150 STRUCTURE OF THE NERVOUS SYSTEM, BY MAX SCHULTZE. 



investment around the axis cylinder, and consists of an oily 

 substance containing protagon, and capable of powerfully 



refracting light. It gives to the 

 nerves their characteristic dark 

 strongly denned borders. Con- 

 sidering the great delicacy of the 

 axis cylinder, and the perfectly 

 fluid nature of the medullary 

 sheath, the consistence of these 

 medullated fibres cannot, it is 

 obvious, be very great. In point 

 of fact, the difficulty of isolating 

 the medullated fibres of the nerve 

 centres is quite as great as in the 

 case of the naked axis cylinder. 

 The isolation of unaltered fresh 

 medullated fibres, obtained from 

 the grey and white substance of 

 the brain and spinal cord, can 

 only be accomplished for short 

 distances. The fibres break up 

 into short pieces as soon as the 

 preparation needles are used, 

 partly in consequence of pressure 

 and tearing, but partly also from 

 the imbibition of fluid. Even 

 when the more indifferent liquids 

 are employed, these fragments 

 of fibres rapidly undergo remark- 

 able changes of form, developing 

 knots and swellings on their 

 surface (see fig. 19), and some- 

 times regular moniliform enlarge- 

 *< nd g ments, though for the most part 

 JJl ft 4 such enlargements appear only 

 "g $ as irregular varicosities, giving a 

 very characteristic appearance to 



the fibre. After a short time numerous spherical and short 

 cylindrical curved masses separate from the medullary invest- 



