STPtUCTUHE OF MEDULLATED XEIIVE-FIEEES. 



127 



bounded on each side by a well-defined, simple and usually gently 

 sinuous outline. Their size differs considerably even in the same nerve, 

 but much more in different parts of the nervous system ; some beino- 

 as small as the ts (joo*^^i ^^^ others upwards of the ToVoth of an inch in 

 diameter; moreover, the same fibre may chance its size in different 

 parts of its course, and it is generally smaller at its central and peri- 

 pheral ends. Very speedily after death, and especially on exposure to 

 the action of water, these seemingly homogeneous fibres become altered: 

 and it is when so altered that they are commonly subjected to examina- 

 tion, as represented in fig. 81, a. In particular instances, and in 



Fk'. SI. 



Fig. SI. — A. WniTE OR IIedullated Nerve-Fibres, showing the sinuous outline 

 and double contours (after Bidder and Yolkmann). 



B. Diagram to s-how the parts of a medullated fibi-e, viz. 1, 1, primitire sheatJi. 2, 2, 

 2i-hite substance or medullary sheath. 3, axis ov primitive band. 



c. Diagram intended to represent the appearances occasionally seen in the tulmlar 

 fibres. 1, 1, membrane of the tube seen at jmrts where the white substance has separated 

 from it. 2, a part where the white substance is interrupted. 3, axis projecting beyond 

 the broken end of the tube. 4, part of the contents of the tube escaped. 



favourable circumstances, it may be discovered that the fibre is com- 

 ]wsed of a fine membranous tube, inclosing a peculiar soft substance, and 

 that this contained substance itself is distinguishable into a central part 

 placed like a sort of axis in the middle of the tube, and a peripheral por- 

 tion surrounding the axis, and occupying the space between it and the 

 tubular inclosing membrane. In the annexed ideal plan (fig. 81, b), 

 the memlranous iiile or primitire sltpcifh, is marked 1, 1 : the central part, 

 marked 3, was named ci/Iinder-axis by Purkinje, who considered it to 

 be identical with the structui-e previously described by Eemak under 

 the name oi the i/rimii ire hand (fibra primitiva) ; the matter surround- 

 ing it, marked 2, 2, is supposed to be the chief cause of the whiteness of 



