320 CENTRAL NERVOUS SYSTKM. 



most used are the chromic salts. Chromic acid was much 

 used at one time, but most workers now agree that its action, 

 though much more rapid than that of the salts, is much more 

 uneven, and frequently causes a disastrous friability of the 

 tissues. Osmic acid is excellent according to BEVAN LEWIS, , 

 it is the best of hardening agents ; but its employment is un- 

 fortunately very restricted, as it can hardly be used for objects 

 of more than a cubic centimetre in size. 



It has already been noted that the liquid of Erlicki has a 

 more rapid action than the other solutions of chromic salts ; 

 for this reason it is one of the most commonly employed 

 solutions. SAHLI, however (I.e.), after having studied the 

 action of the usual solutions, concludes that the best harden- 

 ing agent for fresh tissue is pure bichromate of potash, in 3 

 or 4 per cent, solution, the hardening being done in a cold 

 place. And he does not approve of the addition of sodium 

 sulphate (Miiller), and rejects the liquid of Erlicki on account 

 of the precipitates it so frequently gives rise to. 



OBERSTEINER is of the same opinion, recommending pure bi- 

 chromate for general hardening purposes ; whilst for the study 

 of the most delicate structural relations he recommends fixing 

 in Fol's modification of Flemming's liquid ( 35, p. 24, supra) 

 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 to treat preparations for twenty-four hours 

 or more with alcohol of 80 to 90 per cent, before putting them 

 into the hardening liquid, in order to avoid maceration of the 

 deeper layers of tissue. 



For the question as to how far certain so-called pathological alterations 

 of ganglion-cells should be attributed to putrefactive changes or to the 

 influence of reagents see Neurologisches Centralb. for the years 1881 and 

 1885. 



As to the so-called "pigment spots" produced by the liquid of Erlicki 

 see supra, 71 >. 



661. Strengths of the Reagents. All hardening reagents (ex- 

 cept osmic acid) should at first be taken as weak as is consis- 

 tent with the preservation of the tissue, and be changed by 

 IcL-rees for stronger. 



Osmic ;icid may be taken of 1 per cent, strength, and will 



