XXX.] SPIXAL CORD. 345 



brain, after being hardened in 2 per cent, potassium bichromate for 3-4 days, 

 are then transferred to a stronger solution, say 3 per cent., for 4 days. In- 

 crease successively the quantity of the bichromate until 4-6 per cent, is 

 reached. It takes 30-50 days, according to temperature or other circum- 

 stances, to get the tissues properly hardened. The hardened parts are then 

 transferred for 24-48 hours to .5-. 7 5 per cent, silver nitrate. It is better to 

 place them first of all in a small quantity of silver nitrate, and to wash them 

 in this fluid. The fluid becomes of a dark orange tint from silver chromate. 

 Place the pieces in fresh silver nitrate. Harden in alcohol. Make hand 

 sections, clarify them in the usual way, and mount in balsam, but do not apply 

 a cover-glass. 



(b.) Rapid Method. Make the following mixture : 



Osmico- Bichromate Mixture. 



Potassic bichromate (3 per cent.) . . 20 cc. 



Osmic acid (i per cent.) . . . 5 ,, 



Place small pieces of the fresh organ in 15 cc. of this fluid for 2-3 days, and 

 then place them in the silver solution (.5-1 per cent.; for 1-2 days. I have 

 found the cells of the cerebral cortex stained black two or three hours after 

 immersion of the organ in silver nitrate. 



This is an excellent method, specially useful for young or embryonic nervous 

 system, as the fluid penetrates more readily where the myelin is scanty. It is 

 very capricious in its action. Sometimes only small parts are stained. The 

 nerve-cells and their processes, and nerve-fibres without myelin, and axis 

 cylinders are stained black. 



(c. ) Medium Method. Small pieces of the fresh tissue are placed for 3-5 days 

 in 3 per cent, potassic bichromate ; then 3-4 days in the osmico-bichromate 

 mixture as above, and then in silver nitrate. 



It is not a matter of indifference which method is used. Golgi's method 

 only stains certain elements nerve-cells, neuroglia-cells, and sometimes 

 blood-vessels. Thus, by the rapid method, in the case of the cerebellum, the 

 parallel fibres and the granular layer are the chief parts stained; Purkinje's 

 cells stain best by the slow method. As a general rule, the axis cylinders and 

 their processes stain best by means of the rapid and medium methods, the 

 slow method, as a rule, staining best the protoplasmic processes. 



The rapid silver method has been extensively used by Golgi and his pupils, 

 by Kb'lliker, but above all by Ramon, y Cayal. The black deposit Golgi calls 

 a " black reaction." 



24. Golgi and Mondino's Sublimate Method. Small pieces of the central 

 nervous system, after hardening in bichromate of potash, are placed in a 

 watery solution (.25 per cent.) of corrosive sublimate. The volume of fluid 

 must be large, and renewed frequently, j,.e., as often as it is yellow. After 

 10-15 days the reaction has occurred in small pieces ; but it is better to expose 

 the pieces longer than this to the action of the salt ; in fact, prolonged im- 

 mersion rather improves it. The tissues may remain for months in the fluid 

 without disadvantage. The sections must be very thoroughly washed, else, 

 after being mounted, needle-shaped crystals of the sublimate are apt to form. 

 The sections are mounted in balsam or glycerin ; only they are mounted with- 

 out a cover-glass. 



In Golgi's silver method the cells, and in some preparations the blood- 

 vessels as well, are opaque and black. Sometimes the body of the cell and its 

 finest ramified processes can be seen with the utmost sharpness. The silver is 

 deposited only on the cells and their processes, not on the nerve-fibres. The 

 sections, however, are often dotted over with a black metallic deposit. In the 

 mercury preparations, if the cells do not appear to be very black, they may be 

 darkened by washing them in sodic sulphite. 

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