CHAPTER XXXV. 493 



the silver solution to become cool, the sections are quickly washed 

 in distilled water and then transferred, one by one, into 20 per cent, 

 formalin neutralised with chalk. After one or two minutes, the 

 reduction is complete, and the sections may be washed, toned, fixed 

 and dehydrated, cleared with a mixture of carbolic acid 5 parts, 

 xylol 45 parts, creosote 50 parts, and mounted in balsam. 



Process II, for microglia. Sections are made as above, and then 

 treated for ten or fifteen minutes at 50 or 55 C. with the bromide- 

 formalin solution used for fixing. After washing in two or three 

 changes of water, one continues as in Process I, but warming the 

 ammoniacal silver carbonate solution at 50 or 55 C. until the 

 sections are dark yellow. 



Process III, also for microglia. The pieces are warmed for ten 

 minutes in the fluid used for fixing, and then cut by the freezing 

 method. The sections are washed in distilled water and stained 

 for ten to thirty minutes, either at room temperature or by careful 

 gentle warming, with an ammoniacal silver carbonate solution, 

 prepared by adding to 10 c.c. of 10 per cent, silver nitrate, first, 

 30 c.c. of 5 per cent, sodium carbonate, then ammonia, drop by 

 drop, until the precipitated silver carbonate is dissolved, and, lastly, 

 distilled water up to a total volume of 150 c.c. The sections are 

 kept in the impregnating bath for from ten to thirty minutes at 

 room temperature, but they should nevertheless remain almost 

 colourless. For the reduction, 1 per cent, formalin is used, and 

 sections are left therein until they have taken a greyish-yellow 

 tinge ; wash, tone, etc., as in Process I. 



The above refers to material fixed in Cajal's ammonium bromide- 

 formalin mixture ; if nervous tissues are fixed in 10 per cent, 

 formalin and sections treated as in Process I, nerve cells and axis 

 cylinders become stained as by Bielschowsky's method. If formol 

 sections of non-nervous tissues are treated in the same way, the 

 reticular tissue becomes stained. 



Retina* 



922. Fixation and Hardening. Notwithstanding the Encycl. mik. 

 Technik., 2nd ed., p. 75, I hold that osmic acid is by far the best 

 fixing agent. The retina of small eyes is best prepared by fixing 

 the entire unopened bulb with osmium vapours. 



Besides the sources quoted in the text, see SELIGMANN, Die mikro- 

 skopischen Untersuchungsmethoden des Auges, Berlin, S. Karger (Karl- 



* By A. Bolles Lee. 



