NERVE-FIBRES-AXIS-CYLINDER-MEDULLARY SHEATH. 



der (the primitive band of Remak), a very fine, delicate, 

 pale structure ; and round about it a tolerably firm| 

 dark mass, here and there run- 

 ning together, the nerve-me- Fl - *** 

 dulla or medullary sheath [white 

 substance of Schwann] ; this 

 fills up the space between the 

 axis- cylinder and the external 

 membrane. But the nerve- 

 tube is generally so tightly 

 filled with its contents that, 

 when viewed in the ordinary 

 way, scarcely anything is seen 

 of the separate constituents, 

 the axis cylinder being always 

 with difficulty visible within the 

 medullary substance. Hence 

 the fact may be accounted for, 



that its very existence was disputed for years and the 

 view proclaimed by many, that it was also an appearance 

 due to coagulation, produced by a separation of the ori- 

 ginally homogeneous contents into an internal and exter- 

 nal mass. This view is however unquestionably incor- 

 rect : every mode of examination at last discloses this 

 primitive band ; even in transverse sections of nerves the 

 axis-cylinder is very distinctly seen in the interior, with 

 the medulla round about it. 



It is the so-called nerve-medulla which gives the nerve- 

 fibres in general their white appearance ; wherever the 

 nerves contain this constituent, they look white ; 



Fig. 78. Grey and white nerve-fibres. A. A grey, gelatinous nerve-fasciculus 

 from the root of the mesentery, after the addition of acetic acid. B. A broad white 

 primitive fibre from the crural nerve : a the axis-cylinder laid bare, V, v a varicose 

 state of the fibre with its medullary sheath ; at the end at m, m the medullary mat- 

 ter (myeline) protruding in convoluted forms. 0. A fine, white primitive fibre 

 from the brain, with its axis-cylinder protruding. 300 diameters. 



