OF THE NERVOUS SYSTEM. 



369 



Fig. 118. 



NKRVOCS FILAMESTS from white substance of 



preparation, produces an irregularly bulging or varicose appearance 

 in them at various points, owing to the readiness with which the 

 semi-fluid white substance in their interior is displaced in different 

 directions. (Fig. 118.) Sometimes spots may be seen here and 

 there, where the nervous matter has been entirely pressed apart in 

 the centre of a filament, so 

 that there appears to be an 

 entire break in its continuity, 

 while the investing mem- 

 brane may be still seen, pass- 

 ing across from one portion 

 to the other. When a nerv- 

 ous filament is torn across 

 under the microscope and 

 subjected to pressure, a cer- 

 tain quantity of the semi- 

 fluid white substance is 

 pressed out from its torn 

 extremity, and may be en- 

 tirely separated from it, so 

 as to present itself under the 



. brain. a, a, a. Sott substance of tlie filaments pressed 



form Of irregularly rOUnded O ut, and flojUuig in irregularly ruuuded drops. 



drops of various sizes (a, a, 



a), scattered over the field of the microscope. The varicose appear- 

 ance above alluded to is more frequently seen in the smaller nerv- 

 ous filaments from the brain and spinal cord, owing to their soft 

 consistency and the readiness with which they yield to pressure. 



The second effect produced by the artificial preparation of the 

 nervous matter is a partial coagulation of the white substance of 

 Schwann. , In its natural condition this, substance has the same 

 consistency throughout, and appears perfectly transparent and 

 homogeneous by transmitted light. As soon, however, as the nerv- 

 ous filament is removed from its natural situation, and brought in 

 contact with air, water, or other unnatural fluids, the soft substance 

 immediately under the investing membrane begins to coagulate. 

 It increases in consistency, and at the same time becomes more 

 highly refractive; so that it presents on each side, immediately 

 underneath the investing membrane, a thin layer of a peculiar 

 glistening aspect. (Fig. 119.) At first, this change takes place 

 only in the outer portions of the white substance of Schwann. 

 The coagulating process, however, subsequently goes on, and 

 24 



