THE HISTOLOGICAL TECHNIQUE 7 



impregnation. The manner in which the impregnation is facilitated when the tissue is 

 placed in the silver by anything that stimulates diffusion, is shown in preparations by the 

 fact that wherever a crack or a break in the piece exists, in that vicinity the greatest number 

 of elements are impregnated. The addition of formic acid to the silver nitrate solution (one 

 drop to 200 cc.) as recommended by Cajal, has not proved of any advantage. 



Another modification of the silver solution may be mentioned here. It is at times desirable 

 to double-stain preparations in order to bring out the unimpregnated elements. This may be 

 done by a quick staining of the sections in any suitable dye dissolved in 95 % alcohol. 

 Another method consists in adding the stain to the silver solution, the requirements being 

 that the stain has a neutral reaction and does not affect nor is affected by the silver nitrate 

 solution. Some of the anilines, e.g. acid fuchsin, and a neutral carmine solution, fulfil these 

 requirements. This gives of course an in toto stain. 



Cutting and Mounting. Preparatory to cutting, the piece is transferred directly from the 

 silver solution into 95 % alcohol. It may be left in this an hour, or longer, according to its 

 size. The alcohol should be changed several times to free the tissue as far as possible from 

 the silver nitrate. Pieces of tissue impregnated by the rapid Golgi method cannot be 

 infiltrated with celloidin, as the prolonged immersion in alcohol, etc., is liable to injure the 

 impregnation ; nor, indeed, is it necessary, as sections can be readily cut of the desired thick- 

 ness without imbedding. Consequently the piece is placed for a few minutes in a thin and 

 then in a thick solution of celloidin, to give it support and to gum it upon the microtome 

 block. After placing it upon the block, the celloidin is hardened by immersion in chloro- 

 form for a few minutes. This should not be too prolonged, or the tissue will crack or 

 become friable. 



The sections are cut in 95 % alcohol and kept in the same until cleared. They 

 should be cut as a rule from 50 /A to 100 /A thick. Thinner sections cut the nerve 

 cells and their processes into small pieces. Small sections can be removed from the knife 

 with a brush ; with larger ones, however, the slide must be brought up under the edge 

 of the knife and the section gently washed or pulled down upon it. The upper surface 

 of the knife should be as nearly level as possible, to avoid an accumulation of alcohol at the 

 edge. If it be found impossible, on account of its size, to cut the unimbedded piece 

 without considerable tearing, painting the surface of the block with a medium thick solution 

 of celloidin must be resorted to. Not much time need be lost by this procedure, especially 

 as sections are then more quickly removable from the knife. As the knife is pushed back over 

 the block after a cut, the surface of the block is wiped dry by the knife, and the celloidin 

 can then be applied to it with a brush. While the last section which has just been brought 



