74 



(c) The osmic acid is useful in 1% aqueous solution usually 

 mixed with a potassium dichromate solution. 6 to 10 days suffice; a 

 longer time does no harm (brittleness) . 



(d) See below. 



202. As employed by van Gehuchten [18] : 



1. Harden in 3% potassium dichromate solution for 3 weeks; 



2. Transfer to a mixture of 1% osmic acid solution, 1 part; 

 3% potassium dichromate, 4 parts; for 3 weeks, blocks of tissue 

 not more than 2 mm. thick. Use abundance of the fluid and change 

 2 to 3 times if deemed necessary. 



3. Wash in running water for 12 to 24 hours. 



Avoid the paraffin and celloidin imbedding methods if pos- 

 sible. Of these two the celloidin method is preferable. For further 

 treatment, see 227. 



203. Flemming's or Benda's fluids ( 19, 227) may be used 

 instead of the Marchi method for the same purpose with small objects, 

 and peripheral nerves, etc. 



204. The Golgi Methods, whose field of application is not 

 confined to the nervous system (gland ducts, bile capillaries, blood 

 capillaries, secretory canaliculi, muscle, etc.) consist in (a) mordant- 

 ing the fresh or living tissue for a sufficient length of time in a dichro- 

 mate solution, usually containing as well osmic acid or formalin, 

 and then (b) transferring to a silver nitrate solution, whereupon 

 certain of the nervous elements become outlined more or less com- 

 pletely by impregnation with a chrome-silver combination. 



The reaction probably depends on the presence, in certain "phy- 

 siological states" of the elements of a substance or substances which 

 combines with the chromium salt (with reduction?) and through it 

 with the silver salt. These hypothetical substances, or possibly 

 physical states, seem to disappear more or less rapidly after the 

 death of the animal and their power to hold the silver in combination 

 to decrease with the progress of the dichromate mordantage beyond 

 a certain point. If successful, certain of the cells and their processes, 

 amyelinic, and to a certain extent, myelinic nerve fibers, are out- 

 lined by an impregnation, black by transmitted, brown by reflected 

 light. 



205. The method is, however, capricious; success depends on 

 (a) the kind of animal; different parts and tissues react more satis- 

 factorily in some animals or classes of animals than in others, (b) 



