NERVOUS SYSTEM— GENERAL METHODS 459 



he removes the brains from the skull and puts them back for 

 fifteen to twenty days into the formol. 



Kadyi {Neurol. CentrbL, xx, 1901, p. 687) takes 5 parts of 

 formol, 100 of water, and 2 of bicarbonate of soda, for four to 

 ten days. 



Herdlicka {Proc. U. S. Nat. Mus., xxx, 1906, p. 304) takes 

 3 parts of formol, 25 to 45 of water, and 72 to 52 of 95 per cent, 

 alcohol. 



Landau {Ztschr. wiss. Mikr., xl, 1923, p. 22) recommends 

 fixing large pieces, or even whole brains, in a mixture of 90 parts 

 of 1 per cent, picric acid and 10 of formalin. The picric acid 

 is removed afterwards by adding some lithium iodide to the 

 alcohols of ascending strength used for dehydration. 



The methods for removing the deposits which sometimes 

 become formed in tissues preserved in formalin are described in 

 § 113. 



984. Chromic Salts. The most commonly used is potassium 

 bichromate. The liquid of Erlicki has a more rapid action than 

 other solutions of chromic salts, but it has been generally aban- 

 doned because of the alterations it very often produces. 



Saiili {Ztschr. wiss. Mikr., ii, 1885, p. 1), after investigating the 

 action of the usual solutions, concludes that the best hardening 

 agent for fresh tissues is pure potassium bichromate, in 3 or 4 

 per cent, solution, the hardening being done in a cold j^lace. 

 He rejects the liquid of Erlicki on account of the precipitates to 

 which it so frequently gives rise. 



Obersteiner is of the same opinion, and recommends pure 

 bichromate for general hardening purposes ; whilst for the study 

 of delicate structural details he recommends fixing in Fol's 

 modification of Flemming's fluid (§ 47) for twenty-four hours, 

 followed by washing with water and hardening in 80 per cent, 

 alcohol. 



In view of the slowness of penetration of chromic salts it is 

 often advisable first to place the material for some days in 10 

 per cent, formalin in saline, and then transfer it into a 3 per cent, 

 solution of potassium bichromate or Miiller's fluid. But the 

 formalin should be properly washed away and the bichromate 

 solution changed after twenty-four hours, and again as soon as it 

 becomes even slightly turbid. 



Potassium bichromate should be employed at first in 2 or 2-5 

 per cent, solutions, to be replaced by more concentrated solutions 

 up to 5 per cent. Da Fano recommends leaving jars in an 

 incubator at 22° to 24° C. until the hardening is completed. 

 Instead of pure potassium bichromate in solutions of increasing 

 strength, Miiller's fluid can equally well be employed throughout 

 the process of hardening ; it only needs to be changed from time 

 to time. 



