THE BRAIN AND SPINAL CORD. 243 



CHAPTER XIX. 



THE CENTRAL NEVOUS SYSTEM ; THE BRAIN AND 

 SPINAL CORD. 



SOME experience has already been obtained of tbe 

 methods which are employed for studying the cellu- 

 lar elements of the central nervous system in an 

 isolated condition (see p. 130). The spinal cord was 

 the part then under investigation, but the nerve- 

 cells found in the gray matter of the cerebrum and 

 of the cerebellum may be observed in the same 

 manner with equally satisfactory results. Without 

 delaying, then, to repeat the directions there given, 

 we may pass on to describe the best methods of pre- 

 paring sections of the parts in question. 



Preparation with bichromate of ammonia. 

 To harden any part of the central nervous system 

 the most generally useful reagent is the bichromate 

 of ammonia (2 or 3 per cent, solution). This will of 

 itself render the tissue sufficiently firm for obtain- 

 ing thin sections, but it is always best to finish first 

 with weak and then with strong spirit, especially as 

 this fluid must be used to wet the razor. The pieces 

 to be hardened should not be too large, or at all 

 events not to thick, but the solution has consider- 

 able power of penetration (differing in this respect 

 from chromic acid), and the whole length of the 

 spinal cord of any of the smaller quadrupeds, and 

 even that of man, may be hardened in it intact if 

 put in perfectly fresh. It is always better, however, 

 to cut it into short lengths. The pieces are ready 

 to be transferred from the bichromate of ammonia 

 solution to dilute spirit in three or four days if 

 small ; in a week if rather larger: after twenty-four 

 hours they are placed in strong spirit. 



