to spring from the 

 ends and sides of the muscular plexus also stained blue. The elastic fibres 

 arc fixed to the sarcolemma which, however, is not stained by the methyl- 

 violet and the union is a very firm one. Thus the muscular fibres divide, 

 and arc in elastic sheaths connected with elastic fibres. 



Ranvier has used the same membrane for studying the changes which take 

 place in a striped muscular fibre during contraction. 



LESSON XVII. 

 NERVE-FIBRES, 



NERVE-FIBRES are of two kinds, medullated and non-medullated ; 

 the former are found chiefly in the white matter of the nerve- 

 centres and the cerebro-spinal nerves, while the latter occur in 

 large numbers in the sympathetic system. 



I. Medullated Nerve-Fibre. The essential part is the axis- 

 cylinder, a soft, transparent rod or thread running from end to end 

 of the fibre, and composed of primitive fibrils. It is covered by 

 the myelin, or white substance of Schwann, or medullary sheath, 

 which envelops the axis-cylinder everywhere except at the termina- 

 tion of the fibre and at the nodes of Ranvier (fig. 181). The 

 myelin gives the nerve-fibre its highly refractive appearance and 

 its double contour, and it can be shown to consist of a gtroma or 

 network of fibrils of a peculiar chemical substance called neuro- 

 keratin, enclosing a semifluid fatty-like substance, containing, 

 amongst other chemical substances, protagon, a complex phos- 

 phorised fat. Histologically it consists of cylinder-cones or medul- 

 lary segments, whose ends are bevelled and fit one into the other, 

 but separated from each other by oblique clefts or incisures. 



Outside the medullary sheath is a thin, transparent, tough elastic 

 sheath, the primitive sheath, sheath of Schwann, or neurilemma. 

 It is not present in all nerve-fibres, being absent from the fibres of 

 the central nervous system. 



Between the axis-cylinder and the myelin is a thin layer of 

 matter, called by Kiihne axilemma. By others it is regarded as 

 an albuminous cement. 



At fairly regular intervals about i mm. along the course of a 

 fibre are constrictions, the nodes of Ranvier, where the myelin is 

 absent, so that the neurilemma appears to produce a constriction 

 at these points. The part between any two successive nodes of 



